Taxonomy of sweet potato batata (Ipomoea batatas L.) sec. il Cronquist SystemNature
Superdomain: Biota or Vitae or Eobiontae
Dominium/Superkingdom or Domain: Eucariotae
Regnum/Kingdom: Plantae (Plants)
Subregnum/Subkingdom: Tracheobionta (Vascular plants)
Superdivisio/Superdivision: Spermatophyta (Seed plants)
Divisio/Division: Magnoliophyta Cronquist, 1996 (Flowering plants)
Subdivisio/Subdivision: Magnoliophytina Frohne & U. Jensen ex Reveal, 1996
Classis/Class: Rosopsida Batsch, 1788 (Dicotyledons)
Subclassis/Subclass: Lamiidae Takht., 1993
Superorder/Superordo: Solananae R. Dahlgren., 1992
Ordo/Order: Solanales Dumortier, 1829
subOrder/subOrdine: Convolvulineae Engl., 1898
Familia/Famiglia: Convolvulaceae A.L. de Jussieu, 1789 (in inglese: morning-glory family)
Subfamilia/Sottofamiglia: Ipomoeeae Hallier (1893)
Tribus/Tribù: Ipomoeeae Dumort., 1827
Genus/Genere: Ipomeae L. 1753
Species/Specie: Ipomeae batatas L. 1753
Taxonomy of the sweet potato (Ipomeae batatas L.) sec. il sistema APG
Clade: Eucariota Whittaker & Margulis,1978
Plantae Haeckel, 1866
Clade: Unassigned monocots
Ordo: Solanales Dumortier, 1829
Familia: Convolvulaceae A.L. de Jussieu, 1789 (in inglese: morning-glory family)
Subfamilia/Sottofamiglia: Ipomoeeae Hallier f. (1893)
Genus/Genere: Ipomeae L.
Specie: Ipomoea batatas (L.) Lam.
Common names of Ipomoea batatas in the world :
- batata or patata dolce or patata americana in Italian;
- sweet potato or tuberous morning glory or kuumara in English;
- yam in the United States;
- patate douce in French;
- batate or Süßkartoffe in German;
- batata or boniato or camote in Spanish;
- batata-doce in Portuguese;
- batat in Swedish;
- zoete aardappel in Dutch;
- batata-da-terr in Portuguese (Brazil);
- Kkumara or tuktuka to Aymara (Peru);
- papa dulce in Spanish (Venezuela);
- apichu a Quichua (Peru);
- gogum in Korean.
The sweet potato, also known as tuberous morning glory and less commonly as sweet potato is a species in the family Convolvulaceae, cultivated in tropical regions for its edible tubers, sweet and starchy. This species has nothing to do with the potato which, among other things, belongs to a different botanical family and the organ that is used is not a tuber but a root.
Origin and distribution
The species is native to the tropical areas of the Americas where the cultivation was practiced 5000 years ago. It spread quickly throughout the region, including the Caribbean. Imported after the colonization of the Americas, is widespread in Europe and also in Asia, where its presence in China was already documented in the late sixteenth century. The sweet potatoes were also known in Polynesia before the explorations from the west.
How exactly the species has come up in Oceania is the subject of a lively debate involving observations archaeological, linguistic and genetic.
From South America came in the old continent thanks to Christopher Columbus, the sweet potato contains many useful substances to our body as fiber, vitamins A and C (and in smaller quantities B and E), protein, potassium, magnesium, iron and calcium . L 'Ipomoea batatas is also rich in flavonoids and anthocyanins and has great antioxidant and anti-aging. It is no coincidence that some wrinkle creams contain an extract of this tuber.
The American Association CSPI (Center for Science in the Public Interest) in a ranking on the plants healthier put the red batata first. This tuber has obtained the highest score thanks to the high concentration of beneficial substances for our body, not only inside but also and especially in the peel.
The oldest finds show that the consumption of sweet potato (probably after harvest) have been identified in Peru and date from 8,000 BC. Peru is the oldest place where they have found traces of consumption and domestication. Studies show that was cultivated for more than 10 thousand years in the region of Ayacucho.
Different cultures have adopted the potato as the predominant food in your diet. In Peru testify that the iconography allude to this food found in the Paracas culture sheets, drawings on the ceramics of the Moche civilization, as well as alluding to the sweet potato prints found in the caves of Huarochiri, located west of the Lima.
On the contrary, it seems that the first crops have taken place in Central and South America only in 3,000 BC. & Egrave; conceivable that the area of origin of the sweet potato is between the Yucatan Peninsula (Mexico) and the mouth of the Orinoco (Venezuela).
Today, thanks to import-export, the sweet potato is grown in all temperate zones of the world, wherever there is a sufficient amount of water to the development and maturation. In all probability, the first areas NOT originate that have hosted are the North America, Polynesia, Japan and later across Asia Continental.
According to statistics of the FAO (Food and Agriculture Organization), the production of sweet potato in 1996-1998 was 133 million tons and in 2004 of 127 million tons, most of which are in China (105 million tonnes.) Which is also used for power livestock (Table 1).
Table 1 - Sweetpotato area, production, and yield by continent (19961998)
Sweetpotato is an important global food crop in terms of both area and production (Table 1). It is grown in over 100 countries, almost entirely developing
countries in Asia (78% of the global sweetpotato area) and Africa. China has 73% of the global area and 84% of the global production. Thus,
sweetpotato distribution is extremely concentrated, yet sweetpotato production is
spread over many countries. Vietnam, Indonesia, India, and the
Philippines are other important sweetpotato- producing countries in Asia. There
is a conspicuous concentration of sweetpotato in the East African Highlands
(Uganda, Rwanda, Burundi, Kenya). In Papua New Guinea and in other parts of
sweetpotato has an especially important role in the diet, as demostrated by a high
per capita production: Solomon Islands 178 kg, Rwanda 165 kg, Niue 125 kg, Burundi 102 kg, until 20 kg to Cuba and 19 kg to Barbados and Saint Vincent/Grenadines.
Between the early 1960s and late 1990s, there was a global reduction in
sweetpotato area of about 31%. Due to increases in yield, the associated drop in
production is less, especially in China. In Asia, sweetpotato area decreased
during the 1960s and was stable during the 1970s. Area fell again in
the first part of the 1980s and has remained stable since then. Production
remained nearly constant due to increased yields. Because most sweetpotato is grown in China, the pattern for Asia is driven by changes in China.
In the 1960s Japan was a major producer of sweetpotato, but sweetpotato area
there decreased strongly between the early 1960s and the early 1970s. Since
then sweetpotato area has remained almost constant. Vietnam and Bangladesh are important sweetpotato-producing countries in Asia that have
witnessed an increase in area over the last 30 years.
The role of sweetpotato in China has changed from staple food crop to raw material for industrially processed food (noodles) and to feed, although it remains
a staple food in poor and mountainous districts. In higher-income areas, it has become a supplementary food and snack.
Although the statistics for Africa show a somewhat irregular pattern for the 1960s and 1970s, there is a clear overall trend of a strong increase in area (an
increase of 2.5 times in the past 40 years).
In the USA, there has been a constant decrease in the area with sweetpotato, as in Japan where the crop changed from staple to industrialized food.
Growth habit. The sweetpotato is a herbaceous and perennial plant. However, it is grown as an annual plant by vegetative propagation using either
storage roots or stem cuttings. Its growth habit is predominantly prostrate with a vine system that expands rapidly horizontally on the
ground. The types of growth habit of sweetpotatoes are erect, semierect, spreading, and very spreading (Figure 1).
Figure 1 Types of growth habits in sweet potato. On the left erect plant with its description; on the right spreading plant.
Root system. The sweetpotato root system consists of fibrous roots that absorb nutrients and water, and anchor the plant, and storage roots that are
lateral roots, which store photosynthetic products.
The root system in plants obtained by vegetative propagation starts with adventitious roots that develop into primary fibrous roots, which are branched into lateral roots. As the plant matures, thick pencil
roots that have some lignification are produced. Other roots that have no lignification, are fleshy and thicken a lot, are called storage roots.
Plants grown from true seed form a typical root with a central axle with lateral branches. Later on, the central axle functions as a storage root (Figure 2).
Figure 2 Types of roots. On the left scheme of root with description; on the right roots with red storage roots (tuber-roots).
Tuber-root. It is also called storage root have a skin coloured from red to purple, from brown to white depending on the variety; as well as the pulp that varies from white to yellow, orange or purple (Figure 3).
A tuberous root or storage root, is a modified lateral root, enlarged to function as a storage organ. The enlarged area of the root-tuber, or storage root, can be produced at the end or middle of a root or involve the entire root. It is thus different in origin but similar in function and appearance to a stem tuber. Examples of plants with notable tuberous roots include the sweet potato, as already mentioned, but also cassava, and dahlia.
Root tubers are perennating organs, thickened roots that store nutrients over periods when the plant cannot actively grow, thus permitting survival from one year to the next.
The massive enlargement of secondary roots typically represented by Sweet Potato, have the internal and external cell and tissue structures of a normal root, they produce adventitious roots and stems which again produce adventitious roots.
In root-tubers, such as the stem, there are no nodes and internodes or reduced leaves. Root tubers have one end called the proximal end, which is the end that was attached to the old plant; this end has crown tissue that produces buds which grow into new stems and foliage. The other end of the root tuber is called the distal end, and it normally produces unmodified roots. In stem tubers the order is reversed, with the distal end producing stems.
Tuberous roots are biennial in duration: the first year the plant produces root-tubers, and at the end of the growing season, the plant shoots often die, leaving the newly generated tubers. The next growing season, the root-tubers produce new shoots. As the shoots of the new plant grow, the stored reserves of the root-tuber are consumed in the production of new roots, stems, and reproductive organs; any remaining root tissue dies concurrently to the plant's regeneration of next generation of root-tubers.
Figure 3 Parts of the storage roots and its trasversal section. Tuber roots also called storage roots are differently coloured in the epidermis and in the pulp.
Stem. A sweetpotato stem is cylindrical and its length, like that of the internodes, depends on the growth habit of the cultivar and of the
availability of water in the soil. The erect cultivars are approximately 1 m long, while the very spreading ones can reach more than 5 m long.
Some cultivars have stems with twining characteristics. The internode length can vary from short to very long, and, according to stem diameter, can be thin or very thick.
Depending on the sweetpotato cultivar, the stem color varies from green to totally pigmented with anthocyanins (red-purple color). The
hairiness in the apical shoots, and in some cultivars also in the stems, varies from glabrous (without hairs) to very pubescent.
Leaves. The leaves are simple and spirally arranged alternately on the stem in a pattern known as 2/5 phyllotaxis (there are 5 leaves spirally
arranged in 2 circles around the stem for any two leaves be located in the same vertical plane on the stem).
Depending on the cultivar, the edge of the leave lamina can be entire, toothed or lobed. The base of the leaf lamina generally has two lobes that can be almost straight or rounded. The shape of the general outline of sweetpotato leaves can be rounded, reniform (kidneyshaped), cordate (heart-shaped), triangular, hastate (trilobular and spear-shaped with the two basal loves divergent), lobed and almost divided. Lobed leaves differ in the degree of the cut, ranging from superficial to deeply lobed. The number of lobes generally range from 3 to 7 and can be easily determined by counting the veins that go from the junction of the petiole up to the edge of the leaf lamina.
However, toothed leaves have minute lobes called teeth which could number from 1 to more than 9. Some cultivars show some variation in leaf shape on the same plant (figure 4).
Figure 4 General outline of the leaf (top left); types of leaf lobes (top right); number of leaf lobes (bottom).
The leaf color can be green-yellowish, green or can have purple pigmentation in part or all the leaf blade. Some cultivars show purple young leaves and green mature leaves. The leaf size and the degree of hairiness vary according to the cultivar and environmental conditions. The hairs are glandular and generally are more numerous in the lower surface of the leaf. The leaf veins are palmated and their color, which is very useful to differentiate cultivars, can be green to particularly or totally pigmented with anthocyanins.
The anthocyanin composition of three varieties, "Simon No. 1", "Kyushu No. 119", and "Elegant Summer", in sweetpotato leaves was examined for promoting new uses. Fifteen anthocyanin compounds were identified and measured. HPLC clearly showed quantitative differences, but not qualitative ones. The anthocyanins were acylated cyanidin and peonidin type. The result suggests that the major anthocyanin composition of sweetpotato leaves is cyanidin type.
The length of the petiole ranges from very short to very long. Petioles can be green or with purple pigmentation at the junction with the lamina and/or with the stem or throughout the petiole. On both sides of the insertion with the lamina there are two small nectaries (Figure 5).
Figure 5 Types of leaves.
Flowers. Sweetpotato cultivars differ in their ability of flower. Under normal conditions in the field, some cultivars do not flower, others produce very few flowers, and others flower profusely.
The inflorescence is generally a cyme in which the peduncle is divided in two axillary peduncles; each one is further divided in two after the flower is produced (biparous cyme). In general, buds of first, second, and third order are developed. However, single flowers are also formed. The flower buds are joined to the pedundle through a very short stalk called pedicel. The color of the flower bud pedicel, and
peduncle varies from green to completely purple pigmented.
The flower is bisexual. Besides the calyx and corolla, they contain the stamens that are the male organs or androecium and the pistil that is the female organ or gynoecium.
The calyx consists of 5 sepals, 2 outer and 3 inner, that stay attached to the floral axle after the petals dry up and fall.
The corolla consists of 5 petals that are fused forming a funnel, generally with lilac or pale purple limb and with reddish to purple throat (the inside of the tube). Some cultivars produce white flowers.
The androecium consists of five stamens with filaments that are covered with glandular hairs and that are partly fused to the corolla.
The length of the filaments is variable in relation to the position of the stigma. The anthers are whitish, yellow or pink, with a longitudinal dehiscence. The pollen grains are spherical with the surface covered with very small glandular hairs.
The gynoecium consists of a pistil with a superior ovary, two carpels, and two locules that contain one or two ovules. The style is relatively short and ends in a broad stigma that is divided into two lobes that are covered with glandular hairs. At the base of the ovary there are basal yellow glands that contain insect-attracting nectar. The stigma is
receptive early in the morning and the pollination is mainly by bees.
Figure 6 Flower typically belonging to the family Convolvulaceae and description.
Fruits and seeds. The fruit is a capsule, more or less spherical with a terminal tip, and can be pubescent or glabrous. The capsule turns brown when mature.
Each capsule contains from one to four seeds that are slightly flattened on one side and convex on the other. Seed shape can be irregular, slightly angular or rounded; the color ranges from brown to black; and the size is approximately 3 mm. The embryo and endosperm are protected by a thick, very hard and impermeable testa. Seed germination is difficult and requires scarification by mechanical abrasion or chemical treatment. Sweet potato seeds do not have a dormancy period but maintain their viability for many years.
Figure 7 Scheme of the capsules, ovary (with locules and ovules) and seeds. Seeds and fruits of sweet potato.
Storage-root. The storage roots are the commercial part of the sweetpotato plant, and sometimes are mistakenly named "tubers". Most cultivars develop storage roots at the nodes of the mother stem cuttings that are underground. However, the very spreading cultivars produce storage roots at some of the nodes that come into contact with the soil.
The parts of the storage roots are the proximal end that joins to the stem, through a root stalk, and where many adventitious buds are found from which the sprouts are originated; a central part which is more expanded; and the distal end that is opposite to the root stalk.
The adventitious buds that are located in the central and distal part usually sprout later than those located in the proximal end (Figure 8).
Figure 8 Scheme of the root-tuber parts.
A transverse section of the storage roots shows the protective periderm or skin, the cortex or cortical parenchyma that, depending on the cultivar, varies from very thin to very thick, the cambium ring where the latex vessels are found, and the medulla or central
parenchyma. The amount of the latex formed depends on the maturity of the storage root, the cultivar, and the soil moisture during the growing period. The latex drops are produced when the storage roots are cut and they darken very quickly due to the oxidation (Figure 9).
Figure 9 Scheme of transverse section of the root-tuber.
The formation of the storage roots can be in clusters around the stem.
If the rootstalk that joins the root to the stem is absent or is very short, it forms a closed cluster.
If the stalk is long, it forms an open cluster.
In some other cultivars, the storage roots are formed at a considerable distance from the stem and therefore, the storage root formation is disperse or very disperse (Figure 10, top left).
The storage root surface is usually smooth but some cultivars show some deffects such as alligator-like skin, prominent veins, horizontal constrictions or longitudinal grooves (Figure 10, lower left).
Lenticels are also located on the surface and in some cultivars they can be protuberant due to excess water in the soil.
Storage roots vary in shape and size according to the cultivar, type of soil where the plant is grown and other factors. The outline of their shape can be round, round-elliptic, elliptic, ovate, obovate, oblong,
long oblong, long elliptic, and long irregular or curved (Figure 10, bottom right).
The storage root skin color can be whitish, cream, yellow, orange, brown-orange, pink, red-purple, and very dark purple. The intensity of the color depends on the environmental conditions where the plant is grown. The flesh color can be white, cream, yellow, or orange.
However, some cultivars show red-purple pigmentation in the flesh in very few scattered spots, pigmented rings or, in some cases, throughout the entire flesh of the root (Figure 10, bottom right).
Figure 10 Type of storage-root formation (top left); types of defects on the storage-root surface (lower left); types of storage-root shapes (top right); distribution of anthocyanin pigmentation in storage-root flesh (lower right).
Compared to the potato requires higher temperatures for growing (15 °C for germination and 20 °C for development), has creeping stems climbing or even three meters long, that give beautiful bell-shaped flowers.
The sweet potato is not really a tuber (such as the common potato), but a tuberous root. In addition to Italy, where we know also as sweet potato, in many other countries, the sweet potato is wrongly considered a potato variety (in fact, English is also known by the term sweet potato). In parallel, in other areas (eg. Latin America), the noun "batata" indicates not only the sweet potato, but the "potato" proper. Finally, to further complicate matters, also comes the Oxalis tuberosa, however defined "potato" in areas of New Zealand.
The sweet potato is commonly called sweet potato incorporates high in simple carbohydrates that give the flavor typically sweetened. From the nutritional point of view, provides mainly carbohydrates (mainly complex), has a great amount of fibers and "surprise" to the high concentration of retinol equivalents (vitamin A), slightly lower than that of the carrots.
Plant with tuberous roots similar to sweet potato there are about 50 genres and more than 1,000 different species, but the only significantly exploited for agriculture for human consumption is the Ipomoea batatas. Some edible species are exploited only "local" level, but there are many other poisonous to humans.
Interestingly, the order of the botanical sweet potato (Solanales) is the same of tomatoes, eggplant, peppers, potatoes, etc. Common., But the family is completely different and common ornamental plants - floral type: Morning Glory (translation from Morning Glories).
The sweet potato not tolerate cold weather or dry and requires an average temperature of 24 °C, lots of sunshine and warm nights. Also, irrigation may not cause ponding to avoid rotting of the tuber. The terrain favored by the plant is: draining, light and medium texture with pH of 4.5-7; do not need a massive fertilization. The propagation of sweet potato is by stem cuttings or or adventitious roots; the seeds are used for playback only between plants. The times of cultivation are approximately 2-9 months, between early and late varieties. Usually, you do not need the use of pesticides and the average yield is about 13.2 tons per hectare.
The sweet potato has played a vital role in feeding the North American (especially the southeast) until the mid-twentieth century, when, with the economic recovery, it reduced a lot of consumption. In these areas, the sweet potato was consumed both fresh and dried (preserved).
The sweet potato is a food belonging to the VI group of foods. Contains predominantly complex carbohydrates but the portion of simple sugars is nonetheless significant. The glycemic index of the sweet potato is quite high and this makes it a little food unfit for diabetic and obese.
Proteins are very minor, and even less fat. The fibers instead seem quite abundant and this aspect makes the sweet potato a foodstuff suitable for those who suffer from constipation. As for the minerals, the sweet potato is rich in potassium and manganese.
About ale vitamins, there was a good supply of pyridoxine (vit. B 6 ) and a remarkable content of vit. A (RAE).
According to estimates by the Center for Science in the Public Interest, the nutritional value of sweet potato is an impressive 100 points higher than that of the common potato. Compared to the latter, the sweet potato has a much higher content of retinol equivalents, in particular of β-carotene. A study carried out in Uganda in 2012, which involved about 10,000 households, showed that only 10% of those who eat sweet potatoes dark orange suffering from vitamin deficiencies of retinol equivalents, while as much as 50% of individuals who prefer potatoes American beige or pale yellow shows a significant hypovitaminosis. This is justified by the greater contribution of β-carotene of sweet potatoes dark orange color than lighter ones.
Table 2 shows the nutritional characteristics of batata compared with the recommended daily intake or recommended daily amount that indicates the amount of nutrients (macronutrients and micronutrients) that a healthy person should take to meet its daily needs, according to the current medical knowledge. This value is often misinterpreted as a "minimum requirement".
There was talk of food group. we see what it is. A food group is a collection of foods that share similar nutritional properties or biological classifications. Nutrition guides typically divide foods into food groups and recommend daily servings of each group for a healthy diet. In the United States for instance, USDA has described food as being in from 4 to 11 different groups.
Most Common food groups are:
- Dairy, also called milk products and sometimes categorized with milk alternatives or meat, is typically a smaller category in nutrition guides. Examples of dairy products include milk, butter, ghee, yogurt, cheese, cream and ice cream, is typically a very small category in nutrition guides, if present at all, and is sometimes listed apart from other food groups. The categorization of dairy as a food group with recommended daily servings has been criticized by, for example, the Harvard School of Public Health. The HSPH points out that "research has shown little benefit, and considerable potential for harm, of such high dairy intakes. Moderate consumption of milk or other dairy productsone to two servings a dayis fine, and likely has some benefits for children. But its not essential for adults, for a host of reasons".
- Fruits, sometimes categorized with vegetables, include apples, oranges, bananas, berries and lemons. Fruits are carbohydrates, like sugar, dairy, grains, and starches.
- Grains, beans and legumes, grains are also called cereals and sometimes inclusive of potatoes and other starches, is often the largest category in nutrition guides. Examples include wheat, rice, oats, barley, bread and pasta. An example of beans would be baked beans and soy beans, while an example of legumes would be lentils and chickpeas.
- Meat, sometimes labeled protein and occasionally inclusive of legumes, eggs, meat analogues and/or dairy, is typically a medium- to smaller-sized category in nutrition guides. Examples include chicken, fish, turkey, pork and beef.
- Confections, also called sugary foods and sometimes categorized with fats and oils, is typically a very small category in nutrition guides, if present at all, and is sometimes listed apart from other food groups. Examples include candy, soft drinks, and chocolate.
- Vegetables, sometimes categorized with fruit and occasionally inclusive of legumes, is typically a large category second only to grains, or sometimes equal or superior to grains, in nutrition guides. Examples include spinach, carrots, onions, peppers, and broccoli.
- Water is treated in very different ways by different food guides. Some exclude the category, others list it separately from other food groups, and yet others make it the center or foundation of the guide. Water is sometimes categorized with tea, fruit juice, vegetable juice and even soup, and is typically recommended in plentiful amounts.
| Table 2 Nutritional table of sweet potato
||Quantity per 100 g of product
Total Fat (g)
Dietary fiber (g)
Vitamin K (g)
Vitamin B1 (mg)
Vitamin B2 (mg)
Vitamin B3 (mg)
Vitamin B6 (mg)
Vitamin E (mg)
Vitamin C (mg)
Vitamin A (μg)
Sweet potato with all its natural biodiversity is an implacable enemy of hunger, a fierce advocate for health or a culture steeped in history, sweetpotato is primarily a root with great nutritional and medicinal potential, due to the large presence in it of vitamins , proteins and minerals.>br>
The main vitamins that holds are the A, why the potato is greatly appreciated in the world, it is the species with more vitamin A. Reason also explains the fact that its consumption is widespread in most developing countries. To the extent that their consumption has been allowed to attack frontally childhood blindness that still affects more than 2.5 million children worldwide.
Another important feature of sweet potatos is the presence of vitamin C, an organic substance essential for growth and tissue repair, as for wound healing and maintenance of cartilage, bone and teeth. The presence of potassium is also important to highlight in this species.
Similarly, the sweetpotato contains a considerable amount of iron, starch, sodium and folic acid, among others. It is the presence of the latter that makes this species is recommended for women in gestation, if not to consume these pills folic acid supplementation.
Sweet Potato "Orange Pulp" is rich in vitamin A and C and potassium and iron: "Keys to reduce the risk to develop stomach cancer and liver disease".
The research also states that the Sweet Potato Pulp Residence slows aging, as it has antioxidant properties and high vitamin and protein value. Exceeding of potato (potato). Other studies show that the sweet potato helps reduce depression and counteracts overweight.
According to a research paper on the treatment of cervical cancer, the fresh leaves of purple sweet potato would be helpful to combat the disease in its early stages. One year was investigated the chemical constituents of potato leaves, determining the ethanol extract of these, scientifically known as Ipomoea batatas (L.) Lam are helpful in the treatment of neoplasia.
Was investigated for a year the chemicals in the leaves of the root-tuber, determining the ethanol extract are helpful in the treatment of neoplasia.
In the Andes is used fresh leaf of purple sweet potato infusion therapy for cervical cancer.
The importance of purple sweet potato against cancer is the purplish pigment that has it in greater numbers than other fruits and medicinal plants, for that reason we are dealing exclusively with the purple sweet potato variety, not only in the purple sweet potato anthocyanin is , is also found in blueberries and red cabbage, nature created a great biodiversity of foods containing the same elements cancer healers containing the purple sweet potato, so it is important to eat different foods that nature provides to heal not only cancer but others.
The sweet potato when cooked lose many properties, many items are lost in cooking or frying (a cancer patient should not eat fried food, burnt areas of food and fats are especially bad), so it is recommended to extract the sweet potato pulp having purple (there sweetpotato varieties that have purple pigment in the pulp) and take it with some other juice, can be combined for example with the leaves of wheat germ, etc.. In this case the wild sweet potato do not prepare too much, you only have half a glass every morning combined with any other juice.
If you have the facilities to get the purple sweet potato leaves, you can prepare an infusion, the patient and take it as drinking water, may also be accompanied with other herbal infusions, the latter respect it is advisable to buy herbs of medicinal plants in their natural state, avoid buying products that sell in retail natural, unless you are very sure to be good and to ensure good quality then you could buy those products, the latter perhaps by necessity have to do with the need for natural resources in highly industrialized cities that even the leaves of Aloe Vera is very difficult to find in its natural form, we understand that not all countries are equal, but if you are in a privileged place for its resources natural and easy to access it then take advantage of this great opportunity to cure the patient, depending much on the judgment and will of each person.
Despite the red yam is of American origin and not widely known in our country, even in Italy were born some cooperatives that produce it with the methods of biodynamic agriculture. You can visit the website of Agrilatina, Lazio company that cultivates and ensures a high quality product.
The roots abundant and branched swell forming tubers, up to 30 cm long, similar to the potato, but more tapered. The yam has a reddish yellow rind smooth rutted, inside can take different colarazioni, red, yellow or pale marrone.Il its flavor somewhere between the traditional potato and pumpkin red and lends itself to various preparations and recipes, like bread batata.
How to cultivate it? Its production cycle lasts from 180 to 200 days, then occupy the terrain or long, but it's worth it because on the other hand has an excellent nutritional intake: contains almost three times the daily requirement of vitamin A and has an antioxidant and emollient Soothing.
Land and Climate: Prefers a loose soil, light and organic, a sunny position, sheltered from the wind and a temperate climate.
Cultivation requirements: If you can enrich the soil with organic fertilizer potash and nitrogen and watered regularly during their growth cycle. It 'a good idea to remove the weeds from the base until its vegetation will cover not the surrounding land, also if you do not want to see her lying and prostate have to provide the supports.
The sweet potato has the therapeutic and nutritional details, especially the caiapó extracted from its husk, is used by Japanese people to treat anemia, hypertension and diabetes.
Sweet potatoes are grown from slips, which are sprouts that are grown from stored sweet potatoes. You can buy slips from garden centers, nurseries, or local farmers.
You can also grow your own slips to plant in the spring. In November (this is when the best of the new harvest will be out), go to your supermarket and look for unblemished and uncracked medium sweet potatoes. One potato should yield about 12 plants.
Store these potatoes in a well-lit room with a temperature between 18 and 21 °C. Keep them there until about 90 days before the last spring frost date. They will then need to be embedded in soil for 90 days and kept continuously warm and moist.
Use a 2-4 L pot for every two potatoes. Remember to poke drainage holes in the bottom of the pot and fill it with 5 cm of mulch followed by garden or potting soil. Plant the potatoes in the pot at a 45° angle so that the sprouts will grow above the soil. When the slips are 15 to 30 cm tall, you can plant them outdoors as long as all danger of frost has passed.
After you have grown your own slips or bought them, till area of the garden you will be using to a depth of 20 to 25 cm. Create raised beds 15 to 20 cm tall and about 30 cm wide. Use fertile, well-drained soil.
Plant the slips 30 to 40 cm apart in the bed, after the last spring frost date. Plant the slips deep enough to cover the roots and about 1,4 cm of the stem. Water the slips with a starter solution that is high in phosphorous, then water generously for a few days to make sure that the plants root well.
Propagation by cuttings
Perform the following steps:
- Select parent sweet potato plants that are growing vigorously and appear healthy, with no symptoms of disease or nutrient deficiency.
- Prepare the site or rooting media and container for cuttings. Sweet potatoes prefer a fertile, well-drained soil, so incorporate for good depth an organic-material soil amendment like aged manure or well-rotted compost into the site to improve fertility and drainage, if needed. Break up the top 15 to 20 cm of soil and mound soil into foot-high ridges spaced about 90 cm apart. To root cuttings in a container, choose a container with ample drain holes and fill it partway with a well-drained medium that could contain half peat moss and half sand, perlite or vermiculite.
- Poke a hole into the rooting media in the container with a pencil for each planned cutting, spacing holes far enough apart that leaves from different cuttings will not touch.
- Cut 10-to 30-cm sections of stem from the tips of the sweet potato vines. Make cuttings about 10 cm long if you will root them in a container and longer if you will root them in the ground. Make a clean cut just above a leaf or junction with another branch of the stem.
- Cut leaves off the bottom half of each cutting that you will root in a container and off of the bottom two thirds of longer cuttings you will root directly in the soil.
- Dust the cut end of the cutting with rooting hormone, if desired, to encourage more uniform rooting.
- Insert the end of the sweet potato vine into the prepared hole in the rooting medium and gently firm the medium around the cutting. To root the longer cuttings directly in the soil, bury the entire leafless portion of the vine horizontally under only an inch or two of soil, leaving just the leafy tip above the soil and firm soil around the buried cutting.
- Mist the cuttings gently and thoroughly shortly after placing them in the soil or medium, and mist regularly so the soil remains uniformly moist until new growth emerges from the cutting. Then, gradually decrease irrigation to allow the soil to dry somewhat between waterings.
- Transplant the container-rooted sweet potato cuttings into their permanent containers or positions in the landscape once several roots 2 to 3 cm long develop, typically within a few weeks.
Side-dress the potatoes 3 to 4 weeks after transplanting with 3 pounds of 5-10-10 fertilizer per 100 feet of row. If you have sandy soil, use 5 pounds.
Hoe the beds occasionally to keep weeds down. Remember to reshape the beds with soil or mulch.
For good harvests, do not prune the vines, because they should be vigorous.
Remember to keep the potatoes watered. Deep watering in hot, dry periods will help to increase yields, although if you are planning to store some of the potatoes, do not give the plants extra water late in the season.
Choice of the nursery material
There are two ecotypes used to produce elongated tubers (lobed leaf) or rounded (leaf ovata) even if the structure of the soil influences the shape. and precocity.
In the selection of roots grown for cuttings, it is recommended:
The cuttings necessary to cultivate 1 hectare are obtained from 80-100 kg of sweetpotatoes by putting them to sprout in special bins or greenhouses, tubers of medium size, on a seed bed comprises a first layer of
fresh manure (from 20 cm), a layer of 10-15 cm above the ground to which the tubers are placed covered by a further layer of soil.
- use tubers healthy, free from fungal diseases with the skin clear and supplements;
- prefer those collected at the end of the cycle as physiologically ripe;
- use tubers of medium size with a diameter of 5 to 10 cm;
- keep the seed potatoes in premises not particularly dry, the dark in layers of about 20-25 cm and with a temperature of 12-15 °C. temperatures below 6 °C undermine the conservation.
"Alabama Purple": "Alabama Purple" is called "Purple Delight" in some catalogs. It was a high yielder in our trial and grew beautiful medium to large roots. The roots are long to oblong, with smooth, dark purple skin and deep purple flesh. Despite its gorgeous appearance and heavy yield, this variety has an unappealing flavor when baked.
"Allgold": 90 Days. Allgold was developed by Oklahoma State University in 1952 for resistance to internal cork and soil rot. It is reputed to be a heavy-yielding variety, though it performed only moderately well in our field trial. "Allgold" has medium to large roots that are long to oblong, with golden-brown skin and cream-orange flesh. It features high levels of vitamin C, vitamin A, and carotene, and was a favorite in our taste test.
"Arkansas Red": 90 Days. Arkansas Red produces small to medium size sweet potatoes that are long to oblong, with red-brown skin and cream-orange flesh. It is reported to be a high yielder, but our yields were moderately low. We also observed a high occurrence of soft rot and white grub damage with this variety.
"Beauregard": 90 Days. Louisiana Agricultural Experiment Station release. Light rose or copper skin, dark orange flesh, uniformly shaped. Very good yield that stores well. Resistant to white grub and streptomyces soil rot but is susceptible to root knot nematode. Matures in 105-110 days. Outstanding new release that has really been accepted by farmers. Extremely high yields with very little cracking. Red-orange skin and orange flesh. Quick maturing with good shape.
"Bunch Porto Ricos": also called "Bush" and "Vineless." The favorite of gardners with limited space. Porto Rico has copper-colored outside skin and light red flesh. Delicious "old-fashioned" flavor, an excellent baking potato. "Baby Bakers" in 100 days. The old "red yam."
"Carolina Bunch": developed by USDA-ARS and Clemson University SCAES for use in gardens where the bunch habit requires less space for high yields. Roots are uniformly shaped with a very smooth bright, light copper skin. The flesh is deep orange. Resistant to four races of root knot nematode. Adequate resistance to stem rot, internal cork, sclerotial blight and leaf blight. Low level resistance to soil rot. Adequate level of resistance to wireworms, cucumber beetles and flea beetles but not as high as Regal or Sumor. Not resistant to white grub. Vigorous plants form a dense and high leaf canopy resulting in a bunched appearance. Yields are better than Jewel in a 110-120 day growing season.
"Centennial": soft-fleshed type. Produces a medium to large product. This old favorite is a smooth sweet potato with a deep orange flesh that adds color to every table and the yield is unbelievable! Tolerates clay soil better than the Jewel. This is America's leading sweet potato. Chances are this is the variety you bought at your local market. Carrot color inside. "Baby Bakers" in 90 days. Yields reported of 500 bushels per acre. Chef's favorite because of beautiful color and excellent cooling qualities.
"Cordners Red": 90 Days. Cordners Red was developed in Oklahoma from the breeding material of Dr. Cordner. It produces long, lumpy, medium-size roots with dark red skin and cream-orange flesh. The variety grows as a compact, bush type plant, which makes it well suited for small gardens. Despite its compact growth, "Cordners Red" was a heavy producer in our variety trial. It also did well in our taste test, but is reputed to do poorly in storage.
"Covington": the Covington variety is the newest release from the North Carolina State University's horticultural science department. It has been so well received by growers that is will account for nearly 90% of the sweet potato acreage in North Carolina in the 2009 season. The orange flesh has the look and taste consumers really enjoy.
"Darby": dark red color skin with a deep orange flesh. Produces many uniform potatoes. The Darby has good eating quality when cured. Soft juicy flesh when baked.
"Evangeline": rose colored skin and moist deep orange flesh. New release from Louisiana.
"Excel": attractive light copper skin and orange flesh. Sizes well shaped roots earlier than most cultivars and yields about 15% more than Jewel. Resistance to disease and insects similar to the Regal. Is similar to the Regal in that it has shown better natural insect resistance than could be expected using chemical pesticides. Vine growth is vigorous and ground cover is good. Developed by USDA-ARS and Clemson University.
"Georgia Jet": a spectacular new variety with extremely fast growth (size potatoes in only 90 days) and extra-high yields. Ideal for northern gardens, even New England. Five years of testing in New York shows that Georgia Jet produces 2-1/2 times the yield of standard varieties. Yields in other sections are exceptional. Jets have deep orange inside color with moist flesh and marvelous taste. The outside skin is so red it is almost purple.
"Georgia Red: 110-140 Days. It produces small to medium size roots that are round to oblong, with red-brown skin and cream-orange flesh. It grows vigorous vines and is reported to set above average yields. However, it performed poorly for us. This variety might have benefited from more time in the ground than 128 days the time we gave it. Georgia Red was a favorite in our taste test. It has excellent keeping quality.
"Hernandez": developed by Louisiana Agricultural Experiment Station. This late (125 days) sizing cultivar has high yield and excellent baking and processing qualities. Moderate resistance to fusarium wilt, southern rootknot, nematode, soil rot and internal cork. Roots are fusiform, lightly grooved and red skinned. Flesh is a deep orange.
"Jersey or Yellow Jersey": this variety is an "old-fashioned" sweet potato with a golden yellow skin at harvest time which fades to buff or tan after storage. The flesh color ranges from creamy white to bright yellow with an occasional pink variegation. It has a dry, mealy flesh that is exceptionally sweet and creamy and retains its form when baked. Delicious simply steamed in a pot of rice.
"Jewel": the current Queen of sweet potatoes, was developed by North Carolina State University. Thought to be the most versatile sweet potato, with copper-colored skin and moist, bright orange flesh. They stand up well in salads because of their intense color and are good for baking or steaming. The variety is a yam-type (moist, soft, yellow-fleshed when baked) with a light copper skin and orange flesh. It produces a very high yield (to 6 sweet potatoes per plant) of moderately short, chunky roots. The variety prefers a sandy soil and is resistant to fusarium wilt, southern rootknot nematode, internal cork and sweet potato beetle. It needs 120-135 days growing time for maximum yield.
"Kotobuki (Japanese)": light reddish-brown colored flesh with a nutty flavor similar to a roasted chestnut that is great for baking, salads and tempura. Like the Jersey variety, Kotobuki sweet potatoes are drier and not widely used outside of Japanese cooking. Good for tempura, soups and stews, stir-fries, and baking.
"Kumara (New Zealand)": the kumara has a long history of cultivation in New Zealand. Brought by the early Maori settlers in about the 10th century from its Pacific Island source (Hawaiki), it was widely grown especially in the semi-tropical regions of the North Island. Archaeological diggings at the Waipoua Forest near Kaipara have revealed kumara cultivation from very early Pre-European Maori times. The Maori managed kumara growing with great horticultural skill, making use of the ideal growing climate and controlling kumara caterpillar with the use of tamed black-backed seagulls. Kumara caterpillar could devastate a crop almost overnight, hatching in their thousands. Pre-European Maori grew several different varieties of 'bush' kumara, but compared to modern hybrids, were very small in size, being similar to the modern Anya potato variety. Modern day kumara has evolved from cross breeding with a larger American hybrid imported in the 1850's. A dark red variety was developed which grows on a creeping vine and was named Owairaka Red. The majority of kumara are grown in Northland in the Northern Wairoa region where soil type and climatic conditions suit kumara perfectly. Kumara is the 3rd most popular vegetable in New Zealand and is revered by Maori as a cultural culinary emblem.
"Molokai Purple": It produces small to medium size roots with dark purple skin and deep purple flesh. They are long, veined, and rough-skinned, giving them an unsightly appearance, despite the beautiful purple color. Though Molokai Purple is a heavy producer, its rather ugly appearance and poor ranking in our taste test make us reluctant to recommend the variety.
"Nancy Hall": the "Yellow Yam" of the 30's and 40's. Older gardener's favorite. Light skin, yellow flesh. Juicy, waxy and sweet when baked. If taste is more important than beauty, try Nancy Hall.
"New Jewell": the improved Centennial. The blue ribbon winner for color, taste and yield. Rosy red outside skin, deep orange inside. Bakes quickly with a soft texture.
"Norton": it is an old heirloom variety that predates 1850. It produces medium to large roots that range from long and skinny to thick and blocky. It has golden-brown skin and cream-colored flesh that is sweet and dry when cooked. Nortons flavor can be compared to Nancy Hall, another white sweet potato. Norton performed poorly in the field trial and suffered considerable white grub damage.
"O'Henry": white colored flesh comparable in appearance and flavor to the old Nancy Hall variety. It is a mutation of the Beauregard and was developed by Henry Wayne Bailey. It is high yielding with a maturity of 90 days vs. 120 days for Nancy Hall. It is resistant to disease where other white varieties aren't.
"Okinawa": a native of the Japanese island Okinawa, the Okinawa Sweet Potato with its light brown skin and unusual purple flesh is richly nutritious and surprisingly sweet. It retains its dark purple color when baked and tastes similar to the Kotobuki sweet potato. Its beautiful color lends itself well to stir-fries, tempura and baking. This root vegetable is a staple among the people of Okinawa and Hawaii. Because of its delicate sweet taste, it is often simply boiled, cut into chunks and served, but those with a more creative flair treat it in less traditional ways. Okinawa Sweet Potatoes are often served toward the end of the meal.
"Okinawa Purple": 140 Days. It is a late season Asian type sweet potato, also called Okinawan. It produces roots with purple and cream marbled flesh and cream colored skin. "Okinawa Purple" is dry and sweet when cooked and its flesh turns bright purple. Special health benefits have been attributed to this sweet potato, including its contribution to the longevity of women living on the island of Okinawa. This reputation and its beautiful coloration have led to growing consumer interest. Okinawa Purple requires a full 140 days to set decent sized roots. As mentioned earlier, we tried growing this variety in 2010 and 2011 without success, ostensibly because we did not allow enough time in the ground. We obtained a moderate yield in 2012, but encountered some challenges during harvest. "Okinawa Purple" sets roots in a wide band of soil, which necessitates extra time and care in digging. We estimate that it took about four times as long to harvest "Okinawa Purple" as compared to other varieties. But considering its market appeal and potential for premium pricing, this variety might be worth the extra effort.
"Oklahoma Heirloom": it is a true heritage variety that can be traced to Ralph Mills of Beggs, Oklahoma. Mills has grown the variety for over 30 years after receiving it from a neighbor in Colgate. Oklahoma Heirloom may have originated from another heirloom variety called Mahan, which is similar in appearance. It has compact vines and produces roots in a cluster under the primary stem. The roots are medium to large, oblong, and feature orange-brown skin, with cream-orange flesh. Production is typically early and heavy and the roots keep well in storage. This variety performed poorly this year, but was one of the top yielders in our 2011 trial. Oklahoma Heirloom continues to be a favorite in our taste test.
"Oriental": the Japanese or Oriental Sweet Potato has a beautiful pink to purple skin and is white inside. It has a delicious sweet flavor reminiscent of a chestnut. They are used to make liquor in Japan. They are attractive, colorful, and make a wonderful presentation. They can be grilled, steamed or baked.
"Red Garnet": deep red or purple skin, moist orange flesh with medium-sized roots and short to long spindle. Best used in recipes that call for mashed or grated sweet potatoes such as pies, cakes and breads due to its high moisture content. The soft flesh also makes excellent mashed sweet potatoes.
"Red Jewel": an old favorite. Deep orange inside with very red outside skin. Bakes quickly with soft texture. Real "eye catcher."
"Red Resisto": 90 Days. "Red Resisto" grows as a semi-bush type plant and readily flowers. It produces earlier and yields slightly better than Resisto, but cannot match Resistos superb taste. Red Resisto grows small to medium size sweet potatoes that are round to oblong. It has a smooth, dark red skin and deep orange flesh.
"Porto Rican": copper colored skin with a reddish-orange flesh. Old established moist sweet potato with a very sweet and delicious flavor. Excellent for baking. Its compact growing habits make it an ideal garden variety. It is susceptible to wire worm, fusarium wilt, internal cork and southern root-knot nematodes.
"Regal": brilliant purplish-red skin at harvest, orange flesh and excellent baking quality. It produces abundant sprouts and has excellent yield potential. high level of resistance to internal cork and stem rot (fusarium wilt). Low level of resistance to soil rot (pox) and good resistance to southern rootknot nematode. Regal also has resistance to tobacco and southern potato wireworm, banded cucumber beetle, spotted cucumber beetle, elongate flea beetle, pole striped flea beetle, sweet potato flea beetle and to at least two species of white grubs. Stores well but not as long as Jewel. Developed by USDA-ARS, Clemson University SCAES & Texas A&M.
"Resisto": 110 Days. "Resisto", developed in South Carolina in 1983, was the best tasting sweet potato we grew this year. This variety grows vigorous vines and sets a good yield of small to medium size roots that are round to oblong. Resisto sweet potatoes have smooth, red-brown skin with deeporange flesh. It is reported to have super resistance and good storage quality.
"Satsuma (Japanese)": the name "satsuma-imo" originates from the fact that sweet potatoes were cultivated widely in the Satsuma(Kagoshima) region after they were introduced to Japan in the 17th Century. They are also called kansho, ryukyu-imo, karaimo (Chinese potato), among other names. The Kagoshima region has the largest sweet potato production in Japan. They are sold fresh, processed into shochu (an alcoholic drink), or used as a raw material for processed food. Subtle sweetness and simple flavor makes them a popular variety.
"Southern Delite": has a rose to dark copper skin, dark orange flesh and excellent baking quality and satisfying flavor. Produces excellent yields and stores slightly better than Jewel. Combination of pest resistances is similar to that of Regal. Has moderate soil rot resistance. Has shown better natural insect resistance than could be expected using chemical pesticides. Excellent sprout (plant) production. Developed by USDA-ARS and Clemson University and released in 1986.
"Sumor": "Sumor", which is old English for summer, is considered a novelty as it has similarities to that of an Irish potato. It has a smooth, light tan skin, white to yellow flesh and a high dry matter content. It can be eaten fresh earlier than most cultivars and yields about 15% more than Jewel. Although this variety has only a fraction of the Beta-Catotene found in orange varities, it does contain more Vitamin C than most tomatoes. Resistance to disease and insects similar to Regal. Vine growth is vigorous and ground cover is good. Developed by USDA-ARS and Clemson University SCAES. Sumer should be grown in climates too hot for Irish potatoes. You wont be able to tell the difference.
"Vardaman": a bush variety with deep orange flesh. Perfect for the limited-space garden, where its beautiful deep red and green foliage makes it equally attractive as an ornamental. Released by the Mississippi Agricultural Extension Service in 1981. Is considered the best short-vined variety for eating. Has better resistance to fusarium wilt than older short vines (only 4-5 in length). The latest release and the most spectacular. Golden yellow outside skin that darkens soon after digging. Deepest, brightest orange color of all.
"White Sweet Potato": White sweet potatoes are a variety of regular orange sweet potatoes. Both orange and white sweet potatoes are in the Morning Glory family, Convolvulaceae. White sweet potatoes are also called camote, boniato, or batata. The outside skin of the white sweet potato is a brownish-purple or a reddish-purple color. The inside flesh of the white sweet potato is white or cream colored and is very firm.
"White Triumph": 90-120 Days. White Triumph produces small to medium size sweet potatoes that are long to oblong, with yellow-brown skin and cream colored flesh. We observed significant white grub damage to the roots and only obtained a light yield in the Kerr Center trial. As is common to most white sweet potatoes, "White Triumph" has a sweet flavor but very dry texture. This variety might also go by the name White Yam.
"Yamiamo (Japanese)": this is a sticky sweet potato that the Japanese peel and grate or julienne for salads. It's also fried or used to make soba noodles.
In the case of Peru, potatoes purple type has been the complete replacement of a native variety, the "egg yolk" for other improved, especially the "Trujillo improved", "Canchari" and "Maria Angola, known by producers as "purple legitimate" and "purple bambeado" respectively. Although most ancient varieties planted as "Italian lead", "Ramon Camacho" and "egg yolk" have virtually disappeared in the late 60's, several other prevalent even planted a lesser extent, as the "Japanese Tremesino" "Two in one" and "Mamala".
A variety of sweet potato purple, with purple pigment also in the pulp, is presented as a super food with anticancer and anti-aging properties.
An expert from the University of Kansas, United States, developed the variety (unlike the common purple sweet potato, which has only the color purple on the outside) so that the purple pigment is also present in all the food and so multiply your anticancer properties.
The color contains anthocyanin, which reduces the risk of developing cancer, and may even counteract certain types of this disease.
The scientists used two kinds of anthocyanins (cyanidin and peonidin) to treat colon cancer and found that the purple pigment reduced the growth of cancer cells.
The anthocyanin present in red, blue or purple in different foods, including cherries, red grapes, red cabbage, but especially in this variety of sweet potato purple, which contains the substance in larger amounts.
The purple sweet potato is sweeter than the others and that is good not only to prevent cancer, but has other benefits for its antioxidant properties.
The Batata requires high temperatures (at least 15 ° C for germination and not less than 20 °C for the entire cycle) and high water requirements. Abbisogna of loose soil, well drained and rich in organic matter and nitrogen and potassium.
The planting of the crop is done typically using cuttings or tubers. The apical cuttings of the stems may be located directly before or made germinate in warm bed. The distance is 50 cm between rows and 40 along the row (5 plants per square meter). As regards the tubers, are used smaller ones and with many gems.
Fertilizing should seclude about 60-100 kg / ha of N and up to 150 kg / ha of K2O depending on equipment of the ground.
The cycle lasts 140-160 days; needs support, tamping and adapted pretty inputs of water.
Harvesting and production
Harvesting is done when the leaves begin to turn yellow. The production per hectare varies from 200 to 300 tons of tubers (or more). Before being stored in a dry, dark, well ventilated, must be cleaned from the ground and dried.
Average chemical composition of sweet potato tubers: water 71%, starch 17.5%, other carbohydrates about 7.5%, fiber 1%, protein 1.8%, fat 0.2%, ash 1%.
You can start digging up the potatoes as soon as they are big enough for a meal. Often, this is three to four months from when you planted the slips. The leaves should have started to yellow, but you can leave them in the ground up until the fall frost. Since the roots spread 4 to 6 inches deep in the soil, a spade fork is useful when digging up the potatoes.
Handle the potatoes carefully because they bruise easily.
After digging up the potatoes, shake off any excess dirt but no not wash the roots.
If you want to store sweet potatoes for an extended period of time, you must cure them. Curing the potatoes allows a second skin to form over scratches and bruises that occur when digging up the potatoes. To cure, keep the roots in a warm place (about 27 °C) at high humidity (about 90%) for 10 to 14 days. For best curing, make sure that the potatoes are not touching one another.
After curing, throw out any bruised potatoes, and then wrap each one in newspaper and pack them carefully in a wooden box or basket.
Store the sweet potatoes in a root cellar or other place with a temperature of at least 13 °C. The ideal temperature range is 13 to 16 °C.
The roots should last for about 6 months. When removing the potatoes from storage, remember to be gentle; do not dig around or else you will bruise the potatoes.
The plant is grown throughout the year. Sweet potatoes on the market in Germany come from Brazil and Israel, and are imported mainly from early summer to early autumn.
At home you can keep as potatoes, wine cellars in cool, dry and dark place, at a minimum temperature of 5 °C. As an alternative, also the dry peat keeps them fresh for a certain time, but the high water content limits, however, the shelf life.
For the populations of the countries of origin, the sweet potato is an important staple food, the importance comparable to that of the potato for us, with whom he also shared many of the characteristics and culinary preparations. As rich in carbohydrates, joins some preparations of vegetables, very spicy and aromatic. Boiled or roasted, served with tasty sauces, or worked to make a kind of bread. The leaves and young shoots that grow from tubers, are usually used as ordinary vegetables, but in some areas they serve only as animal fodder.
Industrially the batata is exploited for the production of flour and starch (sweet potato starch, which contains approximately 24-26% starch), but they also derive glucose syrup, alcohol, spirits and other alcoholic beverages.
To cook sweet potatoes in the oven just peel them, bucherellarle on all sides with a fork, brush with oil and wrap in aluminum foil. Also excellent mashed prepared with baked potatoes and cream.
More than 95% of the cultivation of Ipomoea batatas are grown in developing countries, where it is the fifth staple food. It normally takes the rizotuberi of the plant that are cooked boiled, fried or baked. They are also used industrially for the extraction of starch, alcohol and for the production of flours. The leaves and young shoots are edible.
Other uses include culinary not:
Use a red sweet potato in the kitchen, the answer is simple. In all those recipes where you would use the normal potato or pumpkin, given that the taste of this tuber is halfway between these two most known and used foods. You can for example cook a risotto of batata, or invite your friends to enjoy together some great sweet potatoes baked
- the production of dyes. In South America, the juice of sweet potatoes red combined with the lime juice is used as a dye for fabrics;
- feed production. All parts of the plant are used as animal feed;
- an ornamental use because if you sprout and drowns the rizotubero in water can take a good look.
Unlike ordinary potatoes, however, the red sweet potato can be enjoyed raw with his skin well washed (eg in the salad was delicious).
Avoiding cooking, among others, remain intact all the nutritional values and the many properties of this tuber. Is it any sottilineare then that is mainly in the skin which is a substance that appears to have beneficial effects on cholesterol reduction and blood glucose: the Cajapo. Statistics show that in countries where the sweet potato is consumed more frequentamente raw (some regions of Japan) there are less people with diseases such as diabetes, hypertension and anemia.
The yam has the therapeutic and nutritional details, especially the caiapó extracted from its husk, is used by Japanese people to treat anemia, hypertension and diabetes.
But if we do not care to eat, and in the garden spostiamola coltiviamola as decorative plant, especially in the variety "Blackie" in red leaves.
We come to the disadvantages: first of all the difficult availability of this food. The price then it is certainly not for every budget. If we live in a hot area, however, you can try to plant on your terrace or in the garden so you always have it available.
Diseases, pests, and parasites
Viruses are the most damaging group of disease organisms affecting sweet potato. Viruses are amongst the smallest organisms known and can only be seen using a very powerful electron microscope. They are very simple organisms and can only survive and multiply inside their hosts/victims. Most also need to be carried from plant to plant, usually by an insect which feeds on plant sap, such as aphids or whiteflies. Once a virus enters a cell in the body of its host, it will take over part of the management of the cells processes, and force the cell to produce more viruses identical to itself rather than the crop yield we want. These new virus particles then spread through the plant to infect more cells.
Common symptoms of virus infection in plants including sweet potato are:
Viruses affecting sweet potato can be spread by the use of foliar cuttings taken from infected plants. They are also transmitted from plant to plant by sap-sucking aphid and whitefly insects. Here are some viral diseases that may affect the sweetpotato:
- Diminished growth so the plants and leaves remain small (stunted).
- Chlorosis (paleness, even whitening; aidosoikit) of the leaf tissue. This chlorosis may be general or in a pattern, often either between the leaf veins in a mosaic or less well defined mottle, or along the veins to form a chlorotic network.
- Misshapen leaves with an uneven or curled appearance.
- Pigmented leaves, often purple or yellow generally or in spots or rings.
- Reduced production of sweet potato storage roots.
- Sweet potato feathery mottle virus (SPFMV) and Sweetpotato chlorotic stunt virus (SPCSV) are together the most important viruses affecting sweet potato. By themselves, sweet potato viruses may cause only mild or no obvious symptoms. However, plants can be infected by more than one type of virus and, when this happens, the viruses may help each other to multiply with the result that the disease is more severe than expected. The combination of SPCSV+SPFMV is known as sweet potato virus disease (SPVD) and is the most important disease of sweet potato. SPFMV is transmitted by a wide range if aphid species (eiliana) and is spread mainly by winged adults, even of species that do not colonise sweet potato, flying from plant to plant. SPCSV is transmitted by the mobile adult form of white flies (ekwanga), especially Bemisia tabaci, as they fly from plant to plant. Since it is the spread of SPCSV by white flies that synergises SPFMV, whiteflies are also usually the driving force behind the spread of SPVD. Neither disease is spread by insects over very long distances.
Sweetpotato feathery mottle (SPFMV) is a Potyvirus of the Potyviridae family. The synonyms are Sweetpotato chlorotic leaf spot virus, Sweetpotato internal cork virus; Sweetpotato russet crack virus; Sweetpotato vein mosaic virus; Sweetpotato virus A; Sweetpotato vein clearing virus; Sweetpotato ring spot virus.
About the economic importance, although rarely recognised, SPFMV is responsible for considerable yield reduction in sweetpotato crops worldwide. In the field, virus-free sweetpotato plants yield from 20% to over 100% more than infected plants. Sensitive cultivars may suffer significant yield losses in comparison with tolerant ones.
Some isolates of SPFMV cause economic losses by their effect on storage root quality (internal cork and russet crack).
The geographical distribution is Worldwide. SPFMV is found wherever sweetpotato is grown.
SPFMV is the name that has become associated with a serious disease of sweetpotato, symptoms of which typically include chlorosis, discoloration, and stunting (figure 11).
Figure 11 Symptoms observed on plants grown at harvest. A: healthy plant (left); center and right, plants with different degrees of decay (note the strong reduction of the vegetation and the size of the tubers produced). B: longitudinal-necrotic cracks encircling the storage tuber caused by russet crack and internal cork strains.
Symptoms of SPFMV on the foliage are generally slight or absent. The classic irregular chlorotic patterns (feathering) along leaf veins and faint-to-distinct chlorotic spots with or without purple margins occur in some cultivars. Symptom visibility on foliage is influenced by cultivar susceptibility, degree of stress, growth stage, and strain virulence. Increased stress can lead to symptom expression, whereas rapid growth may result in symptom remission (figure 12).
Symptoms on storage roots depend on the strain of SPFMV and the sweetpotato variety. The common strain causes no symptom on storage roots of any variety, but the russet crack and internal cork strains cause external and internal dark necrotic lesions on certain varieties, respectively.
Figure 12 Plant with SPFMV showing chlorotic spots on leaves (left). Chlorosis of leaf veins (center). Leaf with chlorotic spots, chlorosis diffused and yellow veins (right).
Symptoms on storage roots depend on the strain of SPFMV and the sweetpotato variety (Figure 11). The common strain causes no symptom on storage roots of any variety, but the russet crack and internal cork strains cause external and internal dark necrotic lesions on certain varieties, respectively (figure 11).
The virion, ranging from 810 to 865 nm long, contains a single, positive-sense strand of RNA with a Mr of approximately 3.7 x 106 D and a polypeptide with a of 36 to 38 KD. It has shown a serological relationship to some other potyviruses.
SPFMV is the most thoroughly characterized sweetpotato virus. SPFMV has many of the biological and biochemical properties, and cytopathic characteristics of potyviruses, including aphid transmissibility, occurrence of pinwheel inclusions, and a relatively narrow host range. SPFMV is sap-transmissible, and transmitted by a large number of aphid species such as Aphis gossypii, Aphis craccivora, Myzus persicae, in a nonpersistent manner. Vegetative propagation perpetuates the virus.
Several isolates and strains of SPFMV have been characterized in different parts of the world; perhaps the most important ones being the ordinary (O), russet crack (RC) and severe (S) strains because they directly affect storage root quality (Figure 11 B).
SPFMV is found with SPCSV in several countries; the combination usually results in a severe disease known as sweetpotato virus disease (SPVD).
About the host range sweetpotato is the main natural host of SPFMV, although the virus occurs in wild Ipomoea species. The experimental host range of the virus is mainly restricted to the Convolvulaceae and Chenopodiaceae, but a few strains also infect species of the Solanaceae of which Nicotiana benthamiana is a good propagation host for purification of the virus. Several strains cause local lesions on Chenopodium amaranticolor and Chenopodium quinoa.
The main species of cultivated plants susceptible to SPFMV are:
About the control of viruses aphid control is not economically feasible. The main control measures are: avoidance of diseased plants as planting material, sanitation, and the use of resistant varieties. As SPFMV is perpetuated between cropping cycles in infected cuttings, the lack of symptoms in the foliage makes it difficult for farmers to select SPFMV-free cuttings.
- Chenopodiaceae: Chenopodium murale, Chenopodium amaranticolor, Chenopodium quinoa, and Spinacia oleracea (several strains).
- Convolvulaceae: Calonyction aculeatum, Ipomoea hederaceae, Ipomoea incarnata, Ipomoea lacunosa, Ipomoea purpurea, Ipomoea trichocarpa, Ipomoea tricolor, Ipomoea wrightii, Merremia sibirica, and Quamoclit lobata.
- Solanaceae: Datura metel, Nicotiana benthamiana, Nicotiana clevelandii, Nicotiana occidentalis, and Nicotiana tabacum (some strains).
Because viruses spread quickly through the vascular system of a plant to infect the whole plant, any portions of an infected plant that are used as planting materials (vines or roots), are almost always diseased themselves. This then carries the disease to the next generation of plants.
However, often the best and certainly the most convenient means of controlling SPVD is to plant varieties of sweet potato which have resistance to virus diseases.
All these management practices work better if they are done on areas as large as possible, so if communities can work together to manage SPVD they will all benefit. The crop hygiene you do to control sweet potato viruses probably also helps control other pests and diseases.
- Make sure cuttings are collected for planting new sweet potato crops from healthy plants and if possible from healthy plants in crops in which few other plants have the disease. Then there is also less chance of taking cuttings from plants that have just been infected. It may be better to avoid collecting cuttings from very old crops both because SPVD may have built up in these crops and because SPVD is less easy to see in old plants that in vigorously growing crops.
- Remove and burn or feed to livestock any diseased plants as soon as they appear in young crops (plants infected when young wouldnt have yielded much anyway, the neighbouring plants will soon fill up the space and you can replant cuttings If you wish in young crops).
- Avoid planting new crops where you grew sweet potato last season because roots and cuttings from old diseased plants surviving in the soil will produce diseased plants from which infection will easily spread to your new crop.
- Plant your new crop away from old crops so it is difficult for whiteflies and aphids to reach your new crop. Other forms of crop hygiene such as ensuring that the vines and leaves from harvested crops are completely destroyed (they can be fed to livestock) and that all the roots (especially the small ones which may come from diseased plants), are destroyed.
Leaf symptoms of SPFMV are often inconspicuous or absent. If present, leaf symptoms appear as faint, irregular chlorotic spots occasionally bordered by purplish pigment. The classic irregular chlorotic patterns (feathering) along midribs and faint-to-distinct chlorotic spots, with or without purple margins, occur in some cultivars. Symptom intensity on foliage is influenced by cultivar susceptibility, degree of stress, growth stage and strain virulence. Increased stress can lead to symptom expression, whereas rapid growth may result in symptom remission. Symptoms on storage roots depend on the strain of SPFMV and the sweet potato variety. The common strain causes no symptom on any variety, but the "russet crack" strain causes external necrotic lesions or internal cork on certain varieties. Internal cork virus cause brown to black corky areas develop in the flesh of roots and can go undetected until sliced . Modern day varieties are resistant to these two symptoms. Foliar symptoms vary from purple ring spots to vein clearing.
- Sweetpotato caulimo-like virus (SPCaLV). It is a virusof the family Caulimoviridae genus Caulimovirus. This disease is reported in several countries and the impacts on yield are unknown.
Geographical distribution: the virus was first detected in sweetpotato originating from Puerto Rico. It has been found in a complex with sweetpotato feathery mottle virus (SPFMV) from the South Pacific Region (including Tonga, Papua New Guinea, New Zealand, Solomon Islands, Australia and New Zealand), Madeira, Kenya, Uganda, and U.S.A.
Symptoms: sweetpotato plants infected with SPCaLV usually show no obvious viral symptoms. Sometimes infected plants may show interveinal chlorosis (Figure 13, A1) or faint chlorotic spots (Figure 13, A2) which may develop into general chlorosis, wilting and premature death of leaves (Figure 13, A).
Morphology: this virus has isometric particles 50 nm in diameter that contain a major polypeptide of Mr 42-44 KD and dsDNA. SPCaLV is not serologically related to other caulimoviruses: cauliflower mosaic (CaMV), dahlia mosaic (DMV), carnation etched ring (CERV) and soybean chlorotic mottle (SbCMV).
Biology and ecology: virus particles and characteristic intracellular inclusions induced by the virus are readily detected in the cytoplasm of epidermal and vascular parenchyma cells of infected plants. Ultra structural studies have shown that infected vascular parenchyma cells containing inclusions sometimes protrude into, and so cause occlusion of adjacent xylem vessels, which results in wilting and premature abscission of infected leaves. SPCaLV is not mechanically transmitted or by seed or by contact between plants. Its vector is unknown. The virus does not appear to be transmitted by aphids M. persicae and Aphis gossypii.
Host range: the only known natural host of SPCaLV is Ipomoea batatas. In the laboratory, SPCaLV can be made to infect Ipomoea setosa and Nicotiana megalosiphon.
Management: regulatory control and international exchange of virus-free germplasm.
Cultural control: use of healthy planting materials.
- Sweetpotato leaf curl virus (SPLCV). It is a Badnaviridae, genus Badnavirus. Yield losses are unknown and grooving of storage roots may reduce their value. It is Reported in USA, Taiwan and Japan.
Symptoms: the most common symptom is upward curling or rolling of leaves on young plants. The rolled edge tends to be crinkled, and vein swelling may be apparent. An interveinal chlorotic mottle is sometimes observed. Symptoms may appear seasonally and often disappear with time. Storage roots of infected plants have been reported to develop longitudinal grooves or ribs. This appears more pronounced when SPFMV is also present (Figure 13, B).
Morphology: virions are short, rod-shaped (bacilliform) and not enveloped.
Biology and ecology: the virus particles are found in the cytoplasm of phloem cells. Transmitted by the sweetpotato whitefly Bemisia tabaci, in a persistent manner. The virus can be transmitted by grafting,, but not by mechanical inoculation. It is not transmitted by contact between plants, nor by seed.
Host range: the only known natural host of SPLCV is Ipomoea batatas. No secondary host has been reported naturally. The "Japanese morning glory", a variety of the specie Ipomoea nil (L.) Roth 1821 (cultivated as an attractive ornamental plant in many places, and the descendants of garden escapees now grow wild), has been found susceptible through laboratory inoculation, and displays diagnostic upward curling of young leaves.
Management: regulatory control; international exchange of virus-free germplasm.
Cultural control: use of clean, healthy planting materials.
- Sweetpotato mild mottle virus (SPMMV). The virus is of the Potyviriidae family, Ipomovirus genus.
Economic importance: the yield effects of this virus are unknown, but it can reduce the quality of vines for planting material.
Geographical distribution: Burundi, Kenya, Tanzania, Uganda, Philippines.
Symptoms: the predominant symptoms associated with SPMMV are mild leaf mottling and stunting. It induces distinct veinal chlorosis in Ipomea setosa (Figure 13, C). None of these symptoms are diagnostic in the field and the virus can be latent.
Biology and ecology: SPMMV is transmitted non-persistently by the whitefly Bemisia tabaci. It is also perpetuated through planting infected cuttings. SPMMV is often found in sweetpotato plants also infected with sweetpotato feathery mottle virus (SPFMV) and sweetpotato chlorotic stunt virus (SPCSV) but whether there is any underlying link with either or both of these viruses is unclear. SPMMV has been shown to spread in East Africa but no other epidemiological information is available.
Host range: SPMMV has been found naturally only in sweetpotato but can be transmitted experimentally to a wide range of plant species including many Ipomoea spp but also Beta vulgaris, Chenopodium murale, Datura stramonium, Gomphrena globosa, Lycopersicon esculentum, Nicotiana benthamiana, Petunia hybrida and Zinnia elegans.
Detection and inspection: both monoclonal and polyclonal antisera have been developed against SPMMV allowing sensitive ELISA-based assays to be developed. The entire genome of SPMMV has been sequenced, allowing nucleic acid techniques to be utilised. The unusually wide host range and easy sap transmission of SPMMV allows transmission tests to be useful.
Cultural control: sanitation and selection by farmers of symptomless planting material can help achieve control.
Host plant resistance: some sweetpotato cultivars appear to be immune and others are tolerant.
- Sweetpotato ring spot virus (SPRSV). The virus is of the Comoviridae family, Nepovirus genus.
Economic importance: reported only in a few countries in the Pacific and Africa. Crop losses are unknown.
Geographical distribution: SPRSV was isolated from cv "Wanmum" from Papua New Guinea. It has also been detected in plants from Kenya.
Symptoms: sweetpotato plants infected with SPRSV may show chlorotic ring spots (Figure 13, D) but in most infected cultivars plants are symptomless. In graft-inoculated sweetpotato cultivars "Centennial" and "Rose Centennial", the virus induces extensive chlorotic spotting of leaves.
Morphology: the virus has isometric particles measuring 28 nm in diameter. It has three components with sedimentation coefficients of 47, 87, and 130 S and contains a single polypeptide of Mr 56 KD. The middle (87 S) and bottom (130 S) component particles contain ssRNA of c. 6,670 and c. 8,448 nucleotides, respectively. It has a capsid polypeptide of Mr 56.5 KD. Although the virus has some properties typical of nepovirus, it is serologically unrelated to any of the 12 members of the Nepovirus group.
Biology and ecology: the virus is transmitted by mechanical inoculation, but not by contact between plants.
Host range: the only known natural host of SPRSV is Ipomoea batatas. SPRSV can be experimentally induced to infect species in several (3-9) families. The following species are susceptible: Capsicum annum, Chenopodium amaranticolor, C. capitatum, C. murale, C. quinoa, Cucumis sativus, Datura stramonium, Glycine max, Gomphrena globosa, Hibiscus esculentum, Ipomoea nil, I. setosa, Nicotiana benthamiana, N. clevelandii, N. glutinosa, N. megalosiphon, N. tabacum, Phaseolus vulgaris, Tetragonia expansa, and Vigna unguiculata.
Management: regulatory control and international exchange of virus-free germplasm.
Cultural control: use of clean, healthy planting materials.
- Sweetpotato virus disease (SPVD). This is a disease complex caused by dual infection with the whitefly-transmitted the genus in the family Closteroviridae such as the sweetpotato chlorotic stunt viruscrinivirus (SPCSV) and the aphid-transmitted potyvirus sweetpotato feathery mottle virus (SPFMV).
Economic importance: SPVD is the most damaging sweetpotato disease in Africa and perhaps worldwide. It causes virtually total yield loss in affected plants.
Geographical distribution: throughout the tropics worldwide. Particularly prevalent in sub-Saharan Africa but recent reports of damage in South America too.
Symptoms: diseased plants are severely stunted and the leaves are small and narrow (strap-like or fan-like), often with a distorted edge. Puckering, vein clearing, and mottling may occur. The mottling is chlorotic so that the whole plant may appear pale (Figure 13, E). Storage root development is affected resulting to lower yield.
Biology and ecology: this disease is caused by SPCSV synergistically breaking down resistance of sweetpotato against SPFMV, allowing SPFMV to multiply some hundredfold more than when it infects alone. This mechanism of synergism is currently unclear but is an area of active research. SPCSV also synergises the infection of sweetpotato by other viruses including cucumber mosaic virus and sweetpotato mild speckling virus. Although the name SPVD is clearly non-specific, its use is best restricted to describing only the disease caused by dual infection with SPCSV and SPFMV: other names are generally used for diseases caused by other virus combinations. Since SPCSV synergises SPFMV but not vice-versa, the phenomenon of SPVD appears to have no effect on the spread of SPCSV but enormously increases the ability of aphids to acquire and spread SPFMV as suggested by results of studies on cultivar Tanzania and confirmed by observations on other cultivars.
Host range: it is not clear whether SPCSV synergises infection of SPFMV in any plant species other than sweetpotato.
Detection and inspection: the same methods as for SPFMV and SPCSV. SPFMV is much more readily detected in SPVD-affected plants than in plants infected with SPFMV alone.
Host plant resistance: no immunity has been identified in sweetpotato though genotypes, with useful levels of resistance to infection in the field, have been selected by farmers and plant breeders. Sweetpotato cultivars also vary in their tolerance of SPVD.
Cultural control: sanitation and selection by farmers of symptomless planting material can help achieve control.
Figure 13 A: Sweetpotato caulimo-like virus (SPCaLV): discolouration between veins (chlorosis) that may also be described as faint mottling (A1); chlorotic flecks along the secondary veins and interveinal chlorotic spots on leaf of a SPCaLV graft-inoculated Ipomoea setosa plant (A2). B: Sweetpotato leaf curl virus (SPLCV) on grooving of storage roots resulting from SPLCV infection (below); healty storage roots (top). C: Sweetpotato mild mottle virus (SPMMV) on leaf with distinct yellowing of Ipomea setosa leaf veins. D: Sweetpotato ring spot virus (SPRSV) on leaf with chlorotic ring spots. E: Sweetpotato ring spot virus (SPRSV)-affected plant showing small, pale green misshapen leaves (left) with a normal plant (right).
- Sweetpotato chlorotic fleck virus (SPCFV). SPCFV has been detected in plants from Peru, Japan, China, Cuba, Panama, Bolivia, Colombia, Brazil, Uganda, Philippines, Indonesia, Egypt and India.
Symptoms: they vary with cultivar. Chlorotic flecks (fine chlorotic spots) or symptomless infections are common.
Morphology: the virus has filamentous particles around 750-800x12 nm with a capsid polypeptide of Mr 34.5 KD. SPCFV is not serologically related to other filamentous viruses from sweetpotato and potato.
Ecology: no inclusion bodies ("pinwheels") have been observed in cells of Ipomoea nil infected with SPCFV, but only cytological alterations (hypertrophy of chloroplasts). According to serological tests, some strains of SPCFV seem to occur. The vector of SPCFV is unknown. It does not appear to be transmitted by Myzus persicae nor by the botanical seed of Ipomoea nil and Ipomoea setosa.
Host range: the only known natural host of SPCFV is Ipomoea batatas. In the laboratory, SPCFV infects species in the Convolvulaceae and Chenopodiaceae families. Chenopodium murale, Chenopodium quinoa, Ipomoea nil and Ipomoea setosa are susceptible.
Management: regulatory control and international exchange of virus-free germplasm.
Cultural control: use of healthy planting materials.
Host-plant resistance: some genotypes from CIP's germplasm collection showed resistance to SPCFV after graft-inoculation with infected Ipomoea nil scions.
Synthesizing, sweet potato chlorotic fleck virus (SPCFV) is one of several viruses naturally infecting sweet potato, and it has recently been classified as a new member of the genus Carlavirus (family Flexiviridae). However, SPCFV is distantly related to typical carlaviruses, as most of its putative gene products share amino acid sequence identities of over 40% with those of typical carlaviruses. China is the largest sweet potato producing country in the world. So far, SPCFV has been reported in eastern China, such as Jiangsu, Anhui, Henan, and Guangdong provinces, but no reports exist in western China. Sichuan Province, located in southwestern China, is the largest sweet potato producing area in the country. There are big differences between the environment and climate conditions between Sichuan and eastern China. During 2012, a survey was constructed to determine the genetic diversity and distribution of sweetpotato viruses in Sichuan. Forty-seven sweet potato samples exhibiting virus-like symptoms were collected from four different geographic areas of the province. Western blotting using the antisera obtained from the International Potato Center showed that two samples were positive for SPCFV, whereas with reverse transcription (RT)-PCR, only one isolate of SPCFV was obtained from a sample exhibiting symptoms of chlorosis, leaf distortion, and vein clearing. Serological detection indicated that the plant was co-infected with SPCFV, sweet potato feathery mottle virus (SPFMV), and Sweet potato virus G (SPVG). Total RNA was extracted from symptomatic leaves using Trizol reagent (invitrogen) according to the manufacturer's protocol, and RT-PCR was performed by using primer pairs SPCFV-CP F (5′-ATGGCGGCGAAGGAGGCTGATA-3′) and SPCFV-CP R (5′-TCACTTGCACTTCCCATTAC-3′) corresponding to the entire coat protein (CP) gene of SPCFV. Expected DNA fragments of 900 bp were obtained from the symptomatic plant but not from control plants. The obtained fragments were purified and cloned into the PMD19-T vector (TaKaRa). Recombinant plasmids were then transformed into competent cells of Escherichia coli strain DH5α. Nucleotide BLAST analysis revealed that the 900-bp fragment (GenBank Accession No. KC414676) shared 87 to 91% nucleotide identities with other SPCFV isolates available in the GenBank database. To our knowledge, this is the first report of the co-infection between SPCFV and other sweet potato viruses including SPFMV and SPVG in China, and this is the first molecular report of SPCFV in Sichuan, western China. It shows that SPCFV is spreading to a new ecological area of China, and the spread of the virus may affect sweet potato crop yields in western China. Some measures must be carried out quickly to control the virus.
It should also be said that RNA silencing is an important mechanism of antiviral defence in plants. To counteract this resistance mechanism, many viruses have evolved RNA silencing suppressors. In this study, we analysed five proteins encoded by Sweetpotato chlorotic fleck virus (SPCFV) for their abilities to suppress RNA silencing using a green fluorescent protein (GFP)-based transient expression assay in Nicotiana benthamiana line 16c plants. Our results showed that a putative nucleotide-binding protein (NaBp), but not other proteins encoded by the virus, could efficiently suppress local and systemic RNA silencing induced by either sense or double-stranded RNA (dsRNA) molecules. Deletion mutation analysis of NaBp demonstrated that the basic motif (an arginine-rich region) was critical for its RNA silencing suppression activity. Using confocal laser scanning microscopy imaging of transfected protoplasts expressing NaBp fused to GFP, we showed that NaBp accumulated predominantly in the nucleus. Mutational analysis of NaBp demonstrated that the basic motif represented part of the nuclear localization signal. In addition, it has been demonstrated that the basic motif in NaBp was a pathogenicity determinant in the potato virus X (PVX) heterogeneous system. Overall, the results demonstrate that the basic motif of SPCFV NaBp plays a critical role in RNA silencing suppression, nuclear localization and viral pathogenesis.
- Sweetpotato chlorotic stunt virus (SPCSV). This virus is of the family Closteroviridae, genus Crinivirus. Possibly the most damaging virus infecting sweetpotato and the most damaging pathogen of sweetpotato in Africa. By itself, SPCSV causes some yield loss, but its presence breaks resistance in sweetpotato to sweetpotato feathery mottle virus (SPFMV) and the combined infection causes the very severe disease, sweetpotato virus disease (SPVD).
Geographical distribution: tropics worldwide, particularly prevalent in sub-Saharan Africa but recent reports of damage in South America.
Symptoms: by itself, infection by SPCSV typically stunts sweetpotato plants and causes either a reddening or chlorotic yellowing of middle and lower leaves. However, symptoms may also be very mild or even absent, depending perhaps, on the isolate and the conditions. In South America, SPCSV has been reported to induce mosaic symptoms in leaves. SPCSV is most commonly found in combination with SPFMV causing SPVD.
Biology and ecology: SPCSV is a phloem-associated virus transmitted by the whitefly Bemisia tabaci in the semi-persistent manner and needs feeds of several hours to be acquired or transmitted efficiently. It may also be perpetuated through cropping cycles via infected cuttings. SPCSV is generally identified in combination with SPFMV, causing the severe disease SPVD. However, SPCSV seems to gain little or nothing from this relationship as its strength and distribution in sweetpotato plants seems unaffected, though the strength of SPFMV may be some hundredfold greater in dually-infected plants.
Host range: SPCSV has only been reported on sweetpotato. However, wild hosts have been reported to be important in Israel. In addition, SPCSV has been transmitted to a range of Ipomoea spp. including Ipomoea setosa, Ipomoea acuminata, Ipomoea hederacea, Ipomoea hederifolia, Ipomoea nil cv "Scarlet OHara", Ipomoea purpurea, Ipomoea trichocarpa, Ipomoea trifida, Ipomoea wrightii, Ipomoea mexicana, Ipomoea Bona nox and Ipomoea hildebrandtii. It has also been transmitted to Nicotiana clevelandii, N. benthamiana and Amaranthus palmeri and been identified in lisianthus (Eustoma grandiflorum).
Detection and inspection: both monoclonal and polyclonal antisera have been developed against SPCSV allowing sensitive ELISA-based assays to be developed. The coat protein gene has been sequenced and both PCR and cDNA probes have also been used to detect SPCSV.
Management: host plant resistance. No immunity has been identified in sweetpotato though genotypes with useful levels of resistance to infection in the field have been selected by farmers and plant breeders. Sweetpotato varieties also vary in their tolerance to SPVD but it is not clear whether mild symptoms of infection are likely to be beneficial overall in a vegetatively propagated crop.
Cultural control: sanitation and selection of planting material from unaffected parents help achieve control.
- Sweetpotato feathery mottle virus (SPFMV). The synonyms of this virus are: Sweetpotato chlorotic leaf spot virus; Sweetpotato internal cork virus (ICV); Sweetpotato russet crack virus; Sweetpotato vein mosaic virus; Sweetpotato virus A; Sweetpotato vein clearing virus; Sweetpotato ring spot virus. This virus is of the family Potyviridae, genus Potyvirus. Although rarely recognised, SPFMV is responsible for considerable yield reduction in sweetpotato crops worldwide. In the field, virus-free sweetpotato plants yield from 20% to over 100% more than infected plants. Sensitive cultivars may suffer significant yield losses in comparison with tolerant ones. Some isolates of SPFMV cause economic losses by their effect on storage root quality (internal cork and russet crack).
Geographical distribution: worldwide. SPFMV is found wherever sweetpotato is grown.
Symptoms: Leaf symptoms of SPFMV are often inconspicuous or absent. If present, leaf symptoms appear as faint, irregular chlorotic spots occasionally bordered by purplish pigment. The classic irregular chlorotic patterns (feathering) along midribs and faint-to-distinct chlorotic spots, with or without purple margins, occur in some cultivars. The classic irregular chlorotic patterns (feathering) along leaf veins (Figure 14, A) and faint-to-distinct chlorotic spots with or without purple margins occur in some cultivars (Figure 14, B). Symptom visibility on foliage is influenced by cultivar susceptibility, degree of stress, growth stage, and strain virulence. Increased stress can lead to symptom expression, whereas rapid growth may result in symptom remission. Symptoms on storage roots depend on the strain of SPFMV and the sweetpotato variety. The common strain causes no symptom on storage roots of any variety, but the russet crack and internal cork strains cause external and internal dark necrotic lesions on certain varieties, respectively (Figure 14, C, D ed E). Symptom intensity on foliage is influenced by cultivar susceptibility, degree of stress, growth stage and strain virulence. Increased stress can lead to symptom expression, whereas rapid growth may result in symptom remission. Symptoms on storage roots depend on the strain of SPFMV and the sweet potato variety. The common strain causes no symptom on any variety, but the "russet crack" strain causes external necrotic lesions or internal cork on certain varieties.
Morphology: the virion, ranging from 810 to 865 nm long, contains a single, positive-sense strand of RNA with a Mr of approximately 3.7 x 106 D and a polypeptide with a of 36 to38 KD. It has shown a serological relationship to some other potyviruses.
Ecology: SPFMV is the most thoroughly characterized sweetpotato virus. SPFMV has many of the biological and biochemical properties, and cytopathic characteristics of potyviruses, including aphid transmissibility, occurrence of pinwheel inclusions, and a relatively narrow host range. SPFMV is sap-transmissible, and transmitted by a large number of aphid species such as Aphis gossypii, Aphis craccivora, Myzus persicae, in a nonpersistent manner. Vegetative propagation perpetuates the virus. Several isolates and strains of SPFMV have been characterized in different parts of the world; perhaps the most important ones being the ordinary (O), russet crack (RC) and severe (S) strains because they directly affect storage root quality. SPFMV is found with SPCSV in several countries; the combination usually results in a severe disease known as sweetpotato virus disease (SPVD).
Host range: sweetpotato is the main natural host of SPFMV, although the virus occurs in wild Ipomoea species. The experimental host range of the virus is mainly restricted to the Convolvulaceae and Chenopodiaceae, but a few strains also infect species of the Solanaceae of which Nicotiana benthamiana is a good propagation host for purification of the virus. Several strains cause local lesions on Chenopodium amaranticolor and Chenopodium quinoa. The species host are:
Host range: sweetpotato is the main natural host of SPFMV, although the virus occurs in wild Ipomoea species. The experimental host range of the virus is mainly restricted to the Convolvulaceae and Chenopodiaceae, but a few strains also infect species of the Solanaceae of which Nicotiana benthamiana is a good propagation host for purification of the virus. Several strains cause local lesions on Chenopodium amaranticolor and Chenopodium quinoa. The species host are:
Chenopodiaceae: Chenopodium murale, Chenopodium amaranticolor, Chenopodium quinoa, and Spinacia oleracea (several strains).
Convolvulaceae: Calonyction aculeatun, Ipomoea hederaceae, Ipomoea incarnata, Ipomoea lacunosa, Ipomoea purpurea, Ipomoea trichocarpa, Ipomoea tricolor, Ipomoea wrightii, Merremia sibirica, and Quamoclit lobata.
Solanaceae: Datura metel, Nicotiana benthamiana, Nicotiana clevelandii, Nicotiana occidentalis, and Nicotiana tabacum, Capsicum annuum (some strains).
Management: aphid control is not economically feasible. The main control measures are avoidance of diseased plants as planting material, sanitation, and the use of resistant varieties. As SPFMV is perpetuated between cropping cycles in infected cuttings, the lack of symptoms in the foliage makes it difficult for farmers to select SPFMV-free cuttings.
Figure 14 Sweetpotato feathery mottle virus (SPFMV). A: plant with SPFMV showing yellowing of leaf veins ("vein clearing"). B: purpling around chlorotic spots may be separated ring-spots. C: external root lesions induced by the russet crack strain of SPFMV in storage roots. D: detail of russet crack lesions. E: internal root necrosis ("Internal Cork") caused by SPFMV.
- Sweetpotato latent virus (SwPLV). The virus is of the Potyviridae family, Potyvirus genus. It is formerly designated as Sweet potato virus N, was first reported from Taiwan. The virus has flexuous, filamentous particles of approximately 700-750 nm long and induces typical cylindrical inclusion proteins in the cytoplasm of infected cells. The experimental host range of SPLV is wider than that of sweet potato feathery virus (SPFMV), and it induces symptoms on some Chenopodium and Nicotiana species. Other susceptible host species are: Chenopodium amaranticolor, Chenopodium murale, Chenopodium quinoa, Ipomoea nil, Ipomoea setosa, Nicotiana benthamiana, Nicotiana clevelandii, Nicotiana debneyi, Nicotiana megalosiphon, Nicotiana tabacum. Families containing insusceptible hosts are: Amaranthaceae (2 species on 2 tested), Chenopodiaceae (3/5), Compositae (1/1), Cruciferae (3/3), Cucurbitaceae (3/3), Gramineae (5/5), Leguminosae-Papilionoideae (7/7), Solanaceae (8/12), Tetragoniaceae (1/1). SPLV is serologically related to, but distinct from SPFMV. Sequence comparison of the 3-partial sequences showed that SPLV was a distinct species of the genus Potyvirus in the family Potyviridae. The virus is common in China and has been found in Asia and Africa.
- Sweetpotato little leaf phytoplasma (SPLL phytoplasma). Crop losses are unknown. However, the disease causes greatest losses in dry areas, which favour high leafhopper populations. Some infected plants do not survive until harvest or they produce few harvestable roots.
Geographical distribution: widespread in Asia, Australia and the western Pacific. The distribution pattern correlates closely with that of its principal leafhopper vectors. In Asia, witches' broom has been reported in Bangladesh, China (Fujian, Taiwan), India, Indonesia, Japan (Ryukyu Archipelago), Korea, Republic of Korea, Malaysia, and Philippines. In Oceania, it is found in Australia (Northern Territory), Belau, Federated states of Micronesia, New Caledonia, New Zealand, Niue, Papua New Guinea, Solomon Islands, Tonga, and Vanuatu.
Symptoms: the initial symptoms consist of a transient vein clearing followed by the development of new leaves that are distinctly smaller and more chlorotic (paler) than normal. Generally only scattered individual plants are infected, and they are conspicuously smaller than adjacent uninfected plants. Leaves may have a more rounded shape, often curling at the leaf margins. The growth habit tends to be more erect than in healthy plants, internodes are shortened and there is a proliferation of axillary shoots which, together with a greatly reduced root system, result in weak plants with a compressed or bushy appearance (Figure 15, A). The number and quality of storage roots are reduced, with few or no harvestable storage roots being produced on severely affected plants. Production of latex in vines and roots is also reduced (Figure 15, B).
Morphology: characteristic pleomorphic bodies ranging from 0.1 to 1.0 µm in diameter, with a well defined unit membrane.
Biology and ecology: the SPLL phytoplasma can be transmitted by the leafhopper Orosius lotophagorum subsp. ryukyuensis (Hemiptera: Cicadellidae) and Nesophrosyne ryukyuensis Ishihara 1965 (Cicadellidae) in a persistent manner. Low annual rainfall and prolonged dry seasons favour the vector and, under these conditions, the disease can reach epidemic proportions. Infected planting material is also important in the dissemination of the disease. As the disease has an exceptionally long incubation period in sweetpotato (up to 283 days) following graft transmission, infected planting material can appear healthy. In small, local cultivations, vines may be taken from more vigorous plants in order to establish a subsequent crop. While this practice may assist in selecting plants that are better able to tolerate infection by the phytoplasma, it probably contributes to the promulgation of the disease in the crop; this situation may be further exacerbated where crop plantings overlap or where the previous crop is not removed.
Host range: primary natural host is sweetpotato. Wild hosts (Ipomoea nil, Pharbitis purpurea, Ipomoea pes-caprae) act as reservoirs. Alternative experimental hosts of SPLL phytoplasma are Ipomoea setosa, I. triloba, I. indica, I. ericolor, Pharbitis purpurea, P. nil, and Catharanthus roseus.
Cultural control: sanitation (removal of all previous crop debris from the field and use of healthy planting material) has provided the best control to date. The removal and destruction of diseased plants and wild hosts reduces the spread of the disease.
Chemical control: insecticides could prove useful in the production of nursery stock or in commercial plantings, but not when sweetpotato is grown as a subsistence crop. Tetracycline causes remission of symptoms leading to the production of disease-free young shoots, and may be of use in breeding and selection programmes; however, it is unlikely to be commercially significant.
Host-plant resistance: there is little evidence of resistance to the disease. Only one clone from Solomon Islands and another from Taiwan showed moderate resistance, but these clones are affected by SPLL isolates from different geographical regions.
Movement of germplasm: phytoplasma-free vegetative material should be transferred as in-vitro plantlets.
Figure 15 Sweetpotato little leaf phytoplasma (SPLL phytoplasma). A: proliferation of shoots and reduced leaf size in infected plant. B: reduced size of whole plant and of storage roots, in a plant infected with SPLL (right) compared with the adjacent uninfected plant (left).
- Bacterial soft rot Erwinia chrysanthemi caused by Erwinia chrysanthemi Burkholder et al. 1953 (=Dickeya dadantii), belonging to Domain Bacteria (Haeckel 1894) C.R. Woese et al. 1990; Phylum Proteobacteria Garrity et al. 2005; Class Gammaproteobacteria Garrity et al. 2005; Order Enterobacteriales (monotypic); Family Enterobacteriaceae Rahn 1937; Genus Erwinia Winslow et al. 1920. Recent taxonomic revisions have caused the bacteria Erwinia chrysanthemi to be renamed Dickeya dadantii (Burkholder et al. 1953) Samson et al. 2005. Dickeya dadantii is a gram-negative bacillus that belongs to the family Enterobacteriaceae. It is a close relative of Escherichia coli (Migula 1895) Castellani & Chalmers 1919, and other animal pathogens that include Salmonella, Shigella, Klebsiella, Proteus and Yersinia. Members of this family are facultative anaerobes, able to ferment sugars to lactic acid, have nitrate reductase, but lack oxidases. Even though many clinical pathogens are part of the Enterobacteriaceae family, most members of this family are plant pathogens. Dickeya dadantii is a motile, nonsporing, straight rod-shaped cell with rounded ends. Cells range in size from 0.8 to 3.2 μm by 0.5 to 0.8 μm and are surrounded by numerous flagella (peritrichous).
In the natural plant environment, Dickeya dadantii causes plant diseases such as necrosis, blight and soft rot, which is a progressive tissue maceration. Dickeya dadantii contains many pectinases that are able to macerate and break down the plant cell wall material. This exposed part of the plant releases nutrients that can facilitate bacterial growth. Commonly infected plants include potato tubers, bulbs of vegetables, and ornamental crops.
Dickeya dadantii is phytopathogenic bacterium causing soft rot diseases on many host plants including some which are economically important. Dickeya dadantii, more commonly known as soft rot, brown rot or blackleg, causes characteristic symptoms associated with other bacterial wilts, causing final diagnosis to be difficult. The pathogen primarily seeks to attack the plant's xylem vessels located in leaves, stems, blossoms and storage organs of herbaceous plants. Dickeya dadantii is able to infect hosts at any point in its life cycle. In addition to symptoms of wilt, the disease appears as sunken and cracked external lesions also having a brown interior in cross section in root-tubers (Figure 16, right).
Diseased plants will display a variety of symptoms including wilting, stunting and vascular discoloration of the stems up to blackening (Figure 16, left). Early symptoms include water soaked lesions at the site of infection, gradually expanding chlorotic leaves and loss of turgor in tissues. The intensity of Dickeya dadantii colonization relates to the amount of disease and degree of damage. The pathogen is very successful at infiltrating host tissues due to the many pectinases responsible for diassembly of plant cell wall polysaccharides. Once the cell wall is degraded cellular structure collapses and this cell maceration gives a characteristic water-soaked or rotted appearance.
Dickeya dadantii grow intercellularly, continuing to degrade cells and colonize, until it eventually reaches xylem tissues. Upon reaching the xylem vessels Dickeya dadantii possesses the ability to spread to new regions of the host and other areas may begin to display symptoms. Colonization within the xylem restricts flow of water causing loss of turgor pressure and wilting of foliage and stems. Restricted movement of important plant compounds eventually lead to death of the host.
Dickeya dadantii is a pathogen that is spread through water with the splashing of water from infected plants or recycled irrigation water, insects and cultural practices, such as using contaminated tools and machinery or improper storage of vegetables or seeds with infected substances. Insects are an important vector for movement of the pathogen. Insects are able to carry the bacteria externally and internally and are normally unharmed by the bacteria. However, there is continued research in the area of Dickeya dadantii as an insect pathogen to aphids. The pea aphid is able to contract the pathogen from an infected plant and is destroyed in a mode of action similar to Bacillus thuringiensis Berliner 1915, by producing cyt-like entomotoxins that cause septicemia.
The most important factor to disease development is environmental factors consisting of high humidity and temperatures of 22° to 34 °C. In greenhouses, Dickeya dadantii can survive in potting media with or without a host plant for a year or more and in the leaves of host or nonhost plants for 5 to 6 months.
Dickeya dadantii is a member within the genus that is able to produce the pigment indigoidine (blue pigment the production of which conferred an increased resistance to oxidative stress, indicating that indigoidine may protect the bacteria against the reactive oxygen species generated during the plant defense response).
Rapid identification of this species utilizes this water-insoluble blue pigment appearing in the bacterial colonies as a chemotaxonomic trait. The presence of a soft rot may be an indication of a bacterial disease. However, many other organisms and plant disorders may appear as various soft rot or black lesions. Proper identification is important for treatment and control measures. Thus a differential media is used to culture Dickeya species and isolate or identify i>Dickeya dadantii. Researchers at Fu Jen Catholic University in Taiwan developed a medium that differentiates i>Dickeya dadantii from other species. This NGM medium contains nutrient agar (NA) and glycerol medium supplemented with MnCl2·4H2O. To make this media, mix 23 g of nutrient agar, 10 ml glycerol (1% v/v), and 0.4 g MnCl2·4H2O (2 mM) to 1.0 L of water. Note the pH of this media is 6.5 and it has a light brown base colour. The proper temperature for culturing Dickeya dadantii is 28 °C. A positive result occurs when a bacterial streak produces a brownish blue color on the agar plate. Further isolation and extraction of the indigoidine pigment is possible using the methods described by Chatterejee and Brown (1981).
Currently there are no effective chemical controls for Dickeya dadantii. The most important practices involve lowering the prevalence of disease by proper sanitation of materials, exclusion of infected materials, and avoiding environments conducive to disease. Most important to disease management is exclusion because Dickeya dadantii can move through vegetatively propagated tissues asymptomatically. Therefore, it is important to have certified disease-free stock. Some promising biological control research is being done for orchid species. Proper control of humidity and air movement combined with clean, high quality water, in a temperature and light regulated facility are the most commonly employed methods for disease prevention. Other biological controls of Dickeya dadantii include symbiotic fungi known as mycorrhiza and possibly transgenic proteins. Transfer of sweet pepper genes coding for ferredoxin like protein and defensin can shown to reduce Dickeya dadantii disease in some species under cultivation. Under controlled conditions, plants with mycorrhizal fungi such as Rhizoctonia solani G. Kühn, 1858 and Ceratobasidium sp demonstrated resistance to Dickeya dadantii.
In summary in the control of this bacterial perform large rotations; nitrogen fertilizers balanced; do not use or still water containing organic compounds. With regard to the use of pesticides the copper compounds can contain the disease.
Figure 16 Bacterial soft rot Erwinia chrysanthemi=Dickeya dadantii. Blackening of stem caused by Erwinia chrysanthemi (left); a restricted lesion with a typical dark margin on a storage root (right).
- Streptomyces soil rot named also Pox caused by Streptomyces ipomoeae (Person & Martin 1940) Waksman & Henrici 1948 is a widespread and destructive disease of sweetpotato. The disease has caused significant losses in New Jersey, Maryland, Louisiana, North Carolina, and California for many years. The yield and quality of sweetpotatoes are greatly reduced, and infested fields can remain unproductive. Since the pathogen is soilborne, efforts must be made to prevent introduction and avoid build-up of inoculum in the field. Economic and practical controls for growers in North Carolina are possible by following the recommendations described in this information note.
The symptoms are:
The pathogen is difficult to see in infected tissue under the microscope and is difficult to isolate on culture media in the laboratory. The organism produces spores in spiral chains which are believed to be overwintering structures.
- Large "hot spots" (15X60 m or larger) in the field with groups of infected plants usually indicate initial entry points of the pathogen. The disease is often worse in light, well-drained soils. These areas are often grassy.
- Above ground symptoms on sweetpotato plants include stunting and yellowing of the growing three months in foilage. This condition can be confused with soil infertility, residual soil herbicide injury, drainage problems, or wilt (Fusarium) in wilt-susceptible plants. In severe cases plants may die or produce no yield.
- The tips of fibrous roots are rotted. Numerous black lesions of varying size occur on fibrous roots as well as on the underground stem (Figure 17, A). As the lesions develop, they turn black and have distinct margins.
- Storage roots exhibit sunken lesions that are black and crusty (Figure 17, B). One to several pox lesions can appear in rows and are usually associated with the lateral roots. The affected sweetpotato will be misshapen and roots can be severely constricted or indented in one or more places (Figure 17, C). Pox lesions can be confused with circular spot caused by Sclerotium rolfsii. Circular spot lesions are usually not associated with lateral roots and tends to be light brown, round with distinct borders, and bitter to the taste. With both diseases the lesions do not enlarge following harvest.
Disease development is favored by dry soils and increases as the soil pH rises above 5.2. The pathogen can survive many years in the soil, however, the disease potential decreases over the years in the absence of sweetpotatoes, especially in acid soils. The pathogen can infect a number of weeds in the morning glory family. Infection is thought to occur through small lateral roots when soil temperature is above 68F (68 to 108F). Mature storage roots are resistant to direct infection.
The pathogen is spread by movement of contaminated transplants, storage roots, soil, boxes, vehicles, etc. Spread by wind and water erosion is possible. Disease spread does not normally occur on stored sweetpotato roots. However, the pathogen can survive in infected storage roots and infest new fields if these roots are used for seed.
Growers can manage Streptomyces soil rot if integrated control measures are used. These measures are directed at preventing spread to new fields and reducing disease potential in fields where the disease has occurred.
When developing a rotation program, sweetpotato should not follow a crop requiring a high soil pH.
- Select fields where the disease has never occurred. Never plant sweetpotatoes in fields where the disease has recently caused serious losses.
- Select fields with heavier soils that are not especially subject to drought. Early season irrigation may prevent infection of the fibrous roots, reduce disease incidence, and increase yields.
- Prevent spread of the pathogen into new fields by using only certified disease-free storage roots to produce disease-free plants. In this operation, use only non-contaminated equipment, boxes and vehicles. Avoid moving equipment that may carry contaminated soil from infested fields to new fields.
- Consider growing a resistant cultivar. Cultivars "Jasper" and "Beauregard" are resistant to the Streptomyces soil rot. "Beauregard" is susceptible to root-knot nematode.
- Do not lime infested fields until you have sent a soil sample for fertilizer and lime recommendations to specialized Laboratory. Therefore, select fields with a pH of 5.2 or lower to reduce disease.
- Develop a long-range, crop-rotation, soil-management program in infested fields to assure a low disease potential. For example:
- year: check soil pH, lime if needed, and plant soybean;
- year: plant corn;
- year: replant corn;
- year: plant tobacco, and
- year: check soil pH and if at pH 5.2 or under;
- year: fumigate soil with chloropicrin (390 kg/ha) or Telon C-17 (150-250 L/ha) and plant sweetpotatoes.
Treat the soil with a broad spectrum soil fumigant containing chloropicrin 2-4 weeks prior to planting. Fumigants must be used as stated on the label. Proper soil preparation, soil moisture, temperature and depth of application are essential. The materials are injected 15 to 25 cm deep with one chisel per row, and the bed is prepared simultaneously. At planting, cleanly remove two inches from top of the bed and then plant. Do not plant if fumigant residues remain in the soil. Do not rework the beds prior to planting.
Keep in mind that soil fumigation may not give satisfactory control in severely infested fields.
Figure 17 Streptomyces soil rot (pox) caused by Streptomyces ipomoeae. Rootlet rot caused by Streptomyces ipomoeae (A). Misshapen roots due to early infection (B). Lesions with cracked surface, and wound periderm forming, giving a partially healed appearance (C).
- Bacterial wilt caused by Pseudomonas solanacearum (Smith 1896) Smith 1914 (=Ralstonia solanacearum (Smith 1896) Yabuuchi et al., 1996. Taxonomy: Natura; Mundus Plinius; Biota; Domain Bacteria" (Haeckel 1894) C.R. Woese et al., 1990; Phylum Proteobacteria Garrity et al., 2005; Class Betaproteobacteria Garrity et al., 2006; Order Burkholderiales Garrity et al., 2006; Family Burkholderiaceae Garrity et al., 2006; Genus Ralstonia Yabuuchi et al., 1996).
Bacterial wilt can cause severe yield reduction, in the range 30-80%. However, it is geographically restricted.
Geographical distribution: the causal organism is globally distributed as a serious pathogen causing bacterial wilt in a wide range of crops. However, the strain which causes bacterial wilt of sweetpotato has only been recorded in some parts of China. It is the subject of quarantine regulations restricting sweetpotato and stock movement from the affected regions. The sweetpotato strain of Ralstonia solanacearum also infects other crops such as potato, tomato, capsicum and peanut. It is possible that this disease may spread by movement of other crop species. For example, seed potatoes are commonly exported from China to Vietnam, and often carry bacterial wilt. Any suspected case of bacterial wilt in areas exposed to Chinese seed materials should be reported to plant protection authorities.
Symptoms: sprouts from infected mother roots become wilty and the base of the stem shows progressive degeneration: initially the base of the stem becomes water soaked before turning yellowish and brown. The vascular bundles turn brown, and this symptom extends upwards. Infected sprouts usually fail to develop roots after transplanting. Healthy sprouts or stem cuttings may become infected in the field. As for sprouts, the base of the stem becomes watersoaked, then yellowish brown. Brown streaks in the stem (in vascular tissue) may develop resembling Fusarium Wilt. The plants may appear wilted, but may recover as their root system develops. They may appear stunted and hungry, with older leaves turning yellow. Fibrous roots may have brown or watersoaked patches. Storage roots may show no symptoms, or may develop yellowish brown longitudinal streaks (Figure 18, A). In more severe infection, greyish watersoaked lesions may develop on the surface, and the storage root proceeds to decay with a distinctive odour (Figure 18, A).
Morphology: the Pseudomonads are gram-negative, flagellated, aerobic bacteria. Within Ralstonia solonacearum, the sweetpotato-infecting strain has been placed in Race 1, Group 2 on the basis of host range, being pathogenic on peanut and pepper but not on tobacco. It is grouped in Biotype IV on the basis of physiological and biochemical tests. Strains in this group are unable to metabolize lactose, maltose or cellobiose, and were sensitive to the antibiotic oleandomycin.
Biology and ecology: the bacterium may be transferred either in soil or in plant material. A field may become infected through infected planting material, or via irrigation water or composts containing infested plants. Development of the disease is favoured in warm humid weather. It is also more severe on poorly drained clay loam soils than on sandy soils. However, it does not survive well in flooded conditions, such as in rice paddy. It prefers soils that are slightly acidic.
Host range: the sweetpotato strain of Ralstonia solanacearum has been found to infect a number of other solanaceous crops, icluding tomato, potato, eggplant and capsicum, as well as peanut. Cereals are non-hosts and appropriate break crops in rotation with sweetpotato.
- Rotation with non-host crops is the most important measure to reduce infestation on affected fields. The pathogen may survive for over 3 years in upland fields, but is greatly reduced after 1 year in flooded paddy.
- Planting material should be sourced from disease-free crops.
- Cultivars differ in their susceptibility, although no immunity has been found.
- Soil amendments (lime) to reduce soil acidity may help reduce the severity of disease.
- Establishing the crop in the cooler months may reduce infection, which mosty occurs through wounded tissue immediately after transplanting.
- In China, quarantine regulations restrict the movement of sweetpotato and livestock from infested areas.
Figure 18 Bacterial wilt caused by Pseudomonas solanacearum=Ralstonia solanacearum: Bacterial wilt symptoms include wilting and darkening of vascular tissue in the stem and storage roots (A). Crown gall caused by Agrobacterium tumefaciens:
- Crown gall caused by Agrobacterium tumefaciens (Smith & Townsend 1907) Conn 1942; the synonym is Rhizobium radiobacter (Beijerinck & van Delden 1902) Young et al., 2001).
Symptoms: main symptoms are knobbly swollen galls on stems and roots (Figure 18, B). Crown gall is identified by overgrowths appearing as galls on roots and at the base or "crown" of plants. Crown gall is caused by Agrobacterium tumefaciens, bacilliform bacterium that is normally associated with the roots of many different plants in the field. This bacterium can survive in the free-living state in many soils with good aeration such as sandy loams where crown gall diseased plants have grown. The bacterium can also survive on the surface of roots (rhizoplane) of many weeds.
Pathogen biology: the crown gall pathogen Agrobacterium tumefaciens inhabits the soil where it can survive for long periods. Not all strains of this fungus are pathogenic (capable of causing disease) and there are several different pathogenic strains with differing host ranges. The bacteria enter stems or roots through wounds. The bacterial DNA combines with the DNA of the plant host cell where it "transforms" the cell, causing it to become tumour-forming (galls) and also to produce specific new materials on which the bacteria feed. As the gall grows, the plant tissues become disorganised and normal transport processes are disrupted. In herbaceous plants, the gall rots and the bacterial cells return to the soil. On woody plants, the galls are woody and perennial and do not rot away.
Agrobacterium tumefaciens is a rhizoplane bacterium whose characteristics are Gram-negative, strictly aerobic, bacilliform rods measuring 1x3 µm, and whose nutritional requirements are non-fastidious. The rods bear flagella that are arranged subpolarly around the cylindrical circumference of the cell, referred as circumthecal flagellation (Figure 18, C). When Agrobacterium tumefaciens cells perceive plant phenolic compounds, the virulence genes that are located in the resident Ti (tumor-inducing) plasmid are expressed, resulting in the formation of a long flexuous filament called the T pilus. The activation of VirA also shuts off motility of the circumthecal flagella, presumably when Agrobacterium tumefaciens cells attach to plant cells. Attachment to the plant cells is a prerequisite for initiating the transfer of the T-DNA into the plant cell. Both the circumthecal flagella and the T pilus play an essential role in virulence, presumably by bringing the bacterial cell to its target followed by attachment to the plant host, respectively.
Over the past three decades, the advancement in genetic engineering of plants with desired traits have relied heavily on the various strains of Agrobacterium tumefaciens. However, many economically important plant species, and elite cultivars, are highly recalcitrant to Agrobacterium tumefaciens-mediated transformation. Agrobacterium tumefaciens strain K599 is highly infective in a broad range of plant species including legumes (soybean, alfalfa, Medicago) and transformation-recalcitrant crops including sweet potato and Brassica, thus making it a particularly useful strain for transformation purposes. With the use of homologous recombination approach, we have generated a disarmed strain of Agrobacterium tumefaciens strain K599. This disarmed strain is missing the T-DNA encoded region and border sequences of the K599 Ri-plasmid and is completely free of any hairy root inducing properties. It is fully capable of transient and stable transformation of various plant species and posses several appealing characteristics, including higher transformation efficiencies, lower incidence of multiple T-DNA insertions and fewer vector backbone insertions. This disarmed stain of Agrobacterium tumefaciens is an excellent alternative to A. tumefaciens for the genetic engineering of plants.
Transformed sweet potato plants were obtained from embryogenic calli following Agrobacterium tumefaciens-mediated transformation. Agrobacterium tumefaciens strain EHA101/pIG121-Hm used in the present study contained a binary vector with genes for β-glucuronidase (gusA) and hygromycin resistance (hpt). Around ten hygromycin resistant cell clusters were produced from 1g fresh weight of the infected embryogenic calli. The hygromycin resistant plantlets were regenerated from 53.1% of the hygromycin-resistant calli. Histochemical GUS assay and Southern hybridization analysis indicated that these plants were stably transformed with a copy number of introduced genes of one to three. Transgenic plants grew normally and formed storage roots after 3 months of cultivation in a green house.
Control: if crown gall is detected, lift and destroy affected plants. Grow crops of potatoes or other vegetables (except beetroot, which are also susceptible) over the next one or two years to help eliminate the bacteria from the soil, or grass the area over for one or more years. There are no chemicals available for the control of crown gall.
- Ceratocystis fimbriata Ellis & Halst., 1890 is a fungus and a plant pathogen, attacking such plants as the sweet potato (black rot). This fungus belongs to Natura, Mundus Plinius, Naturalia, Biota, Domain Eukaryota Chatton, 1925; Amorphea Adl et al., 2012; Opisthokonta Cavalier-Smith, 1987; Holomycota Liu et al., 2009; Kingdom Fungi R.T. Moore (1980); Subkingdom Dikarya D.S. Hibbett et al., in D.S. Hibbett et al, 2007; Phylum Ascomycota Caval.-Sm. (1998); Subphylum Pezizomycotina O.E. Erikss. & Winka (1997); Class Sordariomycetes O.E. Erikss. & Winka (1997); Subclass Hypocreomycetidae O.E. Eriksson & K. Winka, 1997; Order Microascales Luttr., 1951 ex Benny & Kimbr., 1980; Genus Ceratocystis Ellis & Halst. 1890.
Ceratocystis fimbriata, the type species of the genus Ceratocystis, was originally described on the sweet potato in 1890. It has since been found on a wide variety of annual and perennial plants. It is a large, diverse complex of species that cause wilt-type diseases of many economically important plants. There are thought to be three broad geographic clades, the North American, the Latin American and the Asian clades.
The Ipomoea form of the fungus which attacks the sweet potato, is thought likely to be native to Latin America. It has spread to many locations probably on storage roots (Figure 19, left). The fungus may appear as a dry, black rot, usually with perithecia and ascospores (Figure 19, right). In some countries (such as China and Japan) it is an important constraint to sweet potato production. In other areas (such as south-eastern USA) the damage is less severe due to the use of resistant varieties and sanitary measures. Fungicides can be used in sweet potato fields or as post-harvest dips of sweet potato roots.
The symptoms are small, circular, slightly sunken, dark brown spots are the initial symptoms of black rot. Spots enlarge and appear greenish
black to black when wet and grayish black when dry. Inside the spots are small, black fungal structures (perithecia) with long necks,
which appear to the naked eye as dark bristles. The rot usually remains firm and shallow. However, if secondary fungi or bacteria invade
the tissue, the flesh beneath the spot turns black and may extend to the center of the root. Tissue near the discolored area may have a bitter
taste. Eventually, the entire root may rot. Roots that appear healthy at harvest can rot in storage, during transit, or at the market.
With regard to persistence and transmission, the fungus survives in the soil in crop debris. Infected storage roots escape detection at harvest or bedding.
The fungus either colonizes the young shoots or infects the stem. Transplants are thereby infected and, subsequently, so are the main stem and daughter roots. When slips are pulled for transplanting, the stem carries the pathogen along with the plant.
The black rot fungus can produce tremendous numbers of spores during storage. These spores can contaminate washing machines, crates, and structures as
well as the hands of workers.
Contaminated items or dip tanks can serve as sources of fungal inoculum for new infections.
Washing and packing roots infected with black rot before curing or using contaminated equipment may spread the disease.
Black rot may develop on sweet potatoes during transit or in the marketplace. Entire lots may become infected as the fungus spreads quickly to roots surrounding
a rotting sweet potato. As a result, entire crates of roots may be quickly destroyed in storage.
Insects, such as the sweet potato weevil, also transmit the disease in storage. Development and spread of the disease are rapid at temperatures greater than storage temperatures (13 to 16 °C).
The control of black rot of sweet potatoes can be can be done by the following strategies:
- Rotate sweet potatoes with other crops since most other crops are unaffected by the disease.
- Disinfect seed beds if a clean site is unavailable.
- Propagate plants from healthy stem cuttings.
- Begin curing roots immediately after harvest. Cure at 29 to 35 °C and 85 to 90 percent relative humidity for 5 to 10 days.
- Apply a postharvest fungicide dip.
- Do not wash and package roots showing symptoms of black rot.
- Decontaminate equipment, including washing machines, storage crates, and structures, that comes into contact with an infected crop. Spray empty washing machines and crates with a fungicide. Fumigate storage structures.
Figure 19 Symptoms caused by Ceratocystis fimbriata on root-tuber (left); spherical perithecia with long neck, from which emerge, at maturity, the ascospores which are contained in asci, themselves contained in the ampolla of the fruiting body (right).
- Sclerotial Blight caused by Sclerotium rolfsii Sacc., 1911 is a soilborne disease that causes southern blight on a wide variety of plants. It also causes two distinctly different diseases on sweet potatoes, sclerotial blight and circular spot. These diseases differ both in terms of symptoms and the stage of plant production when they occur.
Sclerotial blight is one of the most common diseases of sweet potatoes in plant beds. This disease also called southern blight, southern stem rot and bed rot develops when the pathogen invades the seed roots and the developing sprouts. On the seed roots it causes a soft rot, and when it invades the developing sprouts at the point they emerge from the seed root it causes them to wilt and die. Under humid conditions, the white fungal mycelium can be seen developing from the base of the infected sprouts. The yellow to brown mustard seedlike sclerotia that serve as survival structures are then produced on the mycelium. The disease generally develops as circular areas of affected plants that may be scattered throughout the seed bed (Figure 20).
Disease development generally occurs during periods of moisture and high humidity especially when temperatures are warm and plants are stressed. Cultivars vary in their susceptibility to this disease, but even Beauregard, which is considered resistant, succumbs when stressed.
Sclerotial blight is managed using several practices. Use only disease-free roots for seed. Choose only cultivars with some resistance to the disease and avoid those that are known to be extremely susceptible. Choose a well-drained site for the plant bed that does not have a history of the disease or has not been planted with sweet potatoes (or other susceptible crops) for at least 3-4 years. Soil fumigation with metam-sodium, Vorlex, methyl bromide or chloropicrin will also aid in reducing inoculum. Treat the seed roots with a fungicide, such as dichloran, prior to or at planting with sufficient water to assure good coverage of the root surface. If the beds are covered with plastic mulch, punch holes in it to allow adequate ventilation and prevent the buildup of excessive moisture.
Circular spot typically develops on storage roots in the field shortly before harvest. Lesions on the roots are circular (1-2 cm in diameter) and are generally very shallow (1-4 mm). The surface of the lesion is dry and brown whereas the decayed tissue below it is yellowish brown. In most instances, lesion development stops as soon as the roots are harvested. In storage, the necrotic surface tissue of the lesion separates from the root following the formation of an abscission zone underneath it.
The occurrence of circular spot is very sporadic, and some cultivars are more susceptible than others. What factors affect incidence of circular spot are not fully known the development of circular spot is often associated with prolonged periods of flooding in the field, especially when the soil is warm. Under these conditions, the lesions tend to be much wider and deeper, and soft rot may destroy the entire root. The only practices for managing circular spot are to choose cultivars that are more resistant to the disease and to avoid fields with a history of diseases caused by Sclerotium rolfsii.
Figure 20 Sclerotial Blight caused by Sclerotium rolfsii. Hyphae and mustard seed sclerotia of Sclerotium rolfsii on infected sprouts and partial decay of the seed root due to sclerotial blight (left). Circular spot lesions on a mature sweet potato root.
- Alternaria blight is a disease caused by Alternaria bataticola Ikata ex W. Yamamoto 1960 appeare with leaf spot and stem blight of sweetpotato that have a foliage and stem disease that is very important in Eastern and Central Africa and Brazil. Although no literature has been found on the losses caused by this disease, it has been known to cause important damage due to plant death.
The disease is first observed as small, brown to black oval lesions with a typical bullseye appearance of concentric rings, on leaves, stems and petioles. Blackened veins are observed on leaf undersurface. As the disease progresses, the lesions become necrotic usually surrounded by a wide yellow halo; soon after the whole leaf blade turns chlorotic and drops. Bases and middle sections are more affected than the vine terminals. The ground under affected vines is often carpeted with blackened leaf debris.
On petioles and stems, lesions are initially grey, but as the lesions enlarge, they become black and sunken (Fifure 21, A). Ultimately, the petioles and stems are girdled (Figure 21, B) and the plant dies (Figure 21).
The mycelium of Alternaria bataticola is fuscous brown to almost hyaline, septate, and branched. Colonies on agar are gray-green. Conidiophores are single or in bundles, unbranched, erect or slightly curved, 2-7 septate, pale brown to fuscous-brown, 47-80 mm long and 7 mm large. Conidia are in chain (Figure 21, C) or free (Figure 21, D), elongate, obclavate muriform, transversely 5-12 septate, longitudinally 0-8 septate, pale to fuscous-brown, smooth walled, conidium body. The conidial beak is long, filiform, colourless to pale brown, septate, often branched once and occasionally up to three times and measures 16-128x3-6 mm.
About the biology and ecology, the fungus survives in soil and plant debris. The air-borne conidia are spread through infected planting material by wind, splashing rain, and water.
High relative humidity or free water is necessary for infection and sporulation, conditions common in tropical regions due to continuous rain. The disease and lesions increase with altitude as observed in Africa. During dry conditions the lesions get a silvery tone.
In other species of Alternaria the presence of different toxins have been reported such as alternaric acid, which might be the reason why leaves drop.
The disease, as well as the causal agent, have been described only few years ago, in Africa (Ethiopia and Burundi) and South America (Brazil).
Sweetpotato is the only known host.
The diagnosis of this disease can be differentiated from other leaf spots and blights because of its severity. There are no other foliage diseases, with the exception of leaf and stem scab (Elsinoe batatas) , which can be so destructive.
There are other Alternaria species that also cause leaf blight but they only attack the basal mature leaves.
The cultural control consists in the use of clean planting materials.
The use of resistant host assumes considerable importance and it has been suggested that red-skinned are more resistant than white-skinned varieties (Table 3).
For chemical control although no records on the use of fungicides have been found, dyrene and mancozeb are specific fungicides to control Alternari spp. in other crops and can probably be effective against Alternaria bataticola.
Figure 21 Alternaria blight on leaves, petioles and stems of sweetpotato (A). Black sunken lesions on stems (B). Conidiophore and conidial of Alternaria bataticola; observe the characteristic muriforme wall of the conidia, their variability in size and shape, their aggregation ranging from catenulate (C) to free (D).
- Cercospora leaf spot caused by Cercospora ipomoeae G. Wint. Synonyms: Cercospora bataticola Ciferri & Bruner (1931) and Phaeoisariopsis bataticola (Cif. & Bruner) M.B. Ellis (1976).
Geographical distribution: the disease is commonly found throughout the tropics, mainly in the Caribbean and South and Central America such as in Antigua y Barbuda, Cuba, Dominican Republic, Costa Rica, French West Indies, Guyana, Panama, Peru, Puerto Rico and Venezuela. It has also been noted in India, Japan, Italia, Netherlands, Antilles and USA.
Morphology: the conidiophores form loose synnemata. They are olivaceous brown, smooth, up to 150 µm long, 2-3 µm thick at the base, and 5-7 µm near the apex. The conidia are cylindrical to obclavate, conico-truncate at the base, pale olivaceous brown, with 4-8 septate, are 55-120 µm long and 4-8 µm thick in the broadest part.
Symptoms: light green spots, approximately 0.5 mm in diameter, initially appear on both surfaces of the leaf. These lesions gradually increase in size and turn brown, with yellow green border advancing at the periphery of the lesion. About two weeks after the first appearance of the symptoms, the lesions further increase to about 3-4 mm in diameter. Two different zones are observed: the centre which is pale brown to whitish grey and the border which is brown to purple black (Figure 22, A). Although the veins limit the lesions, those which are close to each other coalesce and form blighted areas on the leaf. Severely blighted leaves turn yellow leading to defoliation. These symptoms can be found on leaves of different ages throughout the plant, although more spots are found on older leaves.
Morphology: fruiting structures of the fungus, consisting of dark conidiophores bearing conidia, can be observed at the centre of the spots, especially after rainfall. Differences in size, shape and colour of the spots are found. This may be due to the presence of other species of Cercospora attacking sweetpotato such as Cercospora timorensis, and Cercospora batatae.
Biology and ecology: the disease is dispersed by wind or splashing rain. It is most prevalent in the hot and humid tropics and is seldom observed during the dry season.
Host range: the primary host of Cercospora bataticola is Ipomoea batatas. Although no other hosts have been reported it is suggested that this pathogen can overwinter on some weed species.
Diagnosis: the peculiar appearance of the spots with very sharp differences between the centre and the border is one way to recognize the disease in the field. Another way of identifying the disease is to observe the fruiting structures under a compound microscope. For this, it is advisable to collect the samples from the field, early in the morning and to scrape the surface of the spot to observe the conidiophores and conidia.
The management of the disease is based on the cultural control. Control is not usually needed. However, it is suggested that only healthy material should be used for planting.
About the host-pathogen resistance, differences in susceptibility have been found, but these differences could be due to the presence of other Cercospora species.
Figure 22 Cercospora leaf spot caused by Cercospora ipomoeae: leaf lesions with light centres and dark margins (A). Charcoal rot caused by Macrophomina phaseolina: Storage roots mummified (named "philrootcrops") with charcoal rot (B).
- Charcoal rot, Macrophomina phaseolina (Tassi) Goidanich (1947). Synonyms: Macrophoma phaseoli (Maubl. 1905) Ashby 1927; Macrophomina philippines Petr. 1923; Macrophoma corchori Sawada 1916; Macrophoma cajani Syd. P. Syd. & E.J. Butler 1916; Sclerotium bataticola Taub. (1913); Rhizoctonia bataticola (Taub.) Butl. 1927 (The last two names represent the anamorph).
Economic importance: charcoal rot is a minor disease that can damage sweetpotato roots in storage. Losses, however, are seldom serious.
Geographical distribution: the fungus is widespread in tropical and subtropical regions of the world.
Symptoms: the disease only normally affects fleshy roots in storage. However, stem may sometimes show lesions at the soil line during heat stress. Once the fungus enters the root, the disease initially develops in the cortex, then crosses the vascular tissue and finally decays the entire root. Affected fleshy roots dry up, become spongy and hard, but the periderm (skin) remains intact over the decayed tissue. Two zones can be differentiated from a cross section of an infected root: the outer zone which is black due to the presence of mature sclerotia, and the inner zone where the tissue is reddish brown and is in the process of decaying (Figure 22, B).
Morphology: only the anamorph with mycelium and sclerotia has been found in sweetpotato. The later is formed underneath the skin of fleshy roots. Sclerotia are black, smooth, hard, from very small up to 1mm in diameter.
Biology and ecology: the fungus is a saprophyte and survives in the soil for several years as free sclerotia. The pathogen invades the root tissues through wounds by producing black sclerotia confined in the cortex and rot that can consume the entire root. The disease spreads from one field to another by irrigation water, animals, and farm implements. It becomes severe at high temperatures (35-39 °C) and high moisture.
Antagonists: Aspergillus niger, Trichoderma terricola and Trichoderma viride.
Host range: sweetpotato is considered a primary host. However, it has a wide range of hosts and is parasitic to tropical and subtropical crops.
Diagnosis: sclerotia provide the most important diagnostic feature of the disease in storage. These black structures on the infected tissues have a gritty or sandy-like feel and are not present in storage roots infected with Java black rot and black rot, the two storage diseases that charcoal rot can be being confused with.
The management of the disease consists in cultural control:
About the host-plant resistance, certain levels of resistance have been observed in different cultivars.
- Use of clean and disinfected storage facilities and containers.
- Avoidance of leaving harvested fleshy roots under the scalding sun after harvest.
- Avoidance of wounding the roots during harvest and handling.
- Cure of roots before storage. Storage roots should be cured at 28-32°C and 90-95% RH for 10-14 days depending on the cultivar.
- Maintenance of storage temperature at 15-16 °C. High temperature should be avoid ed in storage.
- Dry rot caused by teleomorph (sexual reproductive stage ) Diaporthe phaseolorum Cooke & Ellis) Sacc., 1882, anamorph (asexual reproductive stage) Phomopsis phaseoli (Desmaz.) Sacc., (1915); synonym Diaporthe batatatis Harter & E.C. Field.
Occasionally it is found in plant beds and fields. The firm, dry rot generally progresses from one end and causes the sweetpotato to shrink or wrinkle. Affected tissue is light to dark brown externally and dark brown to black internally. Affected roots become mummified. Sprouts from infected seed roots may develop a reddish-brown to black decay at the base.
The presence of these species on herbaceous hosts raises once more the relevance of weeds as reservoirs for pathogens of economically important plants.
Figure 23 Dry rot caused by teleomoph Diaporthe phaseolorum; anamorph Phomopsis phaseoli. Perithecia on stems in culture (A); asci (B); ascospores (C); pycnidia forming on stems in culture (D); α-conidia (E); β-conidia (F); conidiophores (G); affected tissue of root-tuber is light to dark brown externally and dark brown to black internally (H). Perithecia, asci and ascospores belong to Diaporthe phaseolorum; pycnidia, conidiophores and conidia are of Phomopsis phaseoli.
- Foot rot (Plenodomas destruens Harter (1913), synonym Phomopsis destruens (Harter) Boerema, Loer. & Hamers 1996. The disease is of low importance but it can pose a considerable problem when slightly infected storage roots are stored. In Argentina losses in the field of 25 to 75% have been reported in the past.
Geographical distribution: apparently the disease is present mainly in the western hemisphere. It has been reported in Argentina, Brazil, Caribbean, Peru, and USA.
Symptoms: in seedbeds, seedlings show yellowing, especially, of the lower leaves. Later on, plants wilt and die. In the field, plants show a blackening of the vine around soil level, extending upwards and downwards. The lower portion of the stems rot. Disintegration of the root system occurs. Affected stems may be girdled and the plant dies (Figure 24, A). Black pycnidia develop in affected areas of the stem and roots (Figure 24, E). In storage, roots start rotting from the end that has been attached to the mother root (proximal). Affected roots develop a firm, dry, dark brown decay that covers a large portion of the root (Figure 24, B and C).
Morphology: the conidia are one-celled, oblong, sometimes slightly curved, have rounded ends, hyaline, bigutulated, and measure 6-10x3-4 µm. Long, hyaline, sometimes curved stylospores that measure 5-15 µm are also present. The pycnidia are black, round, partially submerged in affected tissue, and form concentrations of more than 2 structures with straight or curved beaks that measure 100-500x80-200 µm (Figure 24, D).
Biology and ecology: very little is known about the biology of the fungus. It has been observed that survival in the field is very poor, so the main way the disease is perpetuated is when infected cuttings are used for planting. The influence of soil composition, moisture, temperature and rainfall has not been studied. In places where storage roots are used as source of planting material, the disease is transmitted from the sprouts and cuttings.
Host range: Ipomoea batatas is up to now the only economically important host. The disease also attacks some other convolvulaceous hosts.
Diagnosis: in the field, the disease can be diagnosed by the blackening of the stem base and the portion a little below the soil level. In storage the presence of the disease can be observed when the periderm of affected fleshy root is pulled off and the black pycnidia are present.
- Cultural control:
- Plant cuttings from healthy plants.
- Two-year soil rotation is recommended.
- Host-plant resistance
- In Brazil the cultivar "Princesa" have been found to show less disease than other popular cultivars.
- Chemical control:
- Thibendazole can be used in disinfecting planting materials.
Figure 24 Foot rot caused by Plenodomas destruens. A: blackening or rotting of the lower portion of the stem. B: Infected root-tuber with periderm peeled back to reveal pycnidia (fungal agamic fruiting bodies). C: rot spreading from stem end. D: conidial shape of Plenodomas destruens: & alpha;-conidia one-celled, hyaline, oblong or oval, with 2-guttulated (left) and & gamma;-conidia clavate to subcylindrical, one end actue or obtuse, the base somewhat truncate, and one side slightly curved (right). E: black stromatic pycnidia erumpent on infected stem.
- Chlorotic leaf distortion caused by Fusarium denticulatum Nirenberg and ODonnell, 1998. Yield of storage roots was not affected in studies in the USA using cultivars that varied from mild to severe symptom development. The potential effect of chlorotic leaf distortion on quality of vines for use as food or feed has not been reported.
Geographical distribution: the disease has been reported from Brazil, Kenya, Peru, and USA. However, the causal fungus was isolated from botanical seed from a number of additional countries. It appears that the disease is widely distributed and it is likely that the disease occurs in many places from which it has not yet been reported.
Symptoms: the youngest 2-3 leaves at the tips of vines become generally chlorotic (Figure 25, A), sometimes becoming bright yellow, almost bleached in appearance . As the leaves mature, they regain most of their normal colour. On cultivars that normally have purple leaves, the leaves may become pink when affected with chlorotic leaf distortion. Following extended periods of conducive weather (sunny and humid), marginal necrosis may develop on affected leaves (Figure, B). Mycelia of the causal fungus may be seen growing out from between halves of leaves that have not yet opened (Fig. B) or as scattered clumps on the upper surface of more mature leaves (Fig. C). Associated with the mycelia may be phialides that produce typical Fusarium macroconidia and microconidia. The latter clumps may appear similar to salt deposits. On some cultivars affected leaves may be twisted or distorted (Fig. D).
Morphology: conidiophores in aerial mycelium are prostrate, short, and sometimes branched. Sporodochial conidiophores are branched (Figure 25, F). Phialides are mostly monophialidic (Figure 25, D) but occasionally polyphialidic (Figure 25, E) and averaged 25.0×3.0 µm. Microconidia are abundant, long, oval to allantoid, and 0 to 1 septate (Figure 25, G). Macroconidia are fusiform to falcate with a beaked apical cell and a footlike basal cell, 3 to 5 septate, and 38 to 45×3.6 to 4.0 µm (Figure 25, C). Chlamydospores were absent. The fungus is identified as Fusarium denticulatum.
Biology and ecology: the relationship between the causal fungus and plant host is most unusual in that the fungus primarily colonizes the surfaces of the growing vine tip without invading the plant. Mycelia are found on apical meristems and between halves of developing leaves that have not yet opened. Once the leaves open and expose the fungal mycelia, the mycelia appear to stop growing. As a result, individual leaves appear to recover as they mature. Symptom development is strongly favoured by warm, sunny, humid weather. It is not known how the fungus overwinters in temperate areas.
Host range: the disease is only known to occur on sweetpotato, although other Convolvulaceous plants can be infected by artificial inoculation.
Diagnosis: the unique symptoms and the presence of fungal mycelia and characteristic Fusarium macroconidia are adequate for diagnosis.
Management: since chlorotic leaf distortion does not appear to affect storage root production, management efforts have not been considered necessary. It is difficult to eliminate the pathogen from planting material, but it has been successfully eliminated by using the same meristem-tip culture techniques that are used for virus elimination.
Figure 25 Chlorotic leaf distortion caused by Fusarium denticulatum. A: chlorotic leaf. B: leaf with marginal necrosis. C: macroconidia of Fusarium denticulatum. D: monophialide. E: polyphialide. F: sporodochial conidiophores. G: monoconidia.
- Fusarium wilt (stem rot) caused by Fusarium oxysporum Schlechtend. (1824) f. sp. batatas (Wollenweb.) W.C. Snyder & H.N. Hans. (1940).
Economic importance: there are no exact figures published on the damage caused by Fusarium wilt. However, planting susceptible varieties were noted to produce more than 50% losses. The availability of resistant varieties can definitely relegate this important disease into a minor problem.
Geographical distribution: Argentina, Brazil, China, Hawaii, India, Indonesia, Japan, Malawi, New Zealand, Peru, Puerto Rico, Taiwan, USA, Uruguay.
Morphology: Fusarium oxysporum f.sp batatas produces a white aerial mycelium and purple pigment characteristic of the species. Erect, hyaline conidiophores are formed successively producing conidia which accumulate into groups. It produces microconidia, macroconidia (Figure 26, C) and chlamydospores (Figure 26, D) forming bud cells in liquid medium. Microconidia are generally one celled, very seldom two celled, hyaline measuring 2-3.5x5-12 µm. Macroconidia are mostly 3-septate but some have 4-5 septa. They are boat shaped to oblong and 3-4x25-45 µm in size (Figure 26, C). Chlamydospores are formed mainly in the inner cells of macroconidia; they are thick walled, with dense protoplasm, spherical with 7-10 µm in diameter (Figure 26, D). Bud-cells are round or oblong in shape, 1-2x1-1.5 µm.
Symptoms: discolouration of the vascular tissues of the stem is an early and diagnostic symptom. Frequently, this is one-sided with only one portion of the vascular ring discoloured. The most obvious symptom of Fusarium wilt in the field is an interveinal yellowing of leaves followed by wilting (Figure 26, A). The older leaves dry out and can drop from the plant or stay hanging from the stems. The disease may develop at any stage of development. Yellowing of leaves is sometimes one sided; this occurs when only a part of the vascular system is invaded by the fungus. Other symptoms include stunting of the vines, cracking or splitting of stem, or premature flowering. The lower stem may appear purplish and the cortex can rupture and expose brown to black affected tissue (Figure 26, B). The surface of the vine killed by Fusarium wilt has a pinkish extramatricial growth, numerous macroconidia and microconidia. Veinal blackening may extend to at least the proximal end of the storage roots which may appear normal unless cut vertically. It may also cause fibrous root necrosis.
Fusarium solani and Fusarium moniliforme have also been isolated from sweetpotato storage roots infected with Fusarium oxysporum f.sp batatas.
Biology and ecology: Fusarium wilt is a disease mostly present in subtropical regions of the world. The optimum temperature for infection is around 30°C, but the disease can develop at lower temperature and across a wide range of soil moisture- from 28 to 75%. However, the highest damage occurs in fields where the moisture is low. The fungus is soil-borne and can persist in the soil for many years. Infection is usually through vascular wounds such as those obtained when procuring cuttings for planting or when leaves are detached from the stems. The fungus, however, cannot penetrate the callus that has grown over the wounds. The disease can affect vines at any stage of development, but when infected transplants (from mother roots) are used for propagation, plants die at an early stage. Once the soil has been infested, the infection persists in plant refuse, because the fungus produces resistant structures - the chlamydospores - that can survive in the soil for several years. When sweetpotatoes are harvested mechanically, the possibilities of infection are higher, because of wounding. Chlamydospores germinate when enough moisture is present in the soil. They produce a germ tube that enters the root through natural openings, or those produced by nematodes and natural wounds due to root growth. Germination of chlamydospores can be impaired in certain soils, due to the presence of antagonistic microorganisms, alkalinity, and deficient moisture. Disease transmission is through infected plant material and through contaminated soil. The disease can be initiated in a field when infected cuttings are used as planting material. Irrigation water, human movement and use of implements previously used on an infected crop may also cause the spread of the disease. Sometimes the disease is just found in patches across the field.
Host range: the primary host is sweetpotato but the fungus also attacks several Ipomoea species, and a number of other Convolvulaceae. Experimentally, Fusarium oxysporum f.sp. nicotianae, the cause of Fusarium wilt in tobacco was found to cause wilt in sweetpotato and conversely, Fusarium oxysporum f.sp batatas caused wilt in susceptible tobacco.
Detection and inspection: an accurate and easy way to detect the disease in the field is to obtain a transverse section of the stem near the soil surface. Brown to purple discolouration of the vascular system will confirm the presence of Fusarium oxysporum f.sp batatas as the disease develops in the vessels of stems and roots. The same discolouration will be observed in a cross section of the proximal end of a storage root attached to an infected plant. In the laboratory, inspection of thin sections of affected tissue under a compound microscope will show the presence of macroconidia, microconidia or chlamydospores.
- Host-plant resistance: the development of resistant varieties is the most important control measure, and has turned the disease from a major threat to a minor problem in many areas. There are many resistant varieties from breeding programs now available in several countries such as USA, China and Japan. Early examples are "Goldrush" and "Tinian", the latter being widely used in breeding backcrosses that produce moderately resistant F1 material with field-immune progeny. Many additional cultivars with high levels of resistance have been developed since. In Argentina the varieties "Tucumana lisa" and "Brasilera blanca" were also found resistant to the disease (Table 3).
- Cultural control:
- Use of certified healthy planting material. If cuttings would be obtained from sprouts, cut 5 cm above the soil line. Shoots pulled from the mother roots should not be used. Storage roots used for propagation, should come from healthy plants.
- Solarization under plastic kills the mycelium and conidia up to 20 cm deep.
- Crop rotation with non-host plants.
- Proper handling of storage roots after harvest to avoid surface rot during storage.
- Chemical control: fungicides such as carbendazim, benomyl, and thiabendazole are generally used as dip treatments for propagation material.
Figure 26 Fusarium wilt (stem rot) caused by Fusarium oxysporum f. sp. batatas. A: Interveinal yellowing of leaves followed by wilting. B: stem is purplish with brown to black affected tissue. C: macroconidia of Fusarium oxysporum f.sp batatas. D: chlamydospores.
- Fusarium root rot and stem canker caused by Fusarium solani (Mart.) Sacc., 1881. Fusarium oxysporum has been recorded on sweepotatoes in the USA, while Fusarium solani is reported from the USA and China. Both funfi are common throughout the world.
Symptoms: the type of decay is rather variable. End rot, caused by either species, is characterised by a dry decay, at one or both ends of the fleshy root, the lesions being brown with a dark margin. Infected tissue shrivels, somethimes forming cavities filled with mould. Surface rot, again caused by either species, consists of pale brown circular lesions, often centred on a broken rootlet. Decay remains shallow, but the lesions constitute a disfiguring blemish. Fusarium solani can also cause an invasive rot, starting at wounds and eventually affecting the entire root (Figure 27).
Figure 27 Fusarium root rot and stem canker caused by Fusarium solani. Type of decay rather variable.
- Gray mold rot caused by Botrytis cinerea Pers. 1794; teleomorph Botryotinia fuckeliana (de Bary) Whetzel, (1945). Gray mold rot seems to occur only in sweetpotato weakened by chilling injury. It has been recorded on importations from Puerto Rico into the USA, and Jamaica into UK, and the author has observed it in consigments shipped in refrigerated containers (at 8° to 10° C) from Brasil to the UK. Depending on conditions, infected tissue may be soft and wet or dry and crumbly.
Figure 28 Gray mold rot caused by Botrytis cinerea, anemorph and Botryotinia fuckeliana, teleomorph. Sections of an infect root-tuber.
- Mottle necrosis caused by different pathogens such as Pythium Pringsh. (1858) spp.; Pythium scleroteichum Drechsler (1934); Pythium ultimum Trow (1901); Phytophthora de Bary (1876) spp. Externel symptoms on root-tubers consisted of finger-shaped, slightly sunken brown spots. When affected root-tubers were sliced, islands and channel soft dry, crumbly, necrotic tissues formed an interconnected, labyrinth. The extent of internal decay was unrelated to the development of surface lesions, that is, small lesions were often associated with extensive decay inside the root-tuber (Figure 29, A and B). In the infected tissue, non septate hyphae were distinctive, but oogonia and hyphal swellings were not found (Figure 29). The disease was found prior to or just after harvest, and it did not develop during storage.
Figure 29 Symptoms and pathogen morphology of mottle necrosis of sweetpotato caused by Pythium scleroteichum. A: Sunken brown spots of mature root-tubers. B: Cross section of an infected mature root-tuber grown in a naturally infected soil. C, D: Terminal oogonium with monoclinus antheridium of Pythium scleroteichum isolate. E, F: Intercalary oogonium and aplerotic oospore with diclinous antheridium. G: Terminal abortive oogonium with monoclinous antheridium; one of the antheriridial stalks has transverse indentations (arrowheads). H: Intercalary, spherical hyphal swelling produced intercalaryby isolate. I: Appressoria. Bars=10 µm. Pythium scleroteichum had primary hyphae up to 4-8 µm. Oogonia were mostly terminal, occasionally intercalary and 17.2-30.5 µm (mean 24.7 µm) in diameter.
- Phyllosticta leaf blight or Phomopsis leaf spot that is caused by Phomopsis ipomoeae-batatas Punithalingam (1982)=Phyllosticta batatas (Thüm.) Sacc. (1884). The disease is considered to be of minor importance, because it is only present in mature leaves and toward the end of the growing season.
Geographical distribution: probably throughout the humid zones worldwide. Has been reported in Africa (Ethiopia, Ghana, Guinea, Nigeria, Sierra Leone, Sudan, Tanzania, Uganda, Zambia, Zimbabwe), Asia (China, Hong Kong, Japan), Caribbean (Bermuda, Cuba, Jamaica), Pacific Islands (Papua New Guinea), North America (USA), and South America (Argentina, Brazil, Venezuela).
Symptoms: the most obvious symptom is observed in older leaves as whitish to tan brown lesions surrounded by a dark purple to brown margin (Figure 30, left). These lesions are roundish with irregular borders and measure 5-10 mm wide (Figure 30, centre). The centre of the mature lesions have few pinhead-like black to brown structures clearly visible to the naked eye; they are pycnidia at different stages of development (Figure 30, right). Many lesions can coalesce and cover a great portion of the leaf. Usually the lesions are more prominent on the upper surface of leaves. The disease is restricted to the leaves and has not been found in any other part of the plant. However, for some time it has been thought to also attack the fleshy roots in the field and in storage but damage on roots has been caused by Phomopsis batatae, a fungus of the same group that is found to cause root dry rot. It has some resemblance to Phomopsis ipomoea-batatae, but produces B instead of A conidia.
Morphology: the fungus produces black to dark brown, usually solitary pycnidia 120-180 µm wide with septate, occasionally branched conidiophores. The fungi in this group usually produce two kinds of conidia, the so called A and B conidia. However this particular species produces only A conidia that are unicellular, hyaline, oblong or ovoid, and measure 4-8x2.5-3.5 µm. B conidia have not been found.
Biology and ecology: very little is known about the biology of the causal agent. It has been suggested that the fungus overwinters in affected leaves and when the field receives moisture, the pycnidia swell and liberate conidia that become the primary infection agents for the new crop. No environmental factor (eg. temperature and relative humidity) has been reported to favour the development of the disease. It is noted, however, that the disease starts late in the season, and develops mainly in the mature leaves. This phenomenon is very common with fungi that affect the foliage.
Host range: no other hosts of Phomopsis ipomoea-batatae are known.
Diagnosis: a diagnostic feature is the presence of the pathogen's visible fruiting bodies at the centre of lesions in mature leaves. It is easy to recognize these fruiting bodies (pycnidia) and conidia, especially under the microscope. Pycnidia are brown to black structures, with an opening through which the conidia are discharged. Conidia are oblong with two refringent spots near both ends.
Cultural control: although no control measures are mentioned in the literature, sanitation should be practised, since the fungus overwinters in affected crop debris that are left in the field.
Host-plant resistance: resistance to the disease has not been observed.
Chemical control: chemical control is not normally necessary.
Figure 30 Phyllosticta leaf blight=Phomopsis leaf spot caused by Phyllosticta batatas=Phomopsis ipomoeae-batatas (left). Whitish leaf spots (centre). Brown to black pin-head structures (pycnidia) at centre of the lesions (rigth).
- White-rust caused by Albugo ipomoeae-panduratae (Schwein.) Swingle, (1891). Synonyms: Aecidium ipomoeae-panduranae Schwein., 1822; Aecidium ipomoeae Schwein., 1874; Cystopus ipomoeae-panduratae (Schwein.) Stev. & Swing., 1889; Puccinia ipomoeae-panduratae (Schwein.) P. Syd. & Syd. (1904). White rust is a minor disease of sweetpotato, present only on certain cultivars. There are no records about the importance of the disease on yield.
Geographical distribution: Widespread on Ipomoea and Convolvulus spp. in tropical and warmer temperate regions. Main areas incluide North America (USA), tropical and South America (most countries); Africa (Moroco, Sudan and countries south of Sahara); S.W. Europe (S. France, Italy, Malta); S.W. USSR (Azerbaijan, Turkmenistan, Uzbekistan) and Asia (India, Pakistan and Far East); Australia (Queensland) and Pacific. Apparently not recorded in S.E. Europe, nor in Middle Eastern countries from Egypt through to Afghanistan.
Physiological specialization: distinct races occur on Ipomoea batatas and on Ipomoea spp. in the USA and West Indies. Separate races on Ipomoea batatas and Ipomoea pes-caprae in the West Indies are morphologically distinct from each other and also form probable forms on other Convolvulaceae.
Transmission: By air-borne sporangia and by short distance water dispersal of sporangia and zoospores. Initial infection by oospores from overwintered host tissue can occur.
Symptoms: the most obvious symptom is the presence of chlorotic or yellowish blotches, initially roundish to angular where they are limited by veins, on the upper surface of leaves (Figure 31, A). On the lower surface, small pustules develop which later open and expose whitish masses of sporangial pustules (Figure 31, B). After sporulation, the infected tissue dies, forming irregular-shaped brown lesions. In some cultivars or growing conditions, infection induces the development of galls of raised, thickened tissue. Galls may develop on leaves (Figure 31, C), petioles, stems and flowers. Galls turn white as disease develops (Figure 31, D). When pustules erupt, the galls become covered in the white spores. Diseased plants can also present general distortion, defoliation and flower abortion. Distortion may occur where galls form on any part of the stem. The disease may cause twining while twining species may assume an upright habit. In some very sensitive cultivars, symptoms resembling witches broom, with shortening of internodes and bunchy growth habit, have been observed.
Morphology: the mycelium is intracellular with typical knob-like haustoria. The sori are amphigenous or caulicolous, white or light yellow, prominent, superficial, measure 0.5-2.0 mm, rounded, often confluent and frequently producing marked distortion of the host (Figure 31, C). The sporangiophores are hyaline, club shaped, unequally curved at the base and measure 15x30 µm. The sporangia are produced in chains. They are short, cylindrical, with more rounded terminal, hyaline, and smooth; the membrane with an equatorial thickening, is usually very pronounced, measuring 14-20x12-18 µm. The oosporic sori are separated from the sporangial caulicolous, rarely on the petioles, measure 1-2x5-6 cm or even more causing marked distortion of the host. The oospores are light yellowish brown, 25-55 µm in size, and are epispore papillate or with irregular, more or less curved ridges.
Biology and ecology: during the growing season, the disease is spread by air borne sporangia after landing on the plant surface. Germination can be direct such that the sporangia produce an infecting hypha which penetrates the plant tissue, develops intercellulary and sends round haustoria inside the cells. There is indirect germination when the sporangium liberates biflagellate zoospores that swim in a film of water and invade the cells. Oospores overwinter in crop refuse (leaves and stems) in the field and are responsible for primary infection of sweetpotato plants the following growing season. There is no information about the environmental factors that favour white rust development in sweetpotato. The only factor mentioned was rainfall. In other Oomycetes, the free water on plant surfaces determines the way in which the sporangia will germinate. Hence, with direct germination one sporangium is one infection point but if germination is through indirect germination, each zoospore is one infection point and every sporangium contains several zoospores. In the case of Albugo ipomoeae-panduratae, germination directly or indirectly occurs optimally at 12-18 °C. Infection occurs through stomata during periods of rain and cool temperatures. Distinct races of Albugo ipomoeae-panduratae are found on Ipomoea batatas and other Ipomoea spp. These races morphologically differ from each other.
Host range: sweetpotato is the primary host. Other hosts are members of the Convolvulacea family (morning glory) such as: Ipomoea pandurata, Ipomoea pentaphylla, Ipomoea biloba, Ipomoea horsfalliae, Ipomoea purpurea, Ipomoea reptans (water spinach), and Calonyction aculeatum. The disease has also been observed in some members of the family Amaranthaceae such as Amaranthus albus (white pigweed).
Detection and inspection: the symptoms in the field are so evident, that no special diagnosis is needed.
Management: no control methods are mentioned in the literature, apparently because infections are not serious enough. In extreme cases copper fungicides can be used, as for other fungi of the same group.
Figure 31 White-rust caused by Albugo ipomoeae-panduratae. Chlorotic patches and lesions at various stages of development (A). Erupted pustules on the lower leaf surface (B). Yellowish galls as early symptom of the disease (C). Galls turn white as disease develops (D).
- Blue mold rot (Penicillium crustosus Thom, 1930, and others Penicillium Link, 1809, spp.). Penicillium crustosus is reported be very common in sweetpotato imported in UK from Nigeria, while Penicillium cyclopium is important in sweetpotato imported from Ghana. Penicillium gladioli McCulloch and Thom has been found on importations to the USA from Cuba and Puerto Rico. Penicillium oxalicum Currie and Thomhas been recorded in Nigeria, Jamaica, and several island of the French West Indies. Penicillium sclerotigenum Yamam was first described in Japan; subsequently records include India, Nigeria, Brazil and Jamaica
Symptoms: blue or green mould growth is often associated with cut or damaged surfaces (Figure 32, A), but sometimes there may be a serious rot of the interior with no external symptoms. Rotted tissue is pale to dark brown, and may be firm or soft (Figure 32, B). Secondary bacterial infection can result in a wet rot. Penicillium sclerotigenum produces minute brown spherical resting bodies (sclerotia).
Biology: these fungi survuve in the soil on crop debris, and their spores (conidia) are distersed by wind and water. Tubers are likely to be contaminated at harvest, and infection occurs via wounds, including the several "tail" of the root-tubers where it was originally attached to the plants. Decay by these species of Penicillium tends to be expecially rapid at temperatures between 15° and 20 °C.
Conidiophores and conidia on MEA borne from superficial and aerial mycelium; stipes septate, 70-300×3.2-4.0 μm (Figure 32, C and D), distinctly roughened, thin walled; penicilli mostly ter-verticillate (Figure 32, C), occasionally quater-verticillate (Figure 32, C); rami roughened, in group of 1-2, 8.0-30.4×3.2-4.0 μm; ramuli roughened, 11.7-13.6×3.5-4.0 μm; metulae in verticils of 2-6, ampulliform, smooth, 8.8-11.2×2.4-3.2 μm, collula gradually tapering; conidia broadly ellipsoidal, less often subglobose, 2.8-4.2×2.6-3.5 μm (Figure 32, E and F), smooth, thick-walled, borne in defined or occasionally irregular columns.
Control: tubers should be handled as carefully as possible so as to minimize injuries to the skin. Prompt use of a fungicide dip (within 3 days of harvest) can provide effective controlof rotting i store. If fungicides are not available it is recommended that root-tubers be kept for several days in a waem humid atmosphere so as to promote rapid wound-healing.
Figure 32 Blue mold rot caused by Penicillium spp. Blue or green mould growth associated with cut or damaged surfaces (A). Cross section of an infected mature root-tuber (B). conidiophores from superficial and aerial mycelium (C). Penicilli ter-verticillate or, occasionally, quater-verticillate (D). Conidia (E). Conidia enlarged 1000 times more than E (F). Penicilli and conidia; bar=10 μm (G).
- Java black rot caused by the anamorph Lasiodiplodia theobromae (Pat.) Griffon & Maubl. 1909 = Botryodiplodia theobroma Pat. 1892, and teleomorph Botryosphaeria rhodina (Berk. & M.A. Curtis) Arx, 1970 (domain Eukaryota Chatton 1925; Amorphea Adl et al., 2012; Opisthokonta Cavalier-Smith 1987; Holomycota Liu et al., 2009; kingdom Fungi R.T. Moore (1980); subkingdom Dikarya D.S. Hibbett et al., 2007; phylum Ascomycota Caval.-Sm. (1998); subphylum Pezizomycotina O.E. Erikss. & Winka (1997); class Dothideomycetes O.E. Erikss. & Winka (1997); order Botryosphaeriales C.L. Schoch et al., 2006; family Botryosphaeriaceae Theiss. & H. Syd. 1918; genus Botryosphaeria Ces. & De Not. 1863. A synonym of Botryosphaeria rhodina is Physalospora rhodina Berk. & Curt. apud Cooke 1889 (phylum Ascomycota Caval.-Sm. (1998); subphylum Pezizomycotina O.E. Erikss. & Winka (1997); class Sordariomycetes O.E. Erikss. & Winka (1997); subclass Xylariomycetidae O.E. Eriksson & K. Winka, 1997; order Xylariales Nannf., 1932; family Hyponectriaceae Petr., 1923; genus Physalospora Niessl, 1876.
Other synonyms are: Diplodia theobromae (Patouillard) Nowell, Diplodia storage rooticola Ell. & Ev., Diplodia gossypina Cooke, Lasiodiplodia storage rooticola Ell. & Ev. apud Clendenin, and Botryodiplodia theobromae Patouillard 1892.
Geographical distribution: the disease is present worldwide.
Morphology: Lasiodiplodia theobromae is a fungus that grows well in a culture medium (potato-dextrose-agar) in the laboratory, forming fluffy gray colonies initially which become black with age. The mycelium is grey to black. A few days after infection the mycelium forms black stromatic structures containing pycnidia. The pycnidia are round or elongated, ostiolated, generally aggregated, usually setose, up to 5 mm in size. The conidiophores are simple, rarely branched, hyaline, cylindrical, forming a mat in the inner surface of pycnidia. The conidia which arise from the tip of conidiophores are hyaline, unicellular, and granulated when young, becoming cinnamon to fawn, bicellular with longitudinal striae when mature and measure 20-30 x 10-15 µm. They are somewhat subovoid to ellipsoid-oblong in shape.
Symptoms: the most obvious symptoms are those found in fleshy roots few days after harvest. Brown to reddish brown, round sunken lesions with solid black centre, surrounded by soft, pinkish ring of decaying tissues are observed (Figure 33, A). Soon after, the lesions become hard, sunken and completely blackened due to the presence of mature mycelium and stromatic tissue (Figure 33, B). When infection starts in one or both ends of the fleshy root, the entire root dries out and mummifies. During the drying process, black dome-shaped structures bearing pycnidia emerge through root periderm and an abundance of black powdery spores is shed (Figure 33, C). In early stages of infection the symptoms of Java black rot can be confused with those caused by Ceratocystis fimbriata (black rot) and Macrophomina phaseolina (charcoal rot). However, lesions caused black rot are usually confined to the outer layers, in distinct circular lesions. Charcoal rot initially spreads through the outer cambial layer without forming superficial lesions. Java black rot spreads usually from one end of the root, through all layers of tissue. In some cases, the rot is limited to one end of the root and does not spread further.
Biology and ecology: java black rot occurs in seedbeds, attacking underground parts of the plant through soil borne conidia when infected sprouts have been used as propagation material. Fleshy roots are usually infected in the field through wounds made during harvest by inoculum present in the soil or from the infected mother plant. Secondary infections occur in storage when insects carrying the spores infest the storage roots. Lasiodiplodia theobromae requires warm temperatures (20-30 °C). Humidity is not crucial, however, when it is too high the disease does not develop. When infected fleshy roots are stored after harvest, the fungus starts developing and after one week or two. A black pimple-like growth is observed on the surface of the roots. These are pycnidia, containing thousands of conidia, which are the propagation structures of the fungus. Due to its ability to live on decaying matter Lasiodiplodia theobromae survives in the soil on plant refuse for several years.
Host range: cacao (die-back), citrus (stem end rot of the fruit), banana (finger rot), avocado (stem end rot), mango (stem end rot), onion (collar rot), apple (stem rot), cacao (pod rot), yam, cassava, groundnut and melon.
- Cultural control:
- Use of healthy roots or cuttings as propagation material.
- Use of transplants cut above the soil line.
- Avoid wounding the roots during harvest and handling.
- Curing roots before storage. Storage roots should be cured at 30-34oC and 90% relative humidity for 4-7 days depending on the cultivar. After curing, maintain roots at 15-16oC during storage.
- Use of clean and disinfected storage containers.
- Crop rotation for 3-4 years, if possible.
- Host-plant resistance: differences in susceptibility have been noted among lines and cultivars from studies done in different countries.
- Chemical control: dipping propagation material (fleshy roots, sprouts) in Benomyl or Captan. To prevent rotting, the same fungicides are used as a spray for roots before bedding or storing.
Figure 33 Java black rot caused by the anamorph Lasiodiplodia theobromae = Botryodiplodia theobroma. Internal and external views of storage roots with Java black rot: black sunken lesion on storage root (A); cross section of an infected mature root-tuber (B). Spore tendrils exuding from pycnidia (C).
- Aspergillus rot caused by Aspergillus niger van Tieghem, 1867. Synonyms: Aspergillus niger van Tieghem, 1867 var. niger Ann. Sci nut Bot ser. 5(8): 240. 1867.; Aspergillopsis nigra (Tiegh.) Speg.; Rhopalocystis nigra (Tiegh.) Grove, 1911; Sterigmatocystis nigra (Tiegh.) Sacc., (1877). Aspergillus rot appearto be quite common, and has been recorded in Nigeria and India, and also on sweetpotato imported into to UK from Jamaica.
Symptoms: affected flesh is fawn orbrown, sometimes tinged with purple. Lesions generally remain fairly firm, unless there is secondary infection by soft rot bacteria.
Biology: the fungus thrives in tropical conditions, and its conidia are abundant in the atmosphere (Figure 34, C and E). Infection takes place via injuries sustained during harvest, handling or storage. Decay is especially rapid at temperatures between 25° and 35 °C. The fungus was isolated and grows on Czapeks Agar, where produces floccose, granular, and often furrowed colonies (Figure 34, A), or on Malt Extrat Agar, where produces colonies brown (Figure 34, B). The microscopic characteristics of the fungus are shown in figure 34, C, D, and E.
Control: care should be taken to minimise physiological damage, and a post-harvest fungicide dip may be beneficial. The fungus grows slowly at temperatures below 20 °C.
Figure 34 Aspergillus niger that cause Aspergillus rot. Colonies on Czapeks Agar, floccose, granular, often furrowed (A). Colonies on Malt Extrat Agar, 14 days at 25 °C (B). Stipes 220-2460×4.4-24.0 μm, uncolored to pale yellowish brown, smooth (E); vesicles globose, 16.0-80.0 μm wide (E) aspergilla biseriate; metulae covering 3/5 to the whole surface of the vesicle, rarely septate, 5.6-58.0×3.8-13.1 μm; phialides 3.6-14.3×2.4-5.2 μm (C, E). Conidia subglobose to globose 3.2-4.8 μm, roughened or with irregular ridges or bars (D, E).
- Punky rot caused by Trichoderma Pers., 1794 spp.; Trichoderma koningii Oudem. (1902). Trichoderma koningii is a pathogen and an antagonist of fungi and also has a negative effect on certain nematodes. It is being used as a biological fungicide.
Cultures are typically fast growing at 2530 °C, but some species of Trichoderma will grow at 45 °C. Colonies are transparent at first on media such as cornmeal dextrose agar (CMD) or white on richer media such as potato dextrose agar (PDA). Mycelium are not typically obvious on CMD, conidia typically form within one week in compact or loose tufts in shades of green or yellow or less frequently white. A yellow pigment may be secreted into the agar, especially on PDA. Some species produce a characteristic sweet or "coconu" odor. Conidiophores are highly branched and thus difficult to define or measure, loosely or compactly tufted, often formed in distinct concentric rings or borne along the scant aerial hyphae. Main branches of the conidiophores produce lateral side branches that may be paired or not, the longest branches distant from the tip and often phialides arising directly from the main axis near the tip. The branches may rebranch, with the secondary branches often paired and longest secondary branches being closest to the main axis. All primary and secondary branches arise at or near 90° with respect to the main axis. The typical Trichoderma conidiophore, with paired branches assumes a pyramidal aspect. Typically the conidiophore terminates in one or a few phialides. In some species (e.g. T. polysporum) the main branches are terminated by long, simple or branched, hooked, straight or sinuous, septate, thin-walled, sterile or terminally fertile elongations. The main axis may be the same width as the base of the phialide or it may be much wider. Phialides are typically enlarged in the middle but may be cylindrical or nearly subglobose. Phialides may be held in whorls, at an angle of 90° with respect to other members of the whorl, or they may be variously penicillate (gliocladium-like). Phialides may be densely clustered on wide main axis (e.g. T. polysporum, T. hamatum) or they may be solitary (e.g. T. longibrachiatum). Conidia typically appear dry but in some species they may be held in drops of clear green or yellow liquid (e.g. Trichoderma virens, Trichoderma flavofuscum). Conidia of most species are ellipsoidal, 35x24 µm (L/W = > 1.3); globose conidia (L/W < 1.3) are rare. Conidia are typically smooth but tuberculate to finely warted conidia are known in a few species. Synanamorphs are formed by some species that also have typical Trichoderma pustules. Synanamorphs are recognized by their solitary conidiophores that are verticillately branched and that bear conidia in a drop of clear green liquid at the tip of each phialide. Chlamydospores may be produced by all species, but not all species produce chlamydospores on CMD at 20 °C within 10 days. Chlamydospores are typically unicellular subglobose and terminate short hyphae; they may also be formed within hyphal cells. Chlamydospores of some species are multicellula. Trichoderma genomes appear to be in the 3040 Mb range, with approximately 12,000 genes being identifiable.
Figure 35 Microscopic recognition of Trichoderma. Conidia in heads on conidiophores divided into two or three tips, a single head on each tip. You observe the phialides, typically enlarged in the middle (A); spores hyaline, one-celled (B); chlamydospores are typically unicellular subglobose and terminate short hyphae; they may also be formed within hyphal cells (C).
Teleomorphs of Trichoderma are species of the ascomycete genus Hypocrea. These are characterized by the formation of fleshy, stromata in shades of light or dark brown, yellow or orange. Typically the stroma is discoidal to pulvinate and limited in extent but stromata of some species are effused, sometimes covering extensive areas. Stromata of some species (Podostroma) are clavate or turbinate. Perithecia are completely immersed. Ascospores are bicellular but disarticulate at the septum early in development into 16 part-ascospores so that the ascus appears to contain 16 ascospores. Ascospores are hyaline or green and typically spinulose. More than 200 species of Hypocrea have been described but few have been grown in pure culture and even fewer have been described in modern terms.
- Rhizoctonia stem canker (sprout rot) responsible Rhizoctonia solani J.G. Kühn 1858; teleomorph Thanatephorus cucumeris (A.B. Frank) Donk, 1956. Synonyms: Rhizoctonia aderholdi Kolosh, 1984; Sclerotium irregulare Miyake, 1910; Moniliopsis solani (J.G. Kühn) R.T. Moore 1987; Corticium solani (Prill. & Delacr.) Bourd. & Galz., 1911; Corticium vagum Berk. & M.A. Curtis, 1873; Corticium areolatum Stahel, 1940; Pellicularia filamentosa (Pat.) D.P. Rogers 1943; Hypochnus cucumeris A.B. Frank., 1883; Hypochnus solani Prill. & Delacr., 1891; Hypochnus filamentosus Pat., 1891; Hypochnus sasakii Shirai., 1906; and Hypochnus aderholdi Kolosh, 1984. Other common names are: damping off (rapid decline of germinating seed or seedling before or after emergence leading to death due to decomposition of the root and/or lower stem), Rhizoctonia sprout rot, rootlet rot, root rot, Rhizoctonia rot, collar rot.
Thanatephorus cucumeris taxonomy is: phylum Basidiomycota R.T. Moore (1980); Subphylum Agaricomycotina Doweld (2001); class Agaricomycetes Doweld (2001); order Cantharellales Gäum., 1926; family Ceratobasidiaceae G.W. Martin 1948; genus Thanatephorus Donk 1956.
Economic importance: Rhizoctonia stem canker is an economically important disease in many crops. However, very little is known about its economic importance in sweetpotato. The fungus causes more damage in plant beds than in the field. Generally, only a few plants are affected and sometimes the infection heals over after infected sprouts are planted in the field. When the fungus is associated with other root fungi the damage can be serious. This is really what causes most important losses.
Geographical distribution: Rhizoctonia solani is extremely widespread, occurring throughout the world in all arable soils. It has been reported in so many other crops from Africa, Asia, Caribbean, Europe, Pacific Islands, North America and South America. However, effect on sweetpotato had only been recorded in the United states.
Morphology: the mycelia of Rhizoctonia solani are made up of hyaline hyphae when young, becoming brown as they mature. Mature hyphae are 4-6 µm thick and the length of their cells is between 60-200 µm. Hyphae arise at a distinctive straight angle and are constricted at the point of branching with a septum formed in the branch near the constriction (see figure). As hyphae mature they become rigid with branches at right angles. There are also monilioid hyphae made up of short barrel shaped cells. These cells have thick walls and are mostly branched. The sclerotia are compact masses of cells generally formed in affected areas. The cells on the surface of sclerotia are dark brown due to the presence of pigments but the ones in the centre are colourless. Sclerotia are dark in colour, almost black, of various shapes, but mostly flat and of different sizes. Sometimes sclerotia can be as thin as a film.
Symptoms: the symptoms of the disease are found on both above and below ground portions of the plant. Black scurf (Figure 36, A), is the most conspicuous sign of Rhizoctonia disease. In this phase of the disease the fungus forms dark brown to black hard masses on the surface of the root-tuber. These are called sclerotia and are resting bodies of the fungus (Figure 36, A). Sclerotia are superficial and irregularly shaped, ranging from small, flat, barely visible blotches to large, raised lumps. Although these structures adhere tightly to the root-tuber skin, they do not penetrate or damage the tuber, even in storage. However, they will perpetuate the disease and inhibit the establishment of plants if infected root-tubers are used as seed. Early in the season, the fungus attacks germinating sprouts underground before they emerge from the soil (Figure 36, B and C). Reduction in crop vigor results from expenditure of seed energy used to produce secondary or tertiary sprouts to compensate for damage to primary sprouts. Occasionally, heavily infested seed root-tubers are unable to produce stems. Instead, the tubers will produce stems with several small root-tubers (Figure 36 D). The sprout may be killed outright if lesions form near the growing tip (Figure 36, E). Damage at this stage results in delayed emergence and is expressed as poor and uneven stands with weakened plants. This symptom is referred to as "no top" and can be confused with the same symptom caused by physiologically old seed that has been de-sprouted. Late season damage to plants is a direct result of cankers on stolons and stems causing problems with starch translocation. Stem cankers also affect the shape, size and numbers of tubers produced. If stolons and underground stems are severely infected, the flow of starch from the leaves to the developing tubers is interrupted. This results in small, green tubers, called aerial tubers forming on the stem above the soil (Figure 36, D). Formation of aerial tubers may indicate that the plant has no tubers of marketable quality below ground. Interruptions in carbohydrate flow may also result in a stunting or rosetting of the plant. A leaf curl, which can be confused with symptoms of the Potato Leaf Roll Virus, has also been reported in severely infected plants. Although black scurf is the most noticeable sign of Rhizoctonia, stem canker (Figure 36, F), is the most damaging of the disease as it occurs underground and often goes unnoticed. Healthy plants in the field rarely develop symptoms of the disease. The most obvious symptom is the presence of water- soaked sunken lesions, in the stems of seed sprouts, near the soil line. The stem cortex decays causing stunting and yellowing. Occasionally, white powdery growth is observed on the sol surface and on stems of sprouts. When the root system is affected, black cankers can be observed in the taproot and lateral small roots, the whole plant becomes affected and dies. On fleshy roots the fungus causes brown rotted areas or cracked sunken cankers often covered with fungal mycelium.
Ecology: the fungus survives as actively growing mycelium in plant debris, as resting mycelium in dry organic matter, such as plant refuse or in its sclerotial stage, depending on the temperature and moisture of the soil. The incidence and severity of the disease depends on temperature and moisture. The fungus grows well across a wide range of temperatures 20-28 °C. Moisture approaching saturation results in high disease severity. Poorly drained soils have the same effect on severity. Damage could still be moderate even in cases where there is a high soil inoculum level because of good soil composition and aeration. The disease can also be very mild when there are natural enemies of R. solani, such as protozoa, nematodes and earthworms, or when there are antagonistic fungi and bacteria such as Trichoderma spp., Gliocladium spp., Pythium spp., Verticillium sp., Fusarium sp., Pseudomonas sp., Bacillus subtilis and Streptomyces rimosus. Rhizoctonia solani is considered to be a polyphagous fungus and includes many physiological strains grouped as anastomosis groups (AG). The fungus produces pectic, cellulolytic, and other enzymes that hydrolyse cell walls and other cell components.
Rhizoctonia solani does not produce spores (sterile mycelium) and is hence identified only from mycelial characteristics (Figure 36, G) or DNA analysis. Its hyphal cells are multinucleate. It produces white to deep brown mycelium when grown on artificial medium. The hyphae are 415 μm wide and tend to branch at right angles (Figure 36, G). A septum near each hyphal branch and a slight constriction at the branch are diagnostic (Figure 36, G). Rhizoctonia solani is subdivided into anastomosis groups (AG) based on hyphal fusion between compatible strains (Figure 36, G). The teleomorph of Rhizoctonia solani is Thanatephorus cucumeris. It forms club-shaped basidia with four apical sterigmata on which oval, hyaline basidiospores are borne.
Host range: Rhizoctonia solani is world wide in distribution producing root rot, stem rot, and foliage diseases in numerous host crops. Sweetpotato is considered a secondary host, but probably the disease is present wherever sweetpotato is grown.
Cultural control: use of healthy and clean material for transplanting; solarization for 6 weeks; antagonistic microorganisms (biological control).
Chemical control: several fungicides, effective against Rhizoctonia solani, can be used for soil treatment as well as for dipping material to be transplanted. Fungicides recommended are metasodium for soil fumigation, benomyl and tryazole group fungicides as dips before planting.
Figure 36 Rhizoctonia stem canker (sprout rot) caused from Rhizoctonia solani. Black scurf and sign of Rhizoctonia>/i> disease on root-tubers, including dark brown to black hard masses (sclerotia) that are superficial and irregularly shaped, ranging from small, flat, barely visible blotches to large, raised lumps. Although these structures adhere tightly to the root-tuber skin, they do not penetrate or damage the tuber, even in storage (A). Fungus attacks on germinating sprouts underground before they emerge from the soil (B and C). Such as effets of the disease attack the plants produce stems with several small root-tubers (D). The sprout may be killed outright if lesions form near the growing tip (E). Black scurf is the most noticeable sign of Rhizoctonia, stem canker is the most damaging of the disease as it occurs underground (F). Rhizoctonia solani young hypha (hyaline), mature showing brown hypae branched by right angle (diagnostic character), and constrictions at the point of branching (G).
Disease cycle: Rhizoctonia diseases are initiated by seedborne or soilborne inoculum. The pathogen overwinters as sclerotia and mycelium on infected tubers, in plant residue, or in infested soils. When infected seed tubers are planted in the spring, the fungus grows from the seed surface to the developing sprout and infection of root primordia, stolon primordia and leaf primordia can occur. Seed inoculum is particularly effective in causing disease because of its close proximity to developing sprouts and stems. Mycelia and sclerotia of Rhizoctonia solani are endemic to soils, living on organic debris, and can cause disease independently of or in conjunction with seedborne inoculum. Soilborne inoculum is potentially as damaging as seedborne inoculum, but it can cause infection only when the plant organs develop in the proximity of the inoculum. Roots and stolons may be attacked at any time during the growing season, although most infections probably occur in the early part of the plant growth cycle. The plant's resistance to stolon infection increases after emergence, eventually limiting expansion of lesions.
Previous research has shown that soil temperature is a critical factor in the initiation of Rhizoctonia disease, with disease severity being positively correlated with the temperature that is most favorable for pathogen growth. The optimal temperature range for the growth of R. solani AG-3 is 41 to 77°F. Thus, plants will be most susceptible to infection when the soil temperatures are within this critical range. Cool temperatures, high soil moisture, fertility and a neutral to acid soil (pH 7 or less) are thought to favor development of Rhizoctonia disease. Damage is most severe at cool temperatures because of reduced rates of emergence and growth of stems and stolons are slow relative to the growth of the fungus. Wet soils warm up more slowly than dry soils which exasperates damage because excessive soil moisture slows plant development and favors fungal growth. However, it has been shown that high soil temperatures, especially during the early stages of plant development tend to minimize the impacts of Rhizoctonia solani, even when inoculum is abundant. Sclerotia begin to form late in the season, principally after vine death. The mechanisms involved in sclerotial development on daughter tubers are different from those acting in the infection of the mother plant. The mechanisms which trigger sclerotial formation are not well understood, but they may involve products related to plant senescence. However, daughter tubers produced from infected mother plants do not always become infested with sclerotia.
Monitoring and control: currently it is not possible to completely control Rhizoctonia diseases, but severity may be limited by following a combination of cultural and crop protection strategies. Effective management of this disease requires implementation of an integrated disease management approach and knowledge of each stage of the disease. Although the most important measures are cultural, chemical controls should also be utilized. To date, there have been no comparisons of the relative susceptibility of potato varieties currently grown in Michigan.
Cultural control: one of the keys to minimizing disease is to plant certified seed free of sclerotia. If more than 20 sclerotia are visible on one side of washed tubers, consider using a different seed source. Tuber inoculum is more important than the soil inoculum as the primary cause of disease. Seed growers should plant only sclerotia-free seed. Following practices that do not delay emergence in the spring minimizes damage caused to shoots and stolons and lessens the chance for infection. Planting seed tubers in warm soil and covering them with as little soil as possible speeds spout and stem development and emergence reduces the risk of stem canker. Plant fields with coarse-textured soils first because they are less likely to become waterlogged and will warm up faster. Rhizoctonia solani does not compete exceptionally well with other microbes in the soil. Increasing the rate of crop residue decomposition decreases the growth rate of Rhizoctonia. Residue decomposition also releases carbon dioxide, which reduces the competitive ability of the pathogen. Since the fungus is not an efficient cellulose decomposer, soil populations are greatly reduced by competing microflora and less disease is observed. Sweetpotatoes should be harvested as soon as skin is set so minimal bruising will occur. The percent of tubers covered with sclerotia increases as the interval between vine kill and harvest is lengthened. Vine removal or burning also reduces the amount of fungus overwintering and thus the amount of inoculum available to infect future potato crops. Do not dump infested tubers on future potato fields as they can become sources of inoculum.
Biological control: there is growing evidence that a "bio-fumigation" treatment based on incorporating a mustard cover crop is one way to reduce Rhizoctonia incidence. Mustard residues when incorporated into the soil release cyanide-containing compounds that fumigate the soil, but at the same time they also release carbon and nutrients that are the feedstock for soil organisms. Incorporating green cover crop tissues provides energy that supports the complex web of soil organisms that compete with parasite and disease organisms. Thus mustards, and related 'brassica' plant species such as oil-seed radish, do not leave a soil void of organisms. Instead, these cover crops tend to tip the balance in the favor of beneficial organisms and against parasites and pests. Our preliminary research indicates that it is important to maximize growth of the cover crop using a high seed rate (15 lb. acre or more) and irrigation to improve establishment if rainfall is insufficient. A tiny seed such as mustard cannot be drilled too deep. It appears to establish well if broadcast and harrowed or irrigated into sandy soil. The bio-fumigation benefits of mustard residues are maximized if they are incorporated at or just before flowering. We suggest that residues be mowed and incorporated while still green. Mustards are rapid growing species and can become a weed in a subsequent crop, so it is important not to let this cover crop produce seed. We are just beginning to understand the exact mechanisms involved in bio-fumigation using mustard cover crops. Initial results from Michigan research indicate that oriental mustard can be used as a cover crop to improve potato root and tuber health. The growth of Rhizoctonia was slowed by 90% in soil amended with oriental mustard cover crop tissue compared to bare soil. A field experiment indicated that tubers of the tablestock variety Onaway had no observable signs of Rhizoctonia when grown after a spring cover crop of oriental mustard. Further research is required to learn more about management practices that optimize the bio-fumigation action of mustard cover crops, but initial results are promising and farmers are encouraged to experiment with brassica cover crops such as oriental and white mustard or oilseed radish to improve soil health.
Chemical control: several products have been specifically developed for control of seed-borne potato diseases and offer broad-spectrum control for Rhizoctonia, Silver Scurf, Fusarium Dry Rot and to some extent Black Dot (Colletotrichum coccodes). These include Tops MZ, Maxim MZ (and other Maxim formulations + Mancozeb) and Moncoat MZ. The general impact of these seed treatments is noted in improved plant stand and crop vigor but occasionally, application of seed treatments in combination with cold and wet soils can result in delayed emergence. The delay is generally transient and the crop normally compensates. The additional benefit of the inclusion of Mancozeb is for prevention of seed-borne late blight. Application of fungicide in-furrow at planting has resulted in significant improvement in control of Rhizoctonia disease. Products such as Moncut and Amistar applied in-furrow at planting have given consistent and excellent control of Rhizoctonia diseases in trials. However, both seed treatments and in-furrow applications on some occasions have resulted in poor control of Rhizoctonia. This sporadic failure may be due to extensive periods of wet and cold soil shortly after planting or planting in fields with plentiful inoculum. Amistar applied in-furrow has been reported to reduce the symptoms of Black Dot on lower stems and tubers.
- Rhizopus soft rot caused by Rhizopus Ehrenb. 1821 (fungus) spp. and Rhizopus stolonifer (Ehrenb.) Vuill. 1902 = Rhizopus nigricans Ehrenb., 1821. The disease is widespread and important, causing heavy losses. It probably occours in sweetpotatoes from producer countries, but most investigations have been carried out in the USA. Numerous species of Rhizopus have been recorded. Rhizopus stolonifer is by far the most common. Rhizopus oryzae Went & Prinsen Geerlings (1895) and Rhizopus tritici Saito 1904, are among the others studied. Recently, Rhizopus nigricans is synonim of Rhizopus stolonifer; Rhizopus oryzae and Rhizopus tritici are synonims of Rhizopus arrhizus A. Fisch. 1892.
Symptoms: decay often begins at one end of the root (Figure 37, A). Alternatively it may start on the soulder, causing a sunken lesion which tends to encircle the root (Figure). In a dry atmosphere rotting may be arrested; the flesh remains firm but begin toshrivel. Under humid conditions affected tissue is soft and watery. In places where the skin has ruptured there is copius development of coarse white mould, bearing characteristic globular spore-heads (sporangia) which turn from white to black as they mature. Colonization of the entire root can occur within a few days, and mould spread to adjacent roots, causing "nests" decay.
Biology: species of Rhizopus are common in soil and atmosphere, and harvested sweetpotato roots are likely to be contaminated with spores. Wounds predispose the roots to attack, and the root-tip is especially susceptible to invasion because the natural presence of dead tissue is advatageous to the fungus. Once established, it is capable of attacking healthy uninjured tissue. Infection and decay are greatly influenced by environmental conditions during transport and storage. Infection is especially likely if the relative humidity is between 75 and 85%; a drier atmosphere promotes wound-healing whichprotects the root fron attack. If, hewerer, uncured roots are first allowed to wilt and then placed in a humid environment, the mould may be able to invade before the roots can developa defence, and catastrophic rotting usually follows. The optimal remperatures for decay is approximately 20 °C for Rhizopus stolonifer and in the region of 30 °C for Rhizopus oryzae and Rhizopus tritici (both = Rhizopus arrhizus). >br>
Control: pieces or roots used for seed should be dipped in a suitable fungicide before being planted. Hervested roots should be handled with care to minimize injury. They should not be left for too long in the field but removed to a shaded place in order to prevent excessive moisture loss. Immediate curing serves to heal wounds before the fungus can invade, and storage at 13° to 16 °C. is advantageous. When sweetpotatoes are brought out of store for washing and preparation for shipment and marketing, new injuriesare inevitable. It has therefore been pointed out that recuring is necessary, an appropriate strategy being to warm the roots in a hot water bath to enable them to cure subsequently whlst in transit. Cutting off the moribund tips of roots has been suggested as means of reducing the likelihood of infection, and post-harvest fungicide treatments can be very effective. in winter care should be taken to protect sweetpotatoes from chilling injury since this pridisposes them to rhizopus rot.
Figure 37 Rhizopus soft rot caused by Rhizopus. Root-tuber storaged attacked by Rhizopus (A); root-tuber attacked by Rhizopus stolonifer (B) and Rhizopus arrhizus (C). Microphotography of Rhizopus arrhizus, I=x100: sporangiophores, rhizoids,columellae and sprangia (D); sporangiospores, I=x3000 (E); sporangiophora, I=x400 (F).
- Rootlet rot caused by a fungus complex constitued from Fusarium solani (Mart.) Sacc. 1881, and Pythium Pringsh. (1858) spp., and Pythium ultimum Trow (1901), and Rhizoctonia solani J.G. Kühn 1858; Streptomyces ipomoeae (Person & Martin 1940) Waksman & Henrici 1948.
The genus Pythium, certain species of which are known to cause "dampingoff" of a number of plants in the seedling stage, has never been reported on the roots of sweet potatoes so far as the writer is aware. Sweet potato plants, the rootlets of which were partially decayed bv Pythium, were first collected in New Jersey in 1914. Since
then it has frequently been observed and collected from various types of hotbeds in New Jersey, Delaware, Maryland, Virginia, and in many of the States in the South and West. It is not unlikely that under suitable environmental conditions the disease may occur wherever sweet potatoes are grown. Although the Pythium rootlet rot
of sweet potatoes occurs principally in the hotbed or seedbed, it has been found on the roots of plants from the time they were set in the field until they were dug. The amount of injury actually caused by the disease is difficult to estimate. Undoubtedly plants having the ends of the smaller roots
dead are at a disadvantage and would be slow in starting to grow after being set in the field. Furthermore, observations have shown that infected plants, especially if the soil conditions are unfavorable, remain stunted the entire summer. It is probable that a considerable amount of the loss hitherto attributed
to the socalled "sick soils", is actually due to the injury to the root system by Pythium throughout the summer. It is interesting to note in this connection that Rhizoctonia is sometimes associated with the Pythium in the decayed ends of the rootlets. Pythium rootlet rot has been found
in hotbeds or seedbeds prepared by the use of soil which was almost pure sand. It is more prevalent in old beds or beds in which the soil or sand has been used for several years. The amount of infection is apparently increased by an abundance of moisture in the soil. If, on the other hand, the bed later
becomes dry, the injury to the plants is increased as a result of the reduction of the root system. Pythium rootlet rot is primarily a disease of the small rootlets. The infections take place at the tip ends of the rootlets and from there the fungus grows
progressively up the root, killing it for a distance of from 0.5 to S cm or more from the tip. In the spring, twenty-one varieties of sweetpotatoes were bedded in soil that had been used for a sweetpotato bed the two preceding years. The soil contained a considerable amount of organic matter in the form of stable manure and decayed roots and vines. When the plants were pulled
they were carefully examined and Pythium rootlet rot was found on all the varieties. There was, however, considerable variation in the amount of infection among the different varieties, as shown by the following data. The amount of infection on different varieties of sweet potatoes by the Pythium
rootlet rot showed severe infection on six varieties, "Big Stem Jersey", "Key West", "Creóla", "Little Stem Jersey", "Nancy Hall", and "Porto Rico"; moderate infection on 12 varieties, "Red Jersey", "Georgia", "Triumph", "Yellow Belmont", "Gold Skin", "Dooley", "Haiti", "Dahomey", "Red Brazil", "Gen. Grant Vineless", "Pierson", and
"Southern Queen"; and slight infection on three varieties, "Pumpkin", "Yellow Strasburg", and "White Yam". This classification is merely an estimate and was determined by a careful examination of the roots of a number of plants. In many cases it was difficult to determine to which group a certain variety
belonged, and as a matter of fact, there are several varieties in the "moderately infected" class which are on the border line of either the "severely" or "slightly" infected group.
The genus Pythium comprises about eighty-five species. Pythium species are common pathogens causing disease in plants and fishes. The species of this genus are among the most destructive plant pathogens, inflicting serious economic losses of crops by destroying seed, storage organs, roots, and other plant tissues. Pythium insidiosum is the only species reported to cause infections in mammals. The disease caused by this unique microorganism has been termed pythiosis (insidiosi) and can cause life threatening infections in cats, dogs, cattle, equines, captive polar bears, and humans. Members of the genus Pythium have been described as aquatic fungi. However, they are not true fungi (Kingdom Fungi), they belong to the Kingdom Stramenopila, Phylum Oomycota, Class Oomycetes, Family Pythiaceae. In culture, P. insidiosum develops sparsely septate fungal-like hyphae similar to those produced by the Zygomycetes (true fungi). Like other Oomycetes, Pythium insidiosum produces motile zoospores (asexual stage) when exposed to damp conditions. The zoospores are single cells with two lateral flagella that swim to find a new plant host where it completes its lifecycle. Once in contact with the host the zoospores lose their flagella and encyst. It is believed that zoospores act as infecting units once in contact with a mammalian host. Under conditions, still under investigation, P. insidiosum develops globose oogonia (sexual stage) typical of this species, but they are rarely observed in most isolates. Like other oomycetes, P. insidiosum grows relatively well on a variety of media. On corn meal agar and Sabouraud dextrose, colonies are color less to white, submerged with short aerial mycelium and a finely radiate pattern. The coenocytic hyphae range between 4 and 12 mm in diameter with perpendicular lateral branches (Figure 38, left). Septation is only occasionally observed in early hyphae, but they become abundant in old viable hyphae. Hyphal swellings, that mimic sporangia measuring 12 to 28 in diameter, are common in laboratory cultures. Production of zoospores can be induced in water cultures containing minimal quantities of different ions and grass leaves at 37 °C. Zoospore induction in water without ions is rare. Early sporangia can not be differentiated from normal hyphae. These early sporangia, at maturity, flows their cytoplasm into a discharge tube and form a globose sporangium. The cytoplasmic content of the sporangium goes through progressive cleavage and biflagellated zoospores are formed inside a vesicle. The zoospores mechanically break the sporangium vesicle wall and swim approximately 20-30 minutes and then encyst. It is believe that Pythium use a plant to complete its life cycle in nature, but it has not been confirmed. Sexual reproduction in Oomycetes occurs between two dissimilar gametangia: a large round oogonium containing one to several eggs, and a smaller antheridium that fertilizes the oogonium. If the antheridium is located at the side of the oogonium, the arrangement is termed paragynous. In an amphigynous arrangement, the oogonium grows through the antheridium, which remains as a collar at its base (Figure 38, right).
Figure 38 Rootlet rot caused by a fungus complex constitued from Fusarium solani, Pythium spp., Pythium ultimum, Rhizoctonia solani; Streptomyces ipomoeae. Pythium: the coenocytic hyphae range between 4 and 12 mm in diameter with perpendicular lateral branches (Figure 38, left); paragynous arrangement of oogonium and antheridia in Pythium (right).
- Red rust caused by Coleosporium ipomoeae (Schwein.) Burrill (1885) = Uredo ipomoeae Schwein., Schr. Naturf Ges. Leipzig 1:70, 1822 = Uredo ipomoeicola J.C. Lindq. 1966 (accepted name) = Peridermium ipomoeae Hedge. & Hunt, 1917. The taxonomy of Uredo ipomoeicola is: phylum Basidiomycota R.T. Moore (1980); class Pucciniomycetes R. Bauer, Begerow, J.P. Samp., M. Weiss & Oberw. 2006; order Pucciniales Clem. & Shear, 1931; family not assigned; genus Uredo Persoon, 1801.
Red Rust is found on most of the Caribbean islands, including Cuba and Puerto Rico. It is also present in South America and in North America northward as far as Illinois and Pennsylvania. The fungus attacks nearly every species in the sweet potato family and several closely related genera. The disease seldom is destructive. It frequently causes considerable defoliation on morning glory weeds. It has not been observed on commercial sweetpotatoes in the southern United States.
The symptoms consist i minute colorless to deep orange-red pustules that appear on undersides of leaves (Figure 39, A). The pustule-covered leaf tissue died, and sometimes entire leaves turn brown.
The fungus, at different stages during the life cycle, affects two different host plants. Uredial aeciospores globose to ellipsoid, 16-28 µm diam.; wall hyaline to pale yellowish, 1-2,5 µm thick, verrucose with warts 0,5-1,5 µm diam., 0,5-2 µm high, crowded or ± widely spaced. Telia hypophyllous, scattered or clustering around the uredial aecia, appearing as waxy beads or crusts on leaf surface, deep reddish-orange, becoming cinnamon brown when preserved, up to c. 1 mm diam., subepidermal. Teliospores cylindric, clavate, rounded or truncate above, 60-140 x 15-28 µm; wall hyaline, smooth, 1-2 µm thick at sides, up to 25 µm thick above, becoming 3-septate, foot cell elongating at maturity (Figure 39, B). The spores are wind-borne. An aecial, or cup, stage occurs on pine trees; all other stages occur on Ipomeae. Where pine trees are absent, as in Puerto Rico, the fungus lives continuously in the uredial or red-spore stage. Aecia hypophyllous, peridermioid, elongated, like a broad short tongue protruding from the needle, 0,5-2 mm high, 1-4 mm long, 0,5-1 mm wide (Figure 39, C and D).
Hosts: pycnia and aecia on several species of Pinus (especially southern pines), uredial aecia and telia on several genera of the Convolvulaceae (Argyreia, Convolvulus, Ipomoea and Jacquemontia).
Physiological specialization: Observations in USA that sweet potato plants have remained uninfected, although growing close to other infected Ipomoea hosts indicate that physiologic forms occur.
Transmission: transmission of the Coleosporium rusts in Florida has been described by Weber. Short-lived basidiospores, which rarely travel more than 1,6 km in a viable condition, infect pine needles during late summer and autumn probably through the stomata. The aeciospores formed in the spring are very resistant and can travel long distances to infect the alternate host by direct penetration of the cuticle.
No contro measures have been tried.Where necessary, fungicides shuold be evaluated and the use of resistant varieties.
Figure 39 Red rust caused by Uredo ipomoeicola J.C. Lindq. 1966 (accepted name). Orange-red pustules on leave underside (A). Uredospores (B). Mature uredial pustules from that blom-up the uredospores (C). Aecium (D).
- Scab leaf and stem caused by Sphaceloma batatas Sawada (1931), teleomorph Elsinoe batatas Viégas & Jenkins (1943). Scab is the most severe fungal disease of sweetpotato in South East Asia and the Pacific. In Queensland, Australia, 20% yield losses were reported for the cultivar "Puerto Rico". In the Philippines, yield losses reached 50%, and in Papua New Guinea losses of up to 57% were noted. When plants are affected at an early stage, the yield of marketable roots is severely reduced.
Geographical distribution: Australia, Brazil, Brunei, Cambodia, Caroline Islands, China, Cook Islands, Fiji, French Polynesia, Guam, Hong Kong, Indonesia, Japan, Malaysia (including Sabah and Sarawak), Mexico, Micronesia, New Caledonia, New Guinea, Niue, Papua New Guinea, Northern Mariana Islands, Puerto Rico, Solomon Islands, Taiwan, The Philippines, Tonga, USA and Vanuatu.
Symptoms: the first evident symptoms are tiny spots or lesions on the leaves and stem. These may be circular, elliptical or elongate, yellowish to reddish brown, and may be initially sunken but become raised and scabby, as they spread and merge with each other (Figure 40, A). The leaf veins on the lower surface are most commonly infected. Scab tissue contains the structures of the causal agent intermingled with the leaf or stem tissue. When expanded leaves are infected, lesions are present without leaf deformity. However, newly formed leaves are most susceptible, and these become greatly distorted as affected tissue stops growing (Figure 40, B). In particular, the petiole does not flex at the base of the leaves, so they retain the upright presentation of unopened leaves. Leaves are generally cupped and shrivelled in a claw-like manner. Brown, raised scaby lesions on veins under leaf (Figure 40, C). Stem lesions at an early stage of infection (Figure 40, D). Early infection leads to considerable yield reduction. Similar symptoms on the leaves are also caused by the thrips Dendrothripoides innoxius.
Morphology: the ascomata formed below the epidermis are dark brown to black and solitary to aggregated. They measure up to 150 µm in diameter, composed of pseudoparenchymatic tissues, and contain numerous monoascus locules. The asci are globose or ovoid, 8-spored, thick walled and measure 18-25 x 12-25 µm. The ascospores are hyaline, smooth, transversely 1-3 septate, constricted at the midseptum, and measure 12-18 x 4.5 µm. The acervulus is colourless, 12-16 µm in diameter. The conidiophores are short, simple to rarely branched and measure 10 x 3 µm; the conidia are hyaline, smooth, aseptate, oblong, and measure 4 - 9 x 2.5-3.5 µm.
Biology and ecology: the disease is transmitted by infected cuttings and through rain splash that carries masses of spores from infected to healthy plant parts of the same plant or to neighbouring plants. The disease is widespread in places with a high incidence of rain, mist and dew or in places where sprinkle irrigation is used. Under controlled conditions, a temperature range between 25° and 30 °C is optimal for fungal growth. The fungus remains from one season to the next in crop refuse in the form of ascomata. When the temperature rises and there is enough moisture, the ascomata release asci and ascospores, which are the structures that initiate infection of young leaves and stems. Once in the plant tissue, the fungus grows and produces conidia, which are the secondary inoculum that spreads the disease in a field.
Host range: sweetpotato is the primary host but the disease has also been found in Ipomoea aquatica, Ipomoea gracilis, and Ipomoea triloba.
Host-plant resistance: the cultivars "Centennial 83" and "Beerwah Gold" were found highly resistant and moderately resistant, respectively. Philippine cultivars V2-1, V2-3 and V2-30 and a number of AVRDC cultivars were also found to be highly resistant in Taiwan. It has been found that varieties with a thicker cuticle and fewer stomata are more resistant to fungus invasion.
- Plant resistant cultivars.
- Burning or burying of infected vines after harvest.
- Use of disease-free healthy planting material.
- Non-use of overhead irrigation.
- Plant rotation crops that are non-host of the disease.
Chemical control: the fungicides benomyl and chlorotalonil reduce disease incidence.
Figure 40 Scab leaf and stem caused by Sphaceloma batatas (anamorph), Elsinoe batatas (teleomorph). Cupped, distorted and upright leaves showing the confluent scabby lesions along veins and petioles (A); early infection showing little leaf distortion, but upright leaf presentation (B); Brown, raised scaby lesions on veins under leaf (C); Stem lesions at an early stage of infection (D).
Scurf caused by Monilochaetes infuscans Ellis & Halst. 1890. The taxonomy of Monilochaetes infuscans is kingdom: Fungi R.T. Moore (1980); phylum: Ascomycota Caval.-Sm. (1998); subphylum: Pezizomycotina O.E. Erikss. & Winka (1997); class: Sordariomycetes O.E. Erikss. & Winka (1997); order: Glomerellales Chadef. ex Reblova, W. Gams & Seifert 2011; family: Australiascaceae Reblova & W. Gams 2011; genus: Monilochaetes Halst. ex Harter 1916.
The disease is a minor one but market losses can be important because of the disfiguring effects of infection. The superficial damage to outer fleshy root tissue layers can lead to moisture loss and shrinking in storage.
Geographical distribution: the disease has been reported from Africa, Asia, the Pacific Islands, Caribbean, North America and South America.
Symptoms: begin as small brown spots on the fleshy root surface. These enlarge and coalesce to give discoloured, superficial, necrotic areas of varying sizes and shapes, without a definite outline (Figure 41,left). The colour of the spot or lesion depends on the skin colour with copper-skinned cultivars having brown lesions and red ones having almost black lesions. There is no general rupture of the epidermis. Only the periderm is affected, but this may cause increased water loss from roots in storage. Infection may cover most of the storage root surface in storage. Above-ground parts of the plant are not affected unless in contact with infected soil. When slightly infected slips are planted, the fungus colonizes the periderm in the root system and spreads on the outer tissue of fleshy roots. In storage the diseased areas increase and can cover the entire root giving a very unpleasant appearance. Affected areas become leathery and dry, there is a loss of moisture and consequently shrinkage of the root.
Biology and ecology: the fungus is soil borne for a short time but may survive longer in soil high in organic matter, such as animal manure. The disease is more severe in heavy soils. Scurf is mostly restricted to more temperate sweetpotato growing areas, presumably due to a temperature effect. It may also be due to the different methods of propagation. In the tropics this is almost always by direct field planting of stem cuttings. In temperate regions propagation is by sprouts or slips obtained by planting smaller storage roots in nursery beds (or hot beds); when the sprouts are some 25 cm long they are transplanted to the field. The fungus is thus spread from infested nursery beds to the field where the developing storage roots are infected. In storage the optimum temperature for disease development is around 24°C, but temperature is not a limiting factor since the fungus can develop up to a certain extent within a wide range of temperatures. Moisture does not limit the growth of the fungus in the soil although the disease is most severe during the rainy season and in low, wet soils (Figure 41, right). The disease further develops during storage but no new infections are observed.
Morphology of the pathogen: on the host, definite vegetative hyphae are lacking. The conidiophores are septate, erect, unbranched, dark, measure 40-300 µm long and 4-6 µm wide, and attached to the host by a bulb-like enlargement. The conidiophores usually emerge from the host either singly or in pairs. The conidia are formed successively into chains that soon bend. They are aseptate, oblong to ovoid, unicellular, 12-20 x 4-7 µm in size, and are initially hyaline, then turn light brown (Figure 42).
Host range: the disease affects only sweetpotato naturally but the pathogen can infect other convolvulaceous plants. No other host has been found.
Host-plant resistance: although cultivars differ in reaction to infection there is apparently no high host resistance.
- Quarantine, avoid mobilising unclean planting material.
- Rotation (2-3 years in lighter soils and 3-4 years on heavier soils).
- Cutting plants at least 2-3 cm above the soil line.
- Dipping of planting material in hot water 0.5 minutes at 55°, or 5 minutes at 49 °C.
Chemical control: treating storage roots with thiabendazole or dichloran before bedding them for plant production.
Figure 41 Scurf caused by Monilochaetes infuscans. Brown spots merge and discolour the storage root surface (left). A hill of sweetpotatoes showing scurf lesions that had spread from the stem of the mother plant onto the daughter roots (centre). The colour of the spot or lesion depends on the skin colour with copper-skinned cultivars having brown lesions and red ones having almost black lesions (right).
Figure 42 Scurf caused by Monilochaetes infuscans. Conidiophores of Monilochaetes infuscans: arrow indicates a percurrent regeneration of the conidiophore (AF); conidia (GI).
Septoria leaf spot caused by Septoria bataticola Taubenhaus (1914). The taxonomy of the pathogenis is kingdom: Fungi R.T. Moore (1980); phylum: Ascomycota Caval.-Sm. (1998); class: Dothideomycetes O.E. Erikss. & Winka (1997); subclass: Dothideomycetidae P.M. Kirk et al., 2001 ex C.L. Schoch et al., 2006; order: Capnodiales Woron., 1925; family: Mycosphaerellaceae Lindau, 1897; genus: Septoria Sacc. (1884).
Septoria leaf spot is caused by a fungus Septoria bataticola remotely similar in general appearence to that causing leaf blight. It is characterized by cìrcular, whìte spots about 3.2 mm in diameter, scattered indiscrìminately over the areas one or more black specks, just visible to the naked eye, may be seen. These specks contaìn numerous spores, which upon escaping may be carried by insects or other agencies to other leaves where new infection may start.
Like the organism causing leaf blight, this fungus in not kniwn to be parasitic on other species or other part of the sweetpotato. It probably overwinters on the dead leaves in the field. Septoria leaf spot is very widely distributed, having been collected in New Jersey, Delaware, Iowa, and other States where sweetpotatoes are grown. This disease is nowhere serious enough to require the application of remedial measures such as micro sulf (sulfur) employ.
Figure 43 Symptoms of Septoria leaf spot on leaf caused by Septoria bataticola (left). Pycnidium of Septoria bataticola containing elongated threadlike spore named scolecospores = filiform spores (right).
Storage rot caused by a complex of fungi such as Epicoccum Link ex Steudel 1824 spp., and Fusarium solani (Mart.) Sacc. 1881 (Figure 27), and Mucor racemosus Bull. 1791 (Figure 44, A), and Sclerotinia Fuckel 1870 spp. Other fungi, such as Aspergillus niger (Figure 34), Fusarium oxysporum (Figure 26)), Rhizopus stolonifer (Figure 37), Botryodiplodia theobroma = Lasiodiplodia theobromae, anamorph (Figure 33 and Figure 44, B); Botryosphaeria rhodina (Berk, & Curt.) v. Arx 1970 (teleomorph) and Penicillium sp. (Figure 32) were found to be associated with this deterioration of sweetpotato root-tubers.
Storage rot prevention consists in the modify of the atmosphere created by the packing the samples inside a polythene bag of 18 μm thickness significantly maintained the quality
of the samples. Inoculation of samples with the four prevalent rot fungi significantly increased crude protein, lipid and ash content. In contrast, there was a significant decrease in
carbohydrate and moisture content when compared with the controls. Modified atmosphere is therefore recommended for control of sweet potato rots and extension of storage life.
Epicoccum is one of the pathogens that causing storage rot (Figure 44). Epicoccum belongs to the Kingdom Fungi; phylum Ascomycota; class Ascomycetes; subclass Incertae sedis; order Incertae sedis; family Incertae sedis; genus Epicoccum; species Epicoccum nigrum. The latter is one of the species that can causes the syndrome of storage rot of sweetpaper. Although Epicoccum can not be regarded as of much economic importance, it was so often isolated from rotted sweetpotatoes held at low temperatures that it can not be passed over without mention. It grows rather slowly and is probably able to cause decay only at such temperatures at which the competition of other fungi is reduced. From a lot of sweet potatoes which had been thoroughly washed and then stored at 0°, 5°, and 10 °C. Epicoccum sp. was about the only fungus isolated. Later a series of experiments were conducted in which sound potatoes were inoculated with pure cultures by the "well" method and exposed to the temperatures of chambers 3 (7.19 °C), 6 (14.4 °C), 9 (20.9 °C.), and 11 (26 °C) of the Altmann thermostat. All the potatoes in chamber 3 were slightly to completely decayed in three weeks. In the other compartments they remained sound. Epicoccum sp. produces a slow, firm rot. The tissue is rendered at first slightly yellowish followed by a reddish brown color.
Epicoccum is a ubiquitous, cosmopolitan. Conidia dimensions 15-25 microns. Epicoccum is a cosmopolitan fungus with a worldwide distribution. The conidiophores of Epicoccum are short, undistinguished and grouped in clusters. Spores are dark brown, globose and muriform with septa in both directions, like a soccer ball (Figure 44, H). Spores are often observed on colonies growing in culture as little black dots.
Mucor racemosus colonies are very fast growing, cottony to fluffy, white to yellow, becoming dark grey with the development of sporangia. Sporangiophores are erect, simple or branched, forming large (60-300 um in diameter) terminal, globose to spherical, multi-spored sporangia, without apophyses and with well developed subtending columellae (Figure 44 A, B). A conspicuous collarette (remnants of the sporangial wall) is usually visible at the base of the columella following sporangiospore dispersal. Sporangiospores (Figure 44, C) are hyaline, grey or brownish, globose to ellipsoidal and smooth-walled or finely ornamented. Stolons and rhizoids are absent, however, chlamydoconidia and zygospores may be present.
Mucor spp is a filamentous fungus found in soil, plants, decaying fruits and vegetables. As well as being ubiquitous in nature and a common laboratory contaminant, Mucor spp. may cause infections in man, frogs, amphibians, cattle, and swine. Most of the Mucor spp. are unable to grow at 37°C and the strains isolated from human infections are usually one of the few thermotolerant Mucor spp.
Mucor can be a dangerous mold that can adversely affect one's respiratory system. Some species can be a possible cause of the dangerous mold disease zygomycosis.
About macroscopic features, colonies of Mucor grow rapidly at 25-30 °C and quickly cover the surface of the agar. Its fluffy appearance with a height of several cm resembles cotton candy. From the front, the color is white initially and becomes grayish brown in time. From the reverse, it is white. Mucor indicus is an aromatic species and may grow at temperatures as high as 40 °C. Mucor racemosus and Mucor ramosissimus, on the other hand, grow poorly or do not grow at all at 37 °C.
About microscopic features, nonseptate or sparsely septate, broad (6-15 µm) hyphae, sporangiophores, sporangia, and spores are visualized (Figure 44, A, B, C). Intercalary or terminal arthrospores (oidia) located through or at the end of the hyphae and few chlamydospores may also be produced by some species. Apophysis, rhizoid and stolon are absent. Sporangiophores are short, erect, taper towards their apices and may form short sympodial branches. Columella are hyaline or dematiaceous and are hardly visible if the sporangium has not been ruptured (Figure 44, A). Smaller sporangia may lack columella. Sporangia are round, 50-300 µm in diameter, gray to black in color, and are filled with sporangiospores. Following the rupture of the sporangia, sporangiospores are freely spread. A collarette may sometimes be left at the base of the sporangium following its rupture. The sporangiospores are round (4-8 µm in diameter) or slightly elongated. Zygospores, if present, arise from the mycelium.
The branching of sporangiophores (branched or unbranched), the shape of the sporangiospores (round or elongated), maximum temperature of growth, presence of chlamydospores, assimilation of ethanol, and molecular analysis aid in differentiation of Mucor spp. from each other.
Colonies of Lasiodiplodia theobromaeare greyish sepia to mouse grey to black, fluffy with abundant aerial mycelium; reverse fuscous black to black. Pycnidia are simple or compound, often aggregated, stromatic, ostiolate, frequently setose, up to 5 mm wide. Conidiophores are hyaline, simple, sometimes septate, rarely branched cylindrical, arising from the inner layers of cells lining the pycnidial cavity. Conidiogenous cells are hyaline, simple, cylindrical to subobpyriform, holoblastic, annellidic. Conidia are initially unicellular, hyaline, granulose, subovoid to ellipsoide-oblong, thick-walled, base truncate; mature conidia one-septate, cinnamon to fawn, often longitudinally striate, 20-30 x 10-15 µm (Figure 44, . Paraphyses when present are hyaline, cylindrical, sometimes septate, up to 50 µm long.
Figure 44 Storage rot. Sporangiophorum (asexual) of Mucor racemosus with dark columella at the sporangium base (A); Mucor racemosus sporangiophora (B); sporangiospores (C). Lasiodiplodia theobromae: periodic acid-Schiff-stained histological sections of the mold, showing aggregates of pycnidia namely "stromatic pycnidia" (D); iycnidium with an obvious neck and conidiogenous cells and paraphyses (sterile filaments among conidia) lining the internal wall (E and F); mature and old two-celled dark brown conidia with typical and ongitudinal striations, released from the pycnidia (G). Epicoccum spp. that which contributes to the syndrome caused by various biological factors: spores dark brown, globose and with muriform wall (H).
Surface rot caused by complex Fusarium oxysporum Schlechtend. (1824) and Fusarium solani (Mart.) Sacc. 1881.
Violet root rot caused by Helicobasidium mompa Nobuj. Tanaka 1891, teleomorph; Rhizoctonia crocorum (Fr.) (Pers.) DC. ex. Mérat. 1915 (anamorph). Taxonomy of Rhizoctonia crocorum is domain Eukaryota Chatton, 1925; Amorphea Adl et al., 2012; Opisthokonta Cavalier-Smith, 1987; Holomycota Liu et al., 2009; Kingdom Fungi; subkingdom Dikarya D.S. Hibbett et al., in D.S. Hibbett et al., 2007; phylum Basidiomycota R.T. Moore (1980); subphylum Agaricomycotina; class Agaricomycetes; order Cantharellales; family Ceratobasidiaceae; genus: Rhizoctonia (Pers.) de Condolle 1816. Taxonomy of Helicobasidium mompa is domain Eukaryota Chatton, 1925; Amorphea Adl et al., 2012; Opisthokonta Cavalier-Smith, 1987; Holomycota Liu et al., 2009; Kingdom Fungi; subkingdom Dikarya D.S. Hibbett et al., in D.S. Hibbett et al., 2007; phylum Basidiomycota R.T. Moore (1980); Kingdom: Fungi; phylum: Basidiomycota R.T. Moore (1980); subphylum Pucciniomycotina R. Bauer, Begerow, J.P. Samp., M. Weiss & Oberw. (2006); class Pucciniomycetes R. Bauer, Begerow, J.P. Samp., M. Weiss & Oberw. 2006; subclass Incertae sedis; order: Helicobasidiales R. Bauer et al., 2006; family Helicobasidiaceae P.M. Kirk 2008; genus Helicobasidium Pat., 1885.
Economic importance: violet root rot caused by Helicobasidium mompa is a root disease that develops in the field during the growing season. Affected plants die toward the end of the growing season. No reports about the effect on yield have been found, but since the disease causes plants to die, there should be a certain reduction in yield. The direct effect of the disease on the fleshy roots can cause further losses.
Geographical distribution: China, India, Japan, Korea Republic and Taiwan.
Morphology: the teleomorph stage of the Helicobasidium mompa consists of an apically coiled, hyaline, cylindrical basidia measuring 6-7 x 25-40 µm, with three septae and four sterigmata that bear kidney shaped to ovoid binucleate basidiospores that measure 6.0-6.4 x 16-19 µm. The anamorph stage forms a thick sterile mycelium branched in straight angle, with a little narrowing on the branching point. The mycelium is initially white, becoming purplish and purplish brown later on. Mature mycelium forms strands and cushions on the soil surface, as well as, sclerotia. Sclerotia are flat to irregular in shape and when transversally cut, show a purple colour outside and white colour inside.
Symptoms: the disease is called violet root rot due to the colour of the mycelial mats or cushions of the fungus that cover the affected parts of the plant, especially at the soil line. H. mompa is present as mycelium on the soil surface of infected fields and it is parasitic on the below-ground parts of the plant. Once the plant is infected and the root system is invaded, the foliage becomes chlorotic and the leaves at the base of the plant abscise prematurely. The fibrous roots rot and are packed together by a purplish brown to violet mycelial mat. The fleshy roots also rot and are covered by bundles of packed mycelia that creep on the root surface, giving a web-like appearance. The most conspicuous characteristic of an infested soil is the presence of mycelial cushions and bundles on the soil surface under the plant. Initially white, they become pink to brown then purple brown or violet with age. Symptoms start developing toward the middle of the growing season.
Biology and ecology: the fungus lives in the soil and spreads from plant to plant through the mycelium that creeps on the soil surface. It can survive in the soil for at least 4 years, mainly as sclerotia but also as mycelial strands. Sclerotia are formed at the end of the growing season, when there are no nutrients available. As soon as the host and enough moisture are present, the sclerotia start developing and invade the host. This occurs during the early part of the growing season. Dispersion is by rain and irrigation water through the movement of infested soil especially if the fields are on a slope (erosion associated with water movement). When the soil is irrigated for a new crop the fungus grows outside the plant on the soil surface during the early part of the growing season, forming infection cushions from which infecting hyphae penetrate the host and invade the middle lamellae of the tissue in the root system. Disease severity depends on environmental factors such as temperature. Even though, the fungus develops in a wide range of temperatures (8-35 °C), its best performance is around 27 °C. Other favourable conditions are high moisture, such as that present during the rainy season, poor drainage and acid soils, such as those prevailing in forest soils due to the presence of partially decomposed organic matter. No information has been found about the role of the basidiospores in the disease cycle. Helicobasidium mompa produces a pigment - helicobasidin - that has toxic effect on some higher plant species and microorganisms.
Host range: the fungus has a wide host range (plurivorous) and also grows well in decomposing organic matter. It has been reported attacking: Morus (mulberry tree), Malus domestica (apple), sugar beet, soybean, potato, cotton, peanuts, tea, plum and grape but probably also infects other hosts.
Detection and inspection: a few weeks after planting and when the plant is developing a thick canopy, it is possible to observe the disease under the plant canopy and on the soil surface as a whitish mycelial growth. Soon after this mycelium becomes pinkish, then purple and finally purple brown. Towards the middle of the growing season, the mycelium starts invading the plant, and the affected parts (the base of the plant at the soil line) show the mycelial mat. But the most obvious sign of the disease can be observed toward the end of the growing season; when the affected plants are dug and all the symptoms described above can be observed, such as the mycelial web growing on rootlets and fleshy roots and the rotting of such organs.
Host-plant resistance: Early maturing varieties can escape the disease.
- Sanitation. Crop residues should be destroyed either by burning or deeply burying infected plants.
- Lime amendments to reduce the acidity of the soil.
- Rotation for more than 3 years with cereals can prevent the disease.
- Early harvesting before the disease becomes severe.
Chemical control: nothing has been written about the use of chemicals to control the disease.
Nematodes are the most numerous multicellular animals on earth. A handful of soil will contain thousands of the microscopic worms, many of them parasites of insects, plants or animals. Free-living species are abundant, including nematodes that feed on bacteria, fungi, and other nematodes, yet the vast majority of species encountered are poorly understood biologically. There are nearly 20,000 described species classified in the phylum Nemata .
Nematodes are structurally simple organisms. Adult nematodes are comprised of approximately 1,000 somatic cells, and potentially hundreds of cells associated with the reproductive system. Nematodes have been characterized as a tube within a tube ; referring to the alimentary canal which extends from the mouth on the anterior end, to the anus located near the tail. Nematodes possess digestive, nervous, excretory, and reproductive systems, but lack a discrete circulatory or respiratory system. In size they range from 0.3 mm to over 8 meters.
Soil Biota: soils are populated by a multitude of microbial and invertebrate organisms, in addition to more complex animal biota. Plant roots, seeds, and fungi, are a large part of this microhabitat. Soil microorganisms play an extensive role in the decomposition of organic matter and production of humus, cycling of nutrients and energy and elemental fixation, soil metabolism, and the production of compounds that cause soil aggregates to form. Many are in symbiotic relationships with plants and animals serving as nitrogen fixers and gut microbes. They function as a substantial part of the food web.
Among the microorganisms found in the soil are bacteria, actinomycetes, fungi, micro-algae, protozoa, nematodes, and other invertebrates (mostly arthropods).
It has been estimated that if you look at one gram of soil you will see the following numbers of organisms - bacteria 108-9 , actinomycetes 1058 , fungi 1056 , micro-algae 1036 , protozoa 1035 , nematodes 1012 , other invertebrates 1035 . A square meter of soil may contain 30300 earthworms. There are more organisms in a gram of soil than there are human beings on this Earth.
Phylum Nemata: the word Nematoda comes from the Greek words nematos, meaning thread, and eidos, meaning form. Over the years, nematodes have been classified in four different phyla, not always under the same name. There are two contending names for the phylum of nematodes.
In 1919, Cobb named the study of nematodes nematology and therefore wished to rename nematodes nemata. Cobb also placed nematodes in their own phylum, the phylum Nemata. However, when nematodes were placed in the phylum Aschelminthes, they were classified as class Nematoda (along with class Rotifera, class Gastrotricha, class Kinorhyncha, class Priapulida and class Nematomorpha). In 1932, Potss elevated class Nematoda to the level of phylum, leaving the name the same. While both names have been used (and are still used today), many believe (including Maggenti, Luc, Raski, Fortuner and Geraert, 1987) that Nemata is a more precise name. When a reference is made to Nemata, there is no doubt that it is the phylum being referred to whereas when an author makes a reference to Nematoda, the author could mean either the phylum or the class. In addition, the name Nemata was used first and therefore should be given priority. While nematodes are generally accepted as being a phylum, debate is still ongoing concerning their relationship to other animals grouped together on the basis of the structure of the body cavity.
Nematode Body Structure: the phrase tube-within-a-tube is a convenient way to think of nematode body structure, and also a term used to refer to a major trend in the evolution of triploblastic metazoa (Brusca and Brusca, 1990 Invertebrates). It refers to the development of a fluid-filled cavity between the outer body wall and the digestive tube. The nature of this body cavity has led to the grouping of metazoa into three grades, acoelomate, pseudocoelomate, and eucoelomate. Nematodes together with Rotifera, Gastrotricha, Kinorhyncha, Nematomorpha, Acanthocephala, and Entoprocta are traditionally grouped together as pseudocoelomates, on the basis of possessing a body cavity that is not formed from the mesoderm or fully lined by peritoneum. However, there are some problems in applying this concept to nematodes. Cell lineage studies on the nematode Caenorhabditis elegans have demonstrated that most tissues in the nematode are of mixed lineage, derived from several different sources of embryonic tissue. Also, not all nematodes retain a spacious fluid-filled cavity, as can be seen in a cross section of the esophageal region of the monohysterid marine nematode, Theristis.
Nematode digestive system: the nematode digestive system is generally divided into three parts, the stomodeum, intestine, and proctodeum. The stomodeum consists of the mouth and lips, buccal cavity, and the pharynx (esophagus). Each of these regions are used extensively in taxonomy and classification of nematodes, as well as providing as indication of feeding habit or trophic group. For example, the buccal cavity of plant parasitic nematodes (and some insect parasites ) is modified in the form of a hollow spear, adapted to penetrate and withdraw the contents of host cells. Predaceous nematodes often have a buccal cavity characterized by teeth or hook-like projections. The buccal cavity of bacterial feeding nematodes is relatively unadorned.
Bacterial feeding nematodes have the least modified or diversified stomodoeum structures. The basic plan is a circular opening surrounded by six lips, sometimes fused into three or less lips. The structures called lips are actually cuticularly lined area of the mouth that are exposed to the outside. This opens into the buccal cavity, a triangular or cylindrical tube that can contain small teeth. Muscles extending from the body wall to the cuticular lining expand the lumen and suck food through the mouth into the buccal cavity. The buccal cavity terminates in a valve-like glottoid apparatus leading to the pharynx, also referred to as the oesophagus.
Nematode reproductive system: while there is much diversity represented in the reproductive structures of the Phylum Nemata, there are many features that are typical of the phylum. Male nematodes are usually smaller than their female counterparts. Basic male reproductive structures include: one testis, a seminal vesicle and a vas deferens opening into a cloaca. One testis is most common, but two testis are found in some species, while in others one testis is reduced. Spermatogonia are produced in the testis and stored in the seminal vesicle until the nematode mates. The presence of one or two copulatory spicules help dialate the vulva and can also serve as a canal for the spermatozoa. The spicules are made from hardened cuticle, terminating in sensory dendrites near the tip. Often the body wall around the cloaca is modified into a bursa, which helps orient the male nematode and then helps hold the two nematodes together. Spermatozoa are amaeboid, and can have many different modifications. Some spermatozoa are round to ovoid in shape while others bear a resemblance to flagellated sperm. Different types of spermatozoa characterize different taxonomic groups of nematodes. Basic female structures include: one or two ovaries, seminal receptacles, uteri, ovijector and a vuvla. The ovary produces oogonia, which later develop into oocytes. The seminal receptacles, sometimes developed into a spermathecea, stores the spermatozoa until they are needed to fertilize an ooctye. The fertilized oocyte then develops into an egg in the uterus. The uteri often ends in an ovijector. The ovijector is very muscular and uses body movement combined with the high internal body pressure of the nematode to expel the egg through the vagina. All nematodes lay eggs. Syngamy, or cross fertilization, is common in most nematodes. Hermaphroditism also occurs, with the nematode gonads producing spermatozoa first and storing them until the eggs are produced. Parthenogenesis is also a normal means of reproduction in some nematodes.
Nematodes are tiny worm-like animals. Many species live in soils, but only a few are serious parasites of plant roots. Parasitic nematodes generally have a wide host range; they are not specific to sweetpotato.
- Brown ring of roots is a nematode injury that varies among species, but can include galls (abnormal growth or corky localised swelling of a plant part such as leaves, shoots or roots, caused by a fungus, bacterium, nematode, insect or some other organisms; insects like gall mites, aphids, gall wasps, and gall midges and the root knot nematode produce galls on plant parts) on roots and tubers (root-knot), necrosis of roots (root-lesion), or a subtle stubby-appearance (stubby-root). Above ground plant symptoms may range from no apparent injury to less vigorous growth to stunting, yellowing, wilting, and death. Some nematodes also feed on tubers directly, causing small, shallow lesions or raised, scab-like lesions (root-lesion), internal brown to black necrotic spots and external raised bumps (root-knot), or brown, necrotic rings that may extend from the periderm to deep within the tuber (Figure 45, A). The nematodes causing brown ring of roots are:
- Ditylenchus dipsaci (Kühn 1857) Filipjev 1936. The taxonomy of this nematode species is the following: Natura; Mundus Plinius; Biota; domain Eukaryota Chatton 1925; Amorphea Adl et al. 2012; Opisthokonta Cavalier-Smith 1987; Holozoa; kingdom Animalia Linnaeus, 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Eubilateria Ax 1987; Protostomia Grobben 1908; Ecdysozoa A.M.A. Aguinaldo et al. 1997; Introverta C. Nielsen 1995; Nematoida A. Schmidt-Rhaesa 1996; phylum Nematoda Cobb 1932 (round worms); class Secernentea Von Linstow, 1905; subclass Tylenchia (Thorne 1949) Inglis 1983; order Tylenchida G. Thorne, 1949; suborder Tylenchina (Thorne, 1949) Chitwood 1950; infraorder Anguinata Siddiqi 2000; superfamily Tylenchoidea Nicoll, 1935 (1926); family Anguinidae Nicoll (1926); subfamily Anguininae Nicoll 1935 (1926); genus Ditylenchus Filipjev 1936.
Ditylenchus dipsaci is one of the most devastating plant parasitic nematodes on a wide range of crops. In heavy infestation crop losses of 60-80% are not unusual; e.g., in Italy up to 60% of onion seedlings died before reaching the transplanting stage and for garlic crop losses of about 50% were recorded from Italy and more than 90% from France and Poland. In Morocco Ditylenchus dipsaci was found in 79% of seed stocks of Vicia faba examined.
Body marked by transverse striae, about 1 µ apart, which are easily visible under the oil immersion at any point on the body. Lateral field marked by four incisures. Deirids usually visible near base of neck. Hemizonid adjacent to excretory pore, about six annules wide. Phasmids rarely visible and then only from a dorsal or ventral view on favorable specimens. Amphid apertures on apices of lateral lips, where they appear as minute refractive dots which can be seen only from a face view. Spear with strongly developed knobs from which protrudor muscles lead to the well-sclerotized cephalic framework. Basal esophageal bulb with the usual three prominent and two inconspicuous, gland nuclei. Intestine connected to esophageal lumen by a very small valvular apparatus. Ovary outstretched, sometimes reaching to median esophageal bulb, but more often near basal bulb, rarely with one or two flexures. Oocytes lie largely in tandem and develop into eggs which are two to three times as long as the body diameter. Rudimentary posterior uterine branch present, extending about half-way back to anus. Vulva-anus distance equal to 1 3/4 to 2 1/4 times tail length. Teminus always acute. Testis outstretched, with spermatocytes arranged in single file except for a short region of multiplication. From a perfectly lateral view the spicula exhibit a sclerotized pattern that apparently is characteristic of the species, but the proper angle of observation is so difficult to obtain that the pattern is rarely of taxonomic value. Bursa rising opposite proximal ends of spicula and extending about three-fourths the length of the tail. Lateral incisures ending in a pattern as illustrated (Figure 45, C).
- Ditylenchus destructor Thorne 1945. Ditylenchus destructor is a plant pathogenic nematode commonly known as the potato rot nematode. Other common names include the iris nematode, the potato tuber eelworm and the potato tuber nematode. It is an endoparasitic, migratory nematode commonly found in areas such as the United States, Europe, central Asia and Southern Africa.
Cuticle near head marked by transverse striae about 1 u apart, while on the remainder of the body the striae are obscure unless the specimens are shrunken by cold fixative. Lateral fields with six incisures, which are reduced to two on the neck and tail. Deirids usually visible near base of neck. Hemizonid slightly anterior to excretory pore. Phasmids not observed. Cephalic papillae and amphids visible only from a face view, arranged in a manner similar to those figured for Ditylenchus dipsaci. Labial framework well sclerotized. Spear typical of the genus, with rounded knobs. Basal bulb of esophagus with three large and two small gland nuclei, generally extended in a lobe reaching back over the dorsal side of the intestine. This lobe may be either shorter or longer than that illustrated. Anterior end of intestine extending into base of esophagus, where it connects with the lumen by an obscure valvular apparatus. Intestine densely granular, ending in a distinct rectum and anus. Anterior ovary outstretched to near base of esophagus, the developing oocytes often arranged in two lines, changing to tandem near the middle of the ovary. Eggs average slightly longer than body diameter and are about one-half as wide as long. Posterior uterine branch rudimentary, not observed to function as a spermatheca. Spermatozoa usually well up in the uterus. Lips of vulva thick, elevated. Vulva-anus distance 1 3/4 to 2 1/2 times tail length. Testis outstretched to near base of esophagus with spermatogonia arranged mostly in single line until near the middle of the body, where they become primary spermatocytes from which the spermatozoa are produced. Spicula with a distinctive sclerotized pattern as illustrated. Bursa extending from a point about opposite the proximal ends of the spicula to about two-thirds the tail length. Lateral incisures usually reduced to four near the tail, forming a pattern similar to that figured for Ditylenchus dipsaci (Figure 45, D).
Their life cycle takes place inside potato tubers and sweetpotato root-tubers where they eat starch grains. This causes the affected tissues to become brown and powdery, and the surface of the tuber becomes covered with dark patches with dry cracking skin. The nematodes live inside the living tissue where they aggregate rapidly as the fecund females each produce up to 250 eggs. They survive in stored tubers during the winter and can infect the stolons of planting material. After infection, the nematodes move throughout the plant tissue producing a pectinase enzyme, which causes cell degeneration and is the main causal agent of the rot observed. The soil plays only a secondary role in the transfer of this nematode.
The life cycle of Ditylenchus destructor lasts approximately 6 days. As Ditylenchus destructor is an endoparasite, a majority of the life cycle occurs inside the host tissue. There are four molting periods and juvenile stages of development for Ditylenchus destructor with the first juvenile stage occurring within the egg. Females deposit eggs inside the tuber from their ovaries at which point the embryos begin undergo a cleavage process, beginning the first juvenile stage. Two and a half hours later the juvenile nematode can be seen through the egg wall, and 48 hours later the first larval stage has completed and hatching occurs. Hatching marks the molting into the second juvenile stage, and development continues until the next molting. In the third juvenile stage the sexual structures begin to develop and become visible. This development of additional structures causes large amounts of growth and elongation is seen in the nematode (especially in females who have more development occur). It is in the fourth stage where the sexual structures fully develop: vaginal development in females and testes development in males. At this point the nematodes undergo their final molting and enter the final, adult stage in their life cycle. After feeding on a host for some time, the females lay eggs inside the tuber, the eggs are fertilized by a male, and the cycle repeats.
As an migratory, endoparasities, Ditylenchus destructor females lay eggs throughout the plant tissue while moving from cell to cell. Once they have hatched, the juvenile nematodes will either move throughout the surrounding plant tissue or out of the plant from which they hatched to a nearby, healthy host. Ditylenchus destructor is not very mobile through soil though, so dispersal primarily occurs during harvest or transportation of the host when healthy tubers are in the immediate vicinity. If the nematodes exit the initial host, they most commonly infect the tubers of a new host; however, it is sometimes possible for them to infect the above ground parts of the plant and migrate to the tubers through the plant cells.
Unlike other nematodes, Ditylenchus destructor does not have a resting form, so environmental conditions have a large impact on the habits of the organism. The most optimal conditions for the nematodes are soils that are around 28 °C. Temperatures above or below this range will inhibit the movement and life cycle of the nematode; therefore, the 28 °C range provides optimal growth conditions. Moist soils are also especially favorable for the development of the nematode as well as its movement through the soil. Given these conditions, the most opportune locations for Ditylenchus destructor are in midwestern and southern North America as well as central parts of Europe and Asia. In these areas, agricultural practices also largely influence the spread of the pathogen. When harvesting and transporting tubers, moving them in large piles allows the nematodes to move from an infected tuber to surrounding healthy ones, encouraging the spread of disease.
In addition to potatoes, potato rot nematodes have a wide range of over 100 species of hosts from numerous families. Some of these include alfalfa, beets, carrots, garlic, hops, mint, parsnips, peanuts, rhubarb, tomatoes, and flowering plants such as irises and tulips. The nematodes only attack the below ground, non-aerial tissues of plants such as the roots, bulbs, rhizomes, and tubers.
The main symptoms of Ditylenchus destructor, common to potatoes and its other hosts, are the rotting and discoloration of subterranean plant tissue. In potatoes, early infection can be detected by small white spots underneath the potatos skin. As the disease progresses, these spots become larger and darker with a spongy or hollow appearance. Tubers develop sunken areas and the skin becomes dry, cracked, and detached from the underlying flesh. There is further discoloration at this stage that is often due to secondary invasions of fungi, bacteria, and free-living nematodes. Above ground symptoms are usually not seen, although heavily infected plants are often weaker, smaller and can have curled or discolored leaves.
Symptoms of Ditylenchus destructor on the bulbs of flowering plants such as irises and tulips are similar to those of potatoes, except infection usually occurs at the bulbs base and moves upwards. The fleshy scales develop discolored yellow to black lesions, the roots become blackened, and leaves can develop yellow tips. Potato rot nematodes in groundnuts, such as peanuts, develop blackened hulls, shrunken kernels, and embryos with a brown discoloration.
Potato rot nematodes are identified and diagnosed by various morphological and molecular methods. The morphological methods are the primary means of diagnosis, with molecular methods being used for when there is a low level of infestation or when only juveniles are present. Nematodes are extracted from infected plant tissue and examined microscopically for distinguishing characteristics such as body and stylet, and tail morphology. Molecular methods for identifying Ditylenchus destructor (especially in distinguishing from other Ditylenchus species) include PCRs (polymerase chain reactions) to find restriction patterns of DNA to identify specific species.
Management of Ditylenchus destructor can be achieved through various methods of preventative and chemical control. Once the nematodes have been established, they are very difficult to eradicate because of the wide range of other hosts on which they can survive. Therefore, preventative measures are generally the first means of control for potato rot nematodes. Planting materials and locations free of these nematodes is crucial, so the soil, seeds, and farm machinery must all be carefully controlled. This is done by disinfecting machinery and removing all potentially infected plant debris from farm equipment when transferring from field to field. Seeds certified to be free of Ditylenchus destructor should be planted. Crop rotation as a form of control is difficult due to the nematodes wide host range; therefore, non-host crops must be selected carefully to use for rotation each season. Weeds must also be eradicated, as they often act as alternative hosts for potato rot nematodes.
Chemical control of Ditylenchus destructor can be achieved with soil-applied nematicides such as carbofuran, ethylene dibromide, VAPAM HL, and TELONE. Fumigation with these nematicides is often paired with mechanical measures to attain optimal control. For example, Wisconsin has eradicated these nematodes from potatoes by repeated use of ethylene dibromide and restricting the movement of infected tubers. Another management method was demonstrated by the control of potato rot nematodes in garlic. The seeds were coated with thiram or benomyl wettable powder before being planted, resulting in very good control of the disease.
Ditylenchus destructor can reproduce at high rates and cause large amounts of damage to hosts and so can be very damaging to a cash crop. One of the most important impacts of Ditylenchus destructor is in South Africa, where it caused major disease amongst the peanut plantations in the early 1990s. In fields where the nematodes have been found, between 40% and 60% of the peanuts had large amounts of symptoms which heavily damaged the production levels. Similarly, the same issues were observed in Sweden in the 1970s when Ditylenchus destructor was found in their potato fields. In this instance it caused disease in between 40% and 70% of the potatoes being grown in the field where the pathogen was found. Finally, one of the more severe cases of the disease was found in Estonia in the 1960s. In this case the nematodes were only found on about 6% of the potato farms but on these farms between 70% and 90% of the potatoes exhibited severe symptoms. This high rate of disease caused immense damage to the fields that were infected and a large amount of crops were lost.
There have also been instances where the nematodes have affected areas within the United States as well. Since 1953, Wisconsin has had to quarantine multiple areas after finding Ditylenchus destructor in local potato fields. The pathogen has also been seen in Idaho where it was a major concern initially, as potatoes are a major state crop. Other states that have had Ditylenchus destructor issues include Arkansas, California, Hawaii, Indiana, New Jersey, North Carolina, Oregon, South Carolina, Virginia, Washington and West Virginia. Fortunately, the United States has used very stringent control laws to avoid any widespread or major damage similar to that seen in the past.
The complete list of countries that have been affected by Ditylenchus destructor include Azerbaijan, Bangladesh, China, India, Iran, Japan, Kazakhstan, Korea, Kyrgyzstan, Malaysia, Pakistan, Saudi Arabia, Tajikistan, Turkey, Uzbekistan, South Africa, Canada, Mexico, United States, Haiti, Ecuador, Peru, Albania, Austria, Belarus, Belgium, Bulgaria, Czech Republic, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Moldova, Netherlands, Norway, Poland, Romania, Russia, Slovakia, Span, Sweden, Switzerland, Ukraine, United Kingdom, Australia, and New Zealand.
Figure 45 Brown ring of roots. Fleshy roots, some time after they are stored, show symptoms as depressed areas (A). In cross sections, initial infections appear as necrotic isles of brown tissue scattered throughout the flesh. In advanced stages, the pulp becomes completely blackened, slightly soft, and corky (B). These nematodes affect fleshy roots only during storage. No symptoms have been found in the field. Microscopic characteristics of Ditylenchus dipsaci (C) and of Ditylenchus destructor (D).
- Burrowing nematode to which it belongs the genus Radopholus similis (Cobb 1893) Thorne 1945. The taxonomy of this nematode species is the following: Natura; Mundus Plinius; Naturalia; Biota; domain Eukaryota Chatton 1925; Amorphea Adl et al. 2012; Opisthokonta Cavalier-Smith 1987; Holozoa; kingdom Animalia Linnaeus, 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Eubilateria Ax 1987; Protostomia Grobben 1908; Ecdysozoa A.M.A. Aguinaldo et al. 1997; Introverta C. Nielsen 1995; Nematoida A. Schmidt-Rhaesa 1996; phylum Nematoda Cobb 1932 (round worms); class Secernentea Von Linstow, 1905; subclass Tylenchia (Thorn, 1949) Inglis 1983; order Tylenchida G. Thorne 1949; suborder Hoplolaimina Chizhov & Berezina 1988; superfamily Hoplolaimoidea (Filipjev 1934) Paramonov 1967; family Pratylenchidae (Thorne, 1949) Siddiqi 1963; subfamily Radopholinae Allen & Sher 1967; genus Radopholus Thorne 1949. Worldwide, the burrowing nematode, Radopholus similis, is considered the most damaging nematode species in the cropping systems.
Dimensions. Females L=0.52-0.88 mm; a=22-30; b=4.7-7.4; b=3.5-5.2; c=8-13; c=2.9-4.0; V=55-61; stylet=17-20 um. Males: L=0.54-0.67 mm; a=31-44; b=6.1-6.6; b=4.1-4.9; c=8-10; c=5.1-6.7; stylet=12-17 um; spicules=18-22 um; gubernaculum=8-12 um.
Specific characters. Female: Head composed of three to four annules. Lateral field with four longitudinal lines. Spermathecae round, with small, rod-shaped sperm. Hyaline part of tail 9-17 um long; terminus striated. Phasmids in anterior third of tail. Male: Head four-lobed, lateral sectors strongly reduced. Gubernaculum with small titillae. Bursa extends over about two-thirds of tail.
Adult males and females are different in appearance (sexual dimorphism), the males having poorly developed stylets and a knob-like head caused by an elevated, constricted lip region (Figure 46, A, B, C). Both males and females have long, tapered tails with rounded or indented ends (Figure 46, A, B and C). The male has a sharp, curved spicule (male reproductive organ), enclosed in a bursa, or sac (Figure 46, D). Females are between 550 and 880 µm (0.55 to 0.88 mm) in length and about 24 µm in diameter (Figure 46, E), with well-developed stylets 16 to 21 µm (average 18 µm) long (Figure 46, F).
Morphology of Radopholus similis: the signs (visible presence of the pathogen) of these diseases are the various stages of Radopholus similis observed in soil and plant root samples. All nematode stages are vermiform (wormlike), colorless and less than 1 mm in length. Adult males and females are different in appearance (sexual dimorphism), the males having poorly developed stylets and a knob-like head caused by an elevated, constricted lip region (Figure 46, B). Both males and females have long, tapered tails with rounded or indented ends (Figure 46). The male has a sharp, curved spicule (male reproductive organ), enclosed in a bursa, or sac (Figure 46, D). Females are between 550 and 880 µm (0.55 to 0.88 mm) in length and about 24 µm in diameter (Figure 46, E), with well-developed stylets 16 to 21 µm (average 18 µm) long (Figure 46, E). Males are smaller than females, 500 to 600 µm in length (Figure 46). Juveniles are often present in both root and soil samples and average between 315 to 400 µm in length with stylets 13 to 14 µm long (Figure 46, F).
Geographic distribution: originally described from the Fiji Islands, but occurs also in Australia, Florida, Central and South America, several Caribbean islands, tropical Africa. In the 1960�s it was imported into several European countries (France, Belgium, the Netherlands, Germany) with ornamental plants.
Hosts: the most important hosts are banana (toppling-over disease) and pepper yellows and during the years after World War II, pepper culture on the Islands of Banka, Indonesia, was almost completely destroyed. More than 250 other plant species are known to be hosts, among them many ornamentals such as Philodendron and Maranta and Convolvulaceae. To what extent it is a major pest of these has not yet been completely determined.
It has long been suspected that there exist different pathotypes within this species. Some indications for the occurrence of three pathotypes in Florida they are given: one parasitizing citrus only, one banana and citrus, and one banana but not citrus. The first was not examined further, but the existence of the two others was proven and these have since been known as the citrus race and the banana race, respectively. The citrus race is known from Florida only; the banana race is widely distributed. Later there came indications that there exists more than one banana race.
During the 1980's several differences between the citrus race and the banana race were discovered. Were found differences in oocyte maturation and in sex pheromones; in enzymes; in proteins. Was found the haploid chromosome number to be four in the banana race, five in the citrus race. Finally, was concluded that the two races were distinct species; the name Radopholus similis was restricted to the banana race; the citrus race was described as Radopholus citrophilus. was found that both Radopholus similis and Radopholus citrophilus are able to propagate by parthenogenesis, though the normal method is by amphimixis.
Most soilborne plant pests and diseases are not evident aboveground until they are well established. Early symptoms caused by root feeding pests are due to impaired water and nutrient uptake. These symptoms include stunted plant growth, decreased vigor and yield, premature leaf drop and an increased tendency to wilt or dieback during dry periods. Radopholus similis causes a slow decline of many plant species.
Figure 46 Radopholus similis cause burrowing. Adult male and females of Radopholus similis are different (sexual dimorphism), the males having poorly developed stylets and a knob-like head caused by an elevated, constricted lip region (A, B and C). The male has a sharp, curved spicule (male reproductive organ), enclosed in a bursa or sac (D). Females are between 550 and 880 µm (0.55 to 0.88 mm) in length and about 24 µm in diameter (E), with well-developed stylets 16 to 21 µm (average 18 µm) long (F).
Management of Radopholus similis in the tropics and subtropics is complicated by several factors. First, the nematodes most important agricultural hosts are perennial crops, making eradication or reduction of nematode populations difficult and expensive. In addition, subsistence agriculture is practiced in most developing countries in these latitudes so fallowing, crop rotation, clean planting material, or nematicidal chemicals may not be practical, affordable, or attainable. Finally, economic thresholds (nematode population at or above which crop loss would cost more than management efforts) for many of these crops have not been established. As a result, crop damage and high nematode populations may occur before the problem is addressed. Nevertheless, there are actions available to exclude, eradicate and avoid Radopholus similis, or to protect crops growing in infested soils.
Efforts between governments to exclude plant pathogens by quarantine are essential, but expensive and often defeated. Radopholus similis is present internationally in most areas where its host plants are grown, though some races are not widespread. For example, rooted citrus from Florida must currently be certified free of burrowing nematode (citrus race) before it can be accepted for import and distribution within the European Union (EU) or other citrus-growing states like California. However, the citrus race is not morphologically or genetically distinct, so all R. similis races are quarantined by the EU. Local contamination of clean fields and greenhouses can be prevented by using nematode-free planting material and controlling the movement of infested soil into new areas on tools, machinery, humans, or in soil or surface waters. Nurseries must use nematode-free planting material and maintain strict sanitary practices, including disinfesting pots and benches.
Destroying all burrowing nematodes in an area is theoretically possible, but practically difficult. Eradication requires removal of all belowground parts of the host crop, volunteers, alternative hosts, and all weed hosts: R. similis does not form survival structures and will eventually starve in the soil. The push and treat method for nematode control in citrus begins with bulldozing and removal of trees, roots and all, followed by nematicide and herbicide applications to reduce or eliminate host plant and nematode population growth. A restriction on the use of nematicides and their degradation by soil microorganisms, however, has reduced the effectiveness of this approach. Host removal is followed by either a clean fallow of 12 months, rotation with non-host crops, or planting a nematode-suppressive cover crop (e.g. Crotalaria). An efficient method of crop destruction in banana plantations is to inject pseudostems with the herbicide glyphosate. This causes roots and rhizomes to die and rot without the need to dig them up. Flooding the field for 8 weeks is effective but expensive and seldom practiced.
Other nematode species such as root-knot (Meloidogyne), lesion (Pratylenchus), citrus (Tylenchulus semipenetrans), or spiral (Helicotylenchus) nematode often occur with Radopholus similis and their populations may increase if Radopholus similis is eliminated. See other disease lessons on nematodes.
Damage by Radopholus similis can be avoided by planting disease-free crops in uninfested soil or a clean, soilless potting medium. Injury may be minimized in previously infested fields by planting after the eradication procedures mentioned above (e.g. fallow, crop rotation) have reduced nematode populations in the soil.
Cultural practices that maintain plant health and vigor may improve yield by allowing plants to rapidly replace roots damaged by Radopholus similis. Incorporating organic material into the soil provides nutrients and improves its cation-exchange and water-holding capacity, which may also increase the number of beneficial microorganisms and nematode antagonists in the root zone. Some cover crops, like sunnhemp (Crotalaria) or marigold (Tagetes), produce chemicals that are toxic to nematodes (allelopathy) and may reduce burrowing nematode populations in the soil.
Soil fumigation with methyl bromide, 1,3 dichloropropene, or chloropicrin can provide good control of burrowing nematodes but these fumigants must be applied pre-plant. Nonfumigant nematicides such as aldicarb and phenamiphos can be used post-plant and usually give systemic plant protection and reasonable control of burrowing nematodes. Chemical control of nematodes in general is decreasing in the U.S., however, due to regulatory restrictions, costs, and environmental concerns. Use of the important pre-plant fumigant methyl bromide, for example, has been restricted to a limited number of crops due to its effect on the ozone layer. Most nonfumigant nematicides are neurotoxic and can also affect humans and useful microorganisms. Some of these concerns may not be regulated in other growing regions of the world where burrowing nematodes exist and nematicide use in these locales can be relatively high. For example, nonfumigant nematicides are routinely applied in commercial banana plantations worldwide, increasing production up to 250%. Nematicides will not kill all parasitic nematodes in the soil and roots, so must be reapplied periodically. With repeated use, however, soil microorganisms become more efficient at degrading nematicides, making them less effective. The high cost of nematicides also limits their use by small growers.
Biological controls of plant-parasitic nematodes target different stages of nematode development using unique modes of action. The fungi Paecilomyces lilacinus and Trichoderma atroviride destroy nematode eggs, while toxic metabolites from non-pathogenic strains of Fusarium oxysporum inhibit or kill juveniles. Mycorrhizal fungi appear to compete with plant-parasitic nematodes for nutrients, such as phosphorus. The bacterium Pseudomonas kills juveniles and adults by producing lethal hydrogen cyanide. Conversely, Pasteuria spp. are bacterial hyperparasites (organisms that parasitize other parasites) that kill nematodes after attaching endospores to their outer cuticle. Despite their potential, the use of biological controls for plant-parasitic nematodes is quite limited at present.
Resistant varieties, when available, are central to any integrated nematode management strategy. Resistance to Radopholus similis is present in some banana hybrids, but their commercial qualities are still not equal to standard cultivated varieties. There are no black pepper varieties resistant to burrowing nematode, so favored cultivars are sometimes grafted onto the nematode-resistant rootstock of Piper colubrinum, a wild relative. Citrus and tea cultivars of high quality and yield are also grafted onto resistant rootstocks. The economic and environmental benefits of developing plant resistance to nematodes cannot be overemphasized and are a compelling area of research.
Temperate climate nematology had its beginnings in 1743 with discovery of the wheat seed gall nematode, Anguina tritici, by John T. Needham. In contrast, one of the first plant-pathogenic nematodes discovered from the tropics, Radopholus similis, was not described until Nathan A. Cobb found it in bananas from Fiji almost 150 years later. Most tropical and subtropical nematology laboratories were not established nor local problems addressed until the latter half of the 20th Century. This late start accumulating basic information, including the presence of burrowing nematode and the need for quarantine regulations, furthered the worldwide spread of Radopholus similis. The number of management strategies also lags behind those for temperate climate nematodes.
Local, state and international quarantine regulations are a significant economic consequence of burrowing nematode disease. Preshipment certification is often needed for plant export from infested areas to noninfested areas. This includes shipping rooted citrus from Florida to California, for example, or potted anthuriums from Hawaii to California and Japan. Certified nurseries also need a phytosanitary inspection and certificate for each shipment. Costs associated with compliance to quarantine regulations seriously impact growers and consumers on both sides of the law. Preventing the spread of burrowing nematode to new growing areas, however, remains a critical part of its management.
- Dagger. Dagger nematodes cause stunting and swelling of roots and poor above-ground growth. They tend to be most important on perennial crops.
- Xiphinema Cobb 1913 spp. are large nematodes, with an adult length between 1.55.0 mm. They have a long protrusible odontostyle, with 3 basal flanges at the posterior end of the stylet and a relatively posterior guiding ring when compared to the genus Longidorus. The odontostyle is lined with cuticle and alongside the esophagus serves as a good surface for viruses such as arabis mosaic virus to form a monolayer, which can be vectored to healthy plants. Xiphinema have a two-part esophagus, which does not contain a metacorpus. A modification in the posterior end of the esophagus forms a muscular posterior bulb, which can generate a pumping action similar to that of a metacorpus in other plant parasitic nematodes (Figure 47, A). The number of males varies from abundant to sparse depending on the species. Males have paired spicules but the gubernaculum and bursa are absent. Males of different species can be characterized using the varying number and arrangement of papillae. Females could have 1 or 2 ovaries. The Xiphinema genus is distributed worldwide. Two economically important Xiphinema species; X.index and X.americanum are both commonly found in California and tend to be problematic in vineyards. Xiphinema diversicaudatum is also found in parts of the U.S, as well as Europe and Australia.
- Xiphinema americanum Cobb (1913). The taxononomy of Xiphinema americanum is: Natura; Mundus Plinius; Naturalia; Biota; domain Eukaryota Chatton 1925; Amorphea Adl et al. 2012; Opisthokonta Cavalier-Smith 1987; Holozoa; kingdom Animalia Linnaeus, 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Eubilateria Ax 1987; Protostomia Grobben 1908; Ecdysozoa A.M.A. Aguinaldo et al. 1997; Introverta C. Nielsen 1995; Nematoida A. Schmidt-Rhaesa 1996; phylum Nematoda Cobb 1932 (round worms); class Adenophorea; subclass Enoplia Pearse 1942; order Dorylaimida; superfamily Dorylaimoidea; family Longidoridae Thorne 1935; subfamily Xiphineminae; genus Xiphinema Cobb 1913.
Xiphinema americanum (American dagger nematode) is a plant pathogenic nematode. It is one of many species that belongs to the genus Xiphinema. It was first described by N. A. Cobb in 1913, who found it on both sides of the United States on the roots of grass, corn, and citrus trees. Not only is Xiphinema americanum known to vector plant viruses, but also Xiphinema americanum has been referred to as "the most destructive plant parasitic nematode in America", and one of the four major nematode pests in the Southeastern United States.
About the morphology and anatomy, the length of the adult Xiphinema americanum ranges from 1.3 to 3.0 mm (Figure 47, B). The dagger nematode is characterized by a 100 μm odontostyle which is used for deep penetration of root tips with its spear-like stylet. The odontostyle is connected to the lining of the cheilostome by a folded membrane called the "guiding ring". The guiding ring is attached to a flanged odontophore.
Females: The body is usually in an "open C" conformation. The shape of the body tapers towards the extremities. The two rings of the odontophore are located 3 μm apart. The Xiphinema americanum esophagus is dorylaimoid with an enlarged posterior portion that occupies roughly 1/3 of its total length. The esophagus contains a muscular bulb which is 80 μm long and 20 μm wide. The valve between the esophagus and the intestine is amorphous. The vulva is 46-54% of the total body length, and is located equatorially with a transverse slit shape, with the vagina having a diameter of 1/3 of the body diameter. The ovaries normally occur in pairs, and are amphidelphic and relexed. The prerectum of Xiphinema americanum measures 120-140 μm long, with a rectum that is roughly the same length as the body diameter at the anus. The tail contains 2-3 pairs of caudal pores, is conoid, and curves dorsally with a subacute terminus.
Males: The males have a similar overall configuration as the females, but are slightly smaller in length. Males of Xiphinema americanum, however, are rarely found in nature. The male has diorchic testes that are connected to the cloaca, with one anterior branch and one posterior branch. It is common to find more coil in the posterior region. The males also have paired spicules but lack a gubernaculum and bursa.
Identifying Xiphinema americanum as a separate species has been a difficult task because of overlapping morphological aspects; however, differences in the life cycles of Xiphinema americanum may differentiate it from other species. Findings may also suggest that two subgroups of Xiphinema americanum should be made due to the finding of either 3 or 4 juvenile stages. The eggs of Xiphinema americanum are laid directly into the soil in water films, and are not associated with an egg mass. No molt occurs within the egg, which means that the first stage juvenile is the stage that enters the soil. Before becoming sexually mature adults, the Xiphinema americanum nematodes undergo three to four juvenile stages with a molt occurring between each.
Measurements of the functional and replacement odontostyles allows for the determination of the current stage in development. Compounding the issue of determining the life cycle of Xiphinema americanum is their difficulty with being grown in culture or greenhouse conditions. It has been suggested that this is due to Xiphinema americanum's sensitivity to moisture tension, temperature fluctuation, physical handling, or oxygen deprivation.
Field evidence taken over a 2-year observation period indicates that Xiphinema americanum are most likely k-selected; they most likely have a long life span and a low reproduction rate. Unpublished results have shown greenhouse observations of Xiphinema americanum to develop from egg to adult in 7 months. Other results have suggested that Xiphinema americanum can live as long as 35 years.
Reproduction by fertilization from a male is rare if not nonexistent due to the lack of male Xiphinema americanum individuals, and therefore females reproduce parthogenetically. All of the stages of Xiphinema americanum occur in the soil, with no particular stage as an important survival stage. In places with low winter temperatures, however, the egg is the primary survival structure.
Xiphinema americanum is a virtually non-specific plant nematode, causing it to have over one hundred different plant hosts. The most common plant hosts infected by Xiphinema americanum are common weeds and grasses, strawberries, soybeans, forest trees (spruce, pine, etc.), perennial orchards, and grapes. This broad host range is due to the genetic diversity within the Xiphinema americanum species.
The distribution of Xiphinema americanum is found widely throughout most of the world and is found on all of the continents, except for Antarctica. The region with the highest population of Xiphinema americanum is thought to be the Eastern United States. The states with the highest population of the American Dagger Nematode are Arkansas, California, Pennsylvania, Rhode Island and Virginia. Other countries where Xiphinema americanum is found include Australia, Belize, Brazil, Chile, Guatemala, India, Japan, Korea Democratic People's Republic, Korea Republic, Mexico, New Zealand,Pakistan, Panama, South Africa, Sri Lanka, Uruguay, and areas of the Caribbean as well.
This species of nematode is also found to be sensitive to soil pH, and they are found most frequently in soils with a pH of 6.0 or higher.
About the feeding habits, Xiphinema americanum is a plant parasite that lives entirely in the soil and is attracted to young, growing roots due to source-sink dynamics. These nematodes are migratory ectoparasites and all of the life stages of the American Dagger Nematode feed at the root tips of plants. Since it is a migratory ectoparasite, they remain outside the root or other feeding areas and feed on epidermal cells or on cells deeper in the root. This ectoparasitism allows the nematode to move freely to different hosts throughout its life cycle. This nematode is considered to be an obligate parasite. It can survive within plant debris, however it needs living plant tissue to feed. Once the nematode arrives at a root tip, it feeds by puncturing several successive layers of the plant's cells with its odontostyle; while penetrating, the nematode secretes enzymes that result in cell hypertrophy and thickening. The nematode is then able to begin extracting the cell's cytoplasm. The feeding period of Xiphinema americanum can last anywhere from several hours to several days, with the average being around 36 hours at each feeding site along the plant's roots. While feeding, it is common for the nematodes to remain still with their bodies either outstretched or curled, and following the feeding period they move slowly along the length of the root with their stylet remaining protruded and in search of a new feeding site. Unlike some species of nematodes, the observation of food passing into the gut of Xiphinema americanum is not seen. Although the nematodes are non-specific in their Host Range, they generally feed on plants that are in poorer condition for a shorter amount of time.
The symptoms that plants exhibit in response to the pathogenicity of Xiphinema americanum are similar to those of other migratory ectoparasitic nematodes of roots. It is common to see poor growth and or stunting of the plant, yellowing or wilting of the foliage, and reduced root systems which can include root necrosis, lack of feeder or secondary roots, and occasional tufts of stubby rootlets.
Young, shortleaved yellow pine trees with moderate swelling of roots with clusters of short, stubby branches were the first demonstration of Xiphinema americanum pathogenicity in 1955. Xiphinema americanum can also cause severe effects on foliage, sometimes causing chlorosis and complete defoliation as seen on Guatemalan coffee trees. The dagger nematode causes the devitalization of root tips and overall root death when they feed at the root tips and root sides of strawberry plants. Reddish-brown lesions that turn black and necrotic with time result at the sites of feeding, and result in reduced root systems and stunted tops.
Xiphinema americanum is listed as a C-rated pest in California due to its wide host range of California crops. C-rated pests are widespread, and are of known economic or environmental detriment, according to The California Department of Food and Agriculture. Due to Xiphinema americanum's difficulty in maintaining high populations in frequently tilled soils (see Control), the dagger nematode is mainly an economic problem on biennial and perennial crops rather than annual crops (except for damage to emerging seedlings).
The nematode Xiphinema americanum is an important transmitter of various plant viruses including tomato ringspot nepovirus (TomRSV), tobacco ringspot nepovirus (TRSV), peach rosette mosaic nepovirus (PRMV), and cherry rasp leaf nepovirus (CRLV). TobRSV is a widespread nepovirus in annual crops in North America that infects tobacco, soybean, blueberry, apple, ash, autumn crocus, blackberry, cherry, dogwood, elderberry, grapevine, spearmint, and in Wisconsin has an economically important impact on cucurbits.
TomRSV is another nepovirus transmitted by Xiphinema americanum, and is generally a problem with perennial plants including apple, grapevine, raspberry, strawberry, birdsfoot-trefoil, dogwood, elderberry, hydrangeas, orchids, and red currants. It is also a problem some annual plants including tomato and cucumber.
Apple, cherry, and peach trees in the Pacific coast states of the United States are infected by CRLV. PRMV causes substantial damage to Prunus spp., grapevine, and blueberry in the Great Lakes area.
Much like the broad host range of Xiphinema americanum, the 4 nepoviruses transmitted by this nematode do as well. They also have the capability of dissemination in wind-blown seeds as well as remaining harbored in natural reservoirs including weeds.>br>
In parallel tests, TomRSV has been shown to transmit more efficiently than TRSV. Primarily, the viruses reside in the regions of the stylet extension, the anterior esophageal lumen, and rarely in the esophageal bulb. TRSV has been shown to prefer the areas of the stylet extension and anterior esophageal lumen, whereas the TomRSV is found mainly in the triradiate lumen of the esophageal bulb. The different locations of viral binding sites for TRSV and TomRSV account for the capability of dual transmission of both viruses, because the different viruses aren't competing for binding sites. TRSV particles can be liberated into the plant during feeding by the dorsal and subventral gland secretions. TomRSV is mainly liberated by the secretions of the subventral glands due to its location in the triradiate lumen. These facts may account for the differences in the experimentally determined transformation efficiency between TomRSV (100%), and TRSV (75% or less). Previous work attempting to identify virus binding sites and release was difficult without the development of immunoflourescent labeling.
Control of the American Dagger Nematode presents problems because Xiphinema americanum is hard to completely remove. Nematicides generally remove up to 95% of the nematodes in soil, however the 5% that remain can reproduce asexually and the viruses that they carry can still infect the roots of young plants. Therefore, to eliminate the nematodes, nematicides should be used along with having a bare soil field for at least a 2-year period. This ensures that the Xiphinema americanum has no food source. At the end of this 2-year period the nematodes should be eradicated.
The spraying of nematicides also causes plants to release allelopathic chemicals. These chemicals then kill the nematodes by active suppression because they are toxic to the nematode. Crop rotation is another form of control for Xiphinema americanum. It has been shown that certain non-host plants may deny the nematode population an adequate food source for reproduction, and thus greatly reduce its population in the soil. This is termed passive suppression.
Xiphinema americanum can only travel via run-off and in damp soil, therefore if soils are kept dry enough the nematodes can be localized and quarantined.
Additionally, if soil is tilled frequently, Xiphinema americanum will likely not be in high enough of a population density to cause any noticeable symptoms in its hosts. There is also evidence of Xiphinema americanum resistance and "tolerance" seen in certain species of grapes that appeared to be better adapted to the parasite.
Figure 47 Xiphinema americanum that cause dagger on sweetpotato. Microscopic features of the Xiphinema genus (A). Microphotograph of the specie Xiphinema americanum (B).
- Root-lesion nematode. Asia, the Pacific, America, and other tropical and temperate areas where sweetpotato is grown.
Economic importance. Pratylenchus is considered second only to Meloidogyne in terms of plant species parasitised and extent of crop damage and loss. The thresholds vary somewhat with species, climate, soil type and host crop but a density of 1-2 nematodes/g soil at planting is a reasonable guide; thresholds range from 0.5-1.8/g soil.
Symptoms. The nematode causes small, necrotic root lesions. Fibrous root necrosis may lead to some stunting of vines and a significant reduction in the quality of fleshy storage roots. Small, brown to black, necrotic lesions are also produced on storage roots, which make the roots unmarketable.
Morphology. Lesion nematodes are small nematodes with adults, being usually less than 1 mm long. The head region is low and flattened, with 2- 4 head annules. It has distinct head skeleton, continuous with the body contour. The stylet is 20 µm or less, and moderately developed with distinct basal knobs. The esophagus has a well developed median bulb, and the posterior gland lobes overlap the intestine ventrally. The female has posterior vulva (V= 70-80%) with a single anterior gonad and a short post vulval sac; It is tall, cylindrical to conoid, and with two to three anal body widths long (?). The male tail is conical with a distinct bursa that reaches the tail tip.
Life cycle. This genus has the typical nematode life cycle with four juvenile stages, and the adults in some species are parthenogenic and in others amphimictic. The juveniles of P. coffeae mature and differentiate within the root, and adult females deposit eggs singly or in small groups within the tissues of the root. Commonly, the duration of the Pratylenchus life cycle is given as about 35-42 days, but this, considerably varies with species, e.g. P. penetrans can range from 30-86 days (Corbett, 1973) while P. coffeae in sweetpotato is from 30-40 days at 25-30oC to 50-60 days at 20oC. The variability is due partly to the length of the life cycle being temperature dependent.
Ecology. Lesion nematodes are migratory root endoparasites. They are also extremely diverse in their adaptive capacity. Individual species show special preferences for temperature. Some are less able than others to tolerate extremes, especially cold, or the extreme drought areas, where vegetable production is less common except under irrigation. The influence of moisture as rain is not as significant. However, Pratylenchus penetrans moves best when soil water has drained so that 8-12% of the soil volume is air-filled and survives a shorter time in wet than in dry soil. Preferred soil types include coarse-textured sandy loam, silt loam or occasionally, organic soil (muck). More P. penetrans penetrate roots in a sandy loam than in silt loam or loam. They are less common in clays and similar fine-textured soils; adults and juveniles of Pratylenchus penetrans move farther in coarse than in fine-textured soil. Pratylenchus may be disseminated by transportation of plant root parts or soil, and by surface or irrigation water.
Cultural control. Crop rotation. Peanut crops resulted in significantly lower population of Pratylenchus coffeae. Paddy rice or increase fertilization with potassium reduced severity of infection. Addition of organic amendments. Chicken manure is very effective in reducing the nematode.
Biological control. Paecilomyces lilacinus, a fungal egg parasite was evaluated and found effective against Pratylenchus sp. in corn.
Chemical control.Use of soil fumigants.
In Japan the principal species is Pratylenchus coffeae, which caused significant losses, while in the U.S., Pratylenchus brachyurus, is the most common.
- Pratylenchus coffeae (Zimmerman) Goodey 1951, is a plant-pathogenic nematode infecting several hosts including potato, banana, sweet potato, strawberry, Persian violet, peanut and citrus.
The distribution is in India; tropics; southeastern US; Central and South America, Caribbean, Zaire. C-rated pest in California pending current initiatives to have it elevated to an A rating. The first report of Pratylenchus coffeae was by Zimmerman in 1898. He reported that the nematode was responsible for destruction of 95% of the Coffeae arabica plantations in Java.
The female in young specimens the ovary extends over one-quarter of body length, in old ones over more than one-half. Spermatheca broadly oval to nearly round. Posterior uterine branch variable in length, sometimes reaching 50 µm. In about 20% of the females examined (especially older ones) it carried a distinct rudimentary ovary. Tail tapering slightly, its length in young specimens 2-2.5 X, in old ones 1.5-2 X anal body diameter; tip broadly rounded, truncate or indented; in some specimens appearing weakly and irregularly annulated. Intra-uterine eggs often contain embryos, as was noted already by Cobb (1920). the male has Spicula very slender, shaft ventrally concave. Gonad extends over about one-half body length. Testis shorter than vas deferens. Bursal edge faintly crenate. This species may be distinguished from Pratylenchus loosi by the more prominent cuticular annulation, the slightly more anterior position of the vulva, the shape of the female tail, and perhaps by its shorter oesophagus and the length of the posterior uterine branch (Figure 48 A, B, C, D, E, F, G, H, I, L).
- Pratylenchus brachyurus (Godfrey, 1929) Filipjev & Schuurmans Stekhoven 1941. The taxonomy of this nematode is: Natura; Mundus Plinius; Naturalia; Biota; domain Eukaryota Chatton 1925; Amorphea Adl et al. 2012; Opisthokonta Cavalier-Smith 1987; Holozoa; kingdom Animalia Linnaeus, 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Eubilateria Ax 1987; Protostomia Grobben 1908; Ecdysozoa A.M.A. Aguinaldo et al. 1997; Introverta C. Nielsen 1995; Nematoida A. Schmidt-Rhaesa 1996; phylum Nematoda Cobb 1932 (round worms); class Secernente Von Linstow 1905; subclass Tylenchia (Thorne 1949) Inglis 1983; order Tylenchida G. Thorne 1949; superfamily Tylenchoidea (Örley 1880) Chitwood & Chitwood 1937; family Pratylenchidae (Thorne, 1949) Siddiqi, 1963; subfamily Pratylenchinae Thorne 1949 ; genus Pratylenchus Filipjev 1936. Nematode is 0.4-0.5 mm long. Lip region is generally low and flat. Head frame sclerotized. Tail conical and rounded to flattened at tip. Nematode has short ventral overlap of esophagus. Monovarial, uterus prodelphic, short post-uterine sac. Females are slender. Males are extremely rare (Figure 48, M).
Widely in the tropics and sub-tropics. Type host was pineapple in Hawaii where it was described by Godfrey in 1929. C-rated pests in California. Migratory endoparasite of roots. There are no males (extremely rare); females reproduce by parthenogenesis.
- Burrows through cortex; necrosis occurs after 24 hours in tobacco, 4 days in pineapple.
- Can stop growth of pineapple roots.
- May result in vessel blocking in corn.
- In peanuts, causes crop loss by weakening pegs so that pods drop off; lesions appear on pegs, pods, and shells.
- Slows growth on young citrus in Florida, but effect diminishes with tree age (O'Bannon).
Figure 48 Root-lesion nematode. Pratylenchus coffeae (A-L). Pratylenchus brachyurus (M).
- Pin nematode to which it belongs the genus Paratylenchus Micoletzky 1922 spp. The classification of Paratylenchus is: order Tylenchida G. Thorne, 1949; suborder Tylenchina (Thorne 1949) Chitwood 1950; superfamily Criconematoidea [Taylor 1936 (1914)] Geraert 1966; family Tylenchulidae (Skarbilovich 1947) Kirjanova 1955; subfamily Paratylenchinae ) Thorne 1949. A synonyms is Paratylenchoides (Raski, 1973). These nematodes in the family Tylenchulidae are among the smallest plant-parasites, 0.18-0.6 mm long. Cuticular annulation is smooth. Females is Usually less than 0.5 mm long, vermiform, not swollen, except that gravid female may swell anterior to the vulva. Labial framework weakly sclerotized (except in Paratylenchus israelensis and Paratylenchus sheri where it is stronger). There is a strong stylet (from 12 to 40 µm, usually about 36 µm) which allows the nematode to feed several cell layers below the root surface. Excretory pore between level of nerve ring and level of esophago-intestinal junction. Females have a single outstretched ovary and the vulva is in the posterior region of the body. Spermatheca appears as a modification of cells, or pouch-like sac, at the anterior end of the uterus. Males have reduced feeding structures, with stylet reduced or absent, and degenerate esophagus. They probably do not feed. However, they may be common in the population. Juvenile stages resemble the female, but with a smaller stylet. The stylet is usually absent in the J4 stage. The J4 is a dauer (survival) stage, and development will not proceed beyond this stage unless a host plant is present. In a 1987 review, Raski and Luc (1987) recognized 64 species.
Life cycle is 30 to 31 days at 25 to 28 °C. J4 is the persistent stage - at least in some species molt of the J4 is stimulated by root diffusates (host presence) - tolerant of cold and desiccation. The J4 do not molt to adults in water, but molt progressively over a 2 week period in root diffusates. In some cases, the root exudates from a host plant did not induce molting of a Paratylenchus species to which it was a host, for example, red clover (Trifolium pratense) is a host for both Paratylenchus projectus and Paratylenchus dianthus but only induced molting in Paratylenchus projectus. In older pot cultures or field soils, the resistant J4 may be 80% of the population. J4 distinguished from other stages by reduced or absent stylet and esophagus and accumulation of opaque granules in esophagus region. J4 male has no stylet and does not feed. Female grows after final molt, and may not feed as J4. Gravid females may become quite swollen; the fattening is restricted to that part of the body anterior to the vulva, so that the body posterior to the anus remains slender.
Pin nematodes are found in high numbers on many plants but appear to cause significant damage to only a few. Effects range from no symptoms to shallow localized lesions. Root tip elongation and lateral root development may be reduced or terminated by the prolonged feeding of many individuals.
- Reniform. Linford and Oliviera established the genus Rotylenchulus in 1940 with Rotylenchulus reniformis as the type species. The generic name was given by Linford and Oliviera because they thought that the nematode species was similar to the genus Rotylenchus, having features of that genus and other Hoplolaimidae. The species name was coined because of the kidney shape of the mature female.
Found in subtropical and tropical regions. Disease and nematode development are favoured at 29.5 °C. This nematode group cause:
Rotylenchulus reniformis Linford & Oliveira 1940. The taxonomy of Rotylenchulus reniformis is: kingdom Animalia Linnaeus 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Protostomia Grobben 1908; Introverta C. Nielsen 1995; Nematoida A. Schmidt-Rhaesa 1996; Nematoida A. Schmidt-Rhaesa 1996; phylum Nematoda Cobb 1932 (round worms); class Secernentea Von Linstow 1905; subclass Tylenchia (Thorne 1949) Inglis 1983; order Tylenchida G. Thorne 1949; suborder Hoplolaimina Chizhov & Berezina 1988; superfamily Hoplolaimoidea (Filipjev 1934) Paramonov 1967; family Rotylenchulidae (Husain & Khan 1967) Husain 1976; subfamily Rotylenchulinae Husain & Khan 1967; genus Rotylenchulus Linford & Oliveira 1940.
- stunting (a reduction in height of a plant resulting from a progressive reduction in the length of successive internodes or a decrease in internode number), yellowing and wilting (a reduction in plant rigidity and freshness resulting to drooping of leaves and stems due to water stress, such as inadequate water supply or excessive water loss);
- fibrous or feeder root shortening and discolouration;
- in some cases, the nematode may produce lesions (Well-marked, dead, brown or black, localised area caused by diseases, pests, disorders or stresses; can be interchangeably used with spot when referring to dead, brown or black distinct colour or mark on a surface), cracks or distortions on storage roots (Figure 49).
Reniform nematode is a very common, damaging pest of many plants in subtropical/tropical soils and sometimes occurs in protected temperate soil.
Economic importance. The first report that Rotylenchulus reniformis is pathogenic to sweetpotato was made by Martin (1960). In sweetpotato, losses range from 44-60%, depending on initial population.
Symptoms. The reniform nematode causes root necrosis resulting in severe root pruning and subsequent dwarfing of plants. Fibrous or feeder roots are mostly attacked which may reduce the absorption ability and other physiological functions of the plant. This may lead to stunting, yellowing and wilting. In some cases, the nematode may produce lesions, distortions or cracks on storage roots which reduce marketability (Figure 49 A, B, C).
Morphology. Rotylenchulus reniformis is sexually dimorphic. The immature female is free in soil, measures 0.3-0.5 mm long, and C-shaped when killed by heat. The lip region is rounded to conoid, continuous with the body contour and moderately sclerotised. The stylet is 10-20 µm long, of moderate strength and with small rounded basal knobs. There is opening of dorsal oesophageal gland, about a stylet length, behind the stylet base. The oesophagus has well developed median bulb and elongated (4-5 body widths) gland lobes, overlapping intestine laterally and ventrally. The vulva posterior (V= 58-72) has paired, opposed gonads, each with a double flexture in the ovary. The tail conoid with rounded terminus is 2-3 anal body widths long. The mature female is semi-endoparasitic in roots, greatly swollen, irregular to kidney-shaped, and has enlarged gonads occupying much of body. The male lip sclerotisation and stylet are much weaker than the immature female. Its oesophagus degenerates and it has a tail similar to immature female, has small adanal bursa, slender and with curved spicules. Particularly, female with body spiral to C-shaped. Labial region offset or continuous wiht body contours, anteriorly rounded or flattened, generally annulated, with or without scattered transverse striae. Labial framework, stylet, and stylet knobs average sized for the family; knobs with rounded to indented anterior surface. DGO often close to stylet (6 um) but with a tendancy to posteriorly directed migration (up to 16um). Esophageal glands overlap intestine dorsallyand laterally; dorsal gland more developed than subventral glands; intestine symmetrically arranged between the subventral glands. Two genital branches outstretched, equally developed; posterior branch rarely degenerated. Epiptygma single or double present. Tail short, hemispherical, rarely with small ventral projection; phasmids pore- like, small, near anus level. Male with caudal alae (cuticular extensions on either side of, or surrounding, male cloaca) enveloping tail, not lobed. Secondary sexual dimorphism not marked, sometime anterior part of male body slightly smaller than female.
In summary, the characteristics of immature female (Figure 49, D) are:
The characteristics of mature female (Figure 49, E) are:
- body vermiform, slender and spiral to C-shaped when heat-killed;
- length about 0.4 mm;
- stylet knobs are rounded and slope posteriorly;
- the median bulb of the esophagus has a distinct valve and the basal glands of esophagus overlap the intestine laterally and ventrally;
- the vulva is not prominent, and occurs at about 70% of the body length;
- ovaries paired and opposed with double flexure;
- tail tapers to a narrow rounded terminus.
The characteristics of male (Figure 49, E) are:
- body swollen, kidney-shaped, with an irregularly neck, 0.38-0.52 mm long;
- the vulva has raised lips;
- body beyond the anus is hemispherical, with a slender terminal portion 5-9 µm long;
- well-developed stylet;
- cuticle thick;
- ovaries very long, convoluted; vulva post-equatorial;
- eggs deposited in a gelatinous matrix.
Life cycle. Juveniles of the reniform nematode are differentiated within the egg and undergo one moult before the second stage juveniles hatch. Three additional moults occur without feeding while the juveniles are free in the soil. Adult stage and egg production occur 16 days after inoculation in susceptible cultivars. The life cycle from egg to egg is from 22-29 days in susceptible cultivars such as "V20-436".
- anterior end reduced;
- stylet reduced.
- the esophagus is degenerate with reduced median bulb and valve.
- males do not feed.
- the spicules are elongate-slender, ventrally curved (Figure 49, H).
- caudal alae present but difficult to see, not quite reaching tail end.
- juveniles and males remain in soil.
Ecology. Male and female nematodes can survive in air-dried soil kept at 20-25 °C for 7 months. Local dissemination is through infested soil. Distribution is limited by low winter temperatures, and nematode and disease development are both greater at 29.5oC than at 15, 21.5 or 36 °C.
Host range. Very wide host range including weeds. Major host are soybean, cowpea, cotton, pineapple, sweetpotato, cassava and other vegetable crops.
Cultural control. Crop rotation; non-host crops or resistant crops can be planted when nematode population is high; use of organic amendments; use of trap and antagonistic crops. Planting Tagetes erecta and Crotolaria spectabilis in nematode infested soil has been found effective against the nematode.
Biological control. Use of Paecilomyces lilacinus, a fungal egg parasite was found effective against the reniform nematode.
Host-plant resistance. The following Philippine cultivars/lines are found resistant to the reniform nematode: "CI 951-6", "BNAS 551", "VSP-1", "V29-1166", "CI 916-455", "UPLB 80", "Kabiti" and "CI-950-27".
Chemical control. Several nematicides have been reported to be effective against the reniform nematode. Examples are Nemagon, Mocap, Dasanit, Nemacur, Furadan, Temik, Vydate.
Figure 49 Rotylenchulus reniformis cause fibrous or feeder roots, distortions or cracks on storage roots which reduce marketability (A, B, C). Immature and mature female (E); female and male head; male tail (F); Spicula elongate-slender, ventrally curved (G, H).
- Root-knot nematode caused by Meloidogyne Goeldi 1892 spp. The taxonomy of this genus is: Kingdom Animalia Linnaeus 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Eubilateria Ax 1987; Protostomia Grobben 1908; Ecdysozoa A.M.A. Aguinaldo et al., 1997; Introverta C. Nielsen 1995; Nematoida A. Schmidt-Rhaesa 1996; phylum Nematoda Cobb 1932 (round worm); class Secernentea Von Linstow 1905; subclass Tylenchia (Thorne 1949) Ingli 1983; order Tylenchida G. Thorne 1949; suborder Hoplolaimina Chizhov & Berezina 1988; superfamily Hoplolaimoidea (Filipjev 1934) Paramonov 1967; family Heteroderidae (Filipjev & Schuurmans Stekhoven 1941) Skarbilovich 1947; subfamily Meloidogyninae Skarbilovich, 1959.
The biometric of Meloidogyne is:
Females (Figure 50 C, right) is length 430-740 um (mean 591 um, standard deviation (SD 60 um); width 344-518 um (422 um SD 42); a = 1.1-1.8 (1.4, SD 0.2); b = 3.8- 5.0 (4.4, SD 0.4); stylet 11.2-12.5 um (11.9 um, SD 0.3); width of stylet of stylet knobs 3.4-4.3 um (3.8 um, SD 0.3); dorsal esophageal gland orifice (DSO) 3.4-5.5 um (4.2 um, SD 0.6); from base of stylet; center of median bulb 52-80 um (63 um, SD 7) from anterior end; excretory pore 10-27 um (18 um, SD 5) from anterior end; vulva slit length 19-32 um (27 um, SD 3); distance from vulva slit to anus 13-22 um (18 um, SD 2).
Males (Figure 50 C, left) is length 887-1268 um (1068 um, SD 100); a = 28-46 (36, SD 4); b = 6-9 (7.2, SD 1); c = 140-226 (162, SD 20); stylet 18.1-18.5 um (18.3 um, SD 0.2); DGO 2.2-3.4 um (3 um, SD 0.4) from base of stylet; center of median bulb 61-77 um (71 um, SD 5); from anterior end; spicules 26-29 um (27 um, SD 1.2); gubernaculum 6.5-8.2 um (7.7 um, SD 0.6); tail 4.7-9.0 um (6.8 um, SD 0.9).
This nematode group cause:
The following species causing disease to cultivated plants, belong to the genus Meloidogyne:
- poor shoot growth, leaf chlorosis and stunting;
- galling of rootlets and severe cracking of roots on some varieties or formation of small bumps or blisters on other varieties;
- severe longitudinal cracking of storage roots;
- brown to black lesions on flesh which are not evident unless storage root is peeled;
- can be diagnosed by presence of pearl-like swollen female nematodes in flesh of storage roots or in fibrous roots;
- worldwide in distribution;
- usually occurs in warmer areas.
- Meloidogyne arenaria (Neal) Chitwood (1949).
- Meloidogyne hapla Chitwood 1949.
- Meloidogyne incognita (Kofoid & White) Chitwood 1949.
- Meloidogyne javanica (Treub) Chitwood 1949.
Economic importance. The degree of damage depends upon the population density of the nematode, taxa present, susceptibility of the crop, and environmental conditions, such as fertility, moisture and presence of other pathogenic organisms, which may interact with nematodes. In sweetpotato an estimated annual yield loss of 10.2% was reported. In susceptible varieties pathogenicity of Meloidogyne incognita showed a 50% storage root reduction at a population density of 20,000/cm3. Aside from yield loss, cracking can make storage roots unmarketable.
Geographical distribution. The root-knot nematode, Meloidogyne incognita, is worldwide in distribution. It is widespread in Asia, Southeast Asia and usually occurs in warmer areas. In some countries, Meloidogyne javanica is more dominant.
Symptoms. Above-ground symptoms exhibited by sweetpotato plants due to root-knot nematode include poor shoot growth, leaf chlorosis and stunting. Galling of rootlets and severe cracking of storage roots on some varieties or formation of small bumps or blisters (refers to raised dark spots which develop on the skin of storage roots, and may be sparsely scattered or numerous, covering a large proportion of the root surface) on other varieties are important below-ground symptoms in sweetpotato. There may also be brown to black spots in the outer layers of flesh which are not evident unless the storage root is peeled. Presence can be diagnosed by the pearl-like swollen female nematodes in flesh of storage roots or in fibrous roots, within the galls or dark spots (Figure 50, A).
Morphology. Meloidogyne incognita is sexually dimorphic. The female is saccate to globose, 0.4-1.3 mm long, and usually embedded in root tissues which are often swollen or galled. Its body is soft, pearl white in colour and does not form a cyst. The neck protrudes anteriorly and the excretory pore is anterior to the median bulb and often near the stylet base. Its vulva and anus are terminal, flush with or slightly raised from the body contour, the cuticle of the terminal region forms a characteristic perineal pattern, which is made up of the stunted tail terminus, phasmids, lateral lines, vulva and anus surrounded by cuticular striae; the pattern is often characteristic for individual species (Figure 50, D). The female stylet is shorter, 10-24 µm usually 14-15 µm, and more delicate with small basal knobs. The paired gonads have extensive convoluted ovaries that fill most of the swollen body cavity. There are six large unicellular rectal glands in the posterior body which produce a gelatinous matrix, which is excreted via the rectum to form an egg sac in which many eggs are deposited (Figure 50, B). The male has long, thin, cylindrical shape of a worm but the lip region has a distinct head cap, which includes a labial disc surrounded by lateral and medial lips. The head skeleton is usually weaker and the stylet less robust and shorter, 18-24 µm long for many species. Infective second stage juveniles, often free in the soil, are usually 0.3-0.5 mm long; they are less robust, the stylet is delicate with small basal knobs, under 20 µm long, and the head skeleton weak. The median oesophageal bulb is well developed and the oesophageal glands are extensive, overlapping the intestine for several body widths, mainly ventrally; the tail is conoid, often ending in a narrow rounded terminus, but tail length is variable, 1.5-7 anal body widths between species, it often ends in a clear hyaline region, the extent of which can help to distinguish species.
Life cycle. In addition to the adult and egg, there are four juvenile stages and four moults in the life cycle of Meloidogyne incognita. The first stage juvenile develops in the egg, and the first moult usually occurs within the eggshell, giving rise to the second-stage juvenile, which emerges free into the soil or plant tissue. Once the nematode begins feeding on tissue of a favourable host, the second, third and fourth moults occur giving rise to the third, fourth and fifth or adult stages, respectively. Between moults, there is further growth and development of the nematode, with concurrent development of the reproductive systems in the two sexes. Upon maturity, the female deposits eggs and the life cycle is repeated. Its life cycle is similar to Heterodera but the generation time, 4-8 weeks, is shorter.
Host Range. Meloidogyne incognita has a very wide host range including weeds. Meloidogyne spp. attack virtually all plants.
Cultural control. Crop rotation and non-host crops or resistant crops can be planted when nematode population is high. Addition of organic amendments. Chicken manure is very effective reducing nematode egg masses by 56%. Use of trap crops and antagonistic crops. Planting Tagetes erecta and Crotolaria spectabilis in nematode infested soil is effective against the root-knot nematode.
Biological control. Paecilomyces lilacinus, a fungal egg parasite, was found effective against root-knot attacking sweetpotato. The parasite reduced egg masses by about 50%.
Host-resistance. There are many varieties of sweetpotato found resistant to the root-knot nematode. Some of these are: "W-86", "L4-89", "BPA-4" and "Sinibastian", "Jasper", "Jewel", "Miracle", "Georgia Red", "Garcia Yellow" and "Travis". However, some populations of M. incognita can infect even some of the resistant cultivars.
Chemical control. Several nematicide have been very effective against the root-knot nematode in sweetpotato. Examples are Nemagon, Mocap, Dasanit, Nemacur, Furadan, Temik, Vydate.
Figure 50 Root-knot nematode of genus Meloidogyne causing galls and egg masses of root-knot nematodes on fibrous roots and severe cracking of storage roots. Galls on sweetpotato roots are often much smaller and difficult to see (A, left) and severe cracking or blisters of storage roots (A, right). Egg masses of Meloidogyne incognita (B). Female has 2 ovaries, prodelphic; adults swollen; eggs deposited in matrix secreted by six rectal glands, eggs not retained in female body (C). Female body does not form cyst. Cuticular striations in posterior of female form a fingerprint-like perineal pattern (D).
- Spiral nematode such as Helicotylenchus Steiner 1945 spp. About Helicotylenchus spp. associated with sweetpotato and edible aroids in southern Florida, the numbers of nematodes were evaluated on three dates over a six-month period on an orange-fleshed cultivar, "Carver", and on three white-fleshed cultivars. Numbers of Rotylenchulus reniformis were very high on all cultivars (228 to 408/100 cm3 of soil). Significantly lower numbers of Helicotylenchus dihystera were present on "Carver" than on "Morado" and "White Triumph" after six months, while significantly lower numbers of Quinisulcius acutus Allen, 1955 (nematode of the Telotylenchinae family), occurred on "Morado" than on "Carver" at that time. Populations of all three nematodes multiplied several times on all cultivars over the test period.
- Sting such as Belonolaimus longicaudatus Rau (1958). Belonolaimus longicaudatus is native to the sandy coastal plains of the south-eastern United States. It is limited to soils with high sand content (>80% sand) and is common in sandy regions along the Gulf of Mexico and Atlantic coasts from Texas to Virginia. Belonolaimus longicaudatus has been spread to sandy areas inland, mostly with infested soil and plant material, and as has been found as far north as southern Ohio. It has been spread on infested golf course sod to California, and to several islands in the Caribbean. An undescribed species of Belonolaimus, morphologically similar to Belonolaimus longicaudatus, has been found infesting turfgrasses west of the Mississippi river in Texas, Kansas, and Oklahoma.
Belonolaimus longicaudatus is an ectoparasite, meaning that its entire life cycle is spent in the soil and it feeds on plant roots from the outside. Eggs are laid in pairs by the female nematode as rapidly as 10 eggs per 10-15 hours. After a few days, the second-stage juvenile hatches from the egg. The juvenile nematode feed on root hairs, and as it matures it will begin to feed on root tips. It will undergo three additional molts into a third-stage juvenile, fourth-stage juvenile, and finally into an adult nematode. The juvenile stages are similar to each other and to the adults, differing only in size and the presence of sexual organs in adults (Figure 51, A). Belonolaimus longicaudatus is an amphimictic and gonochoristic species, meaning that it employs sexual reproduction between distinct males and females. Paired male organs called spicules (Figure 51, C) are inserted into the vagina (Figure 51, B) of the female to deposit sperm. Copulation is aided by paired cuticular flaps called the bursa on the male (Figure 51, C). Mating only needs to occur once because the female nematode stores the sperm in an organ called a spermatheca. Depending on conditions and population, development from an egg to an egg-laying adult takes between 18 and 24 days. Belonolaimus longicaudatus has a long stylet that it uses to feed. The stylet functions like a hypodermic needle that can be used to inject substances into plant cells and also to remove plant cell contents (Figure 51, D). It inserts its stylet into the meristematic tissue of root tips and injects digestive enzymes prior to ingestion. This causes death of the root meristem.
Belonolaimus longicaudatus is considered the most damaging nematode to turfgrasses, forage grasses, strawberry, potato, sugarcane, and cantaloupe in Florida. It also can cause major damage on many other agronomic, horticultural, forage, and tree crops. To avoid severe crop losses from Belonolaimus longicaudatus, chemical nematicides or crop rotations are commonly used. Belonolaimus longicaudatus is a Class A quarantine pest in California, and any plant shipments containing Belonolaimus longicaudatus are banned from that state. Similarly, Belonolaimus longicaudatus is a quarantine pest in many countries and can greatly impact movement of plant and soil material internationally.
Belonolaimus longicaudatus is among the most damaging of all plant-parasitic nematodes. Feeding by this nematode kills the root meristem and halts root growth. Lateral roots will develop, but Belonolaimus longicaudatus will migrate to these lateral roots and damage them as well. This causes an abbreviated and stubby-looking root system. The roots may appear cropped off just below the thatch. Because the roots are damaged, they are unable to supply the plant with water and nutrients. Damaged annuals will become stunted, wilt, and die. Young citrus trees infected by Belonolaimus longicaudatus may require extra years after planting before bearing fruit. Corn and cane may lodge (fall over) as the nematodes damage their brace-roots. Turfgrasses may wilt and decline, and weeds may proliferate. Damage from Belonolaimus longicaudatus typically occurs in patches because the nematodes are generally clumped in distribution.
Figure 51 Belonolaimus longicaudatus causing sting. Juvenile Belonolaimus longicaudatus differ from adult females and male only by size and absence of developed sex organs (A). The vagina of most Belonolaimus longicaudatus females is surrounded by sclerotized pieces (B). Sex organs of male Belonolaimus longicaudatus (C); the bursa is long and narrow. The anterior of a Belonolaimus longicaudatus with its stylet protruding (D).
Identification. Belonolaimus longicaudatus is a very long (2-3 mm) and slender (40 µm) nematode. The stylet is also long (>100 µm) and slender with rounded knobs (Figure 52, A). The anterior region is offset by a prominent constriction (Figure 52, C). The most similar nematode genus in appearance to Belonolaimus spp. in Florida is Dolichodorus spp. These are both common nematode genera that share many of the same hosts and can occur concomitantly. The features that most easily distinguish these two genera are:
While Belonolaimus longicaudatus was originally characterized by having a stylet shorter than its tail, this feature has since been found to be highly variable and not a reliable diagnostic feature.
- the esophageal glands of Belonolaimus overlap the intestine (Figure 52, A), those of Dolichodorus do not (Figure 52, B),
- the tail of juveniles and females of Belonolaimus are rounded (Figure 52, D), those of Dolichodorus have a pointed tip (Figure 52, E), and c) males of Belonolaimus have a long, narrow bursa (Figure 51, C), the bursa of Dolichodorus is wide and round. Belonolaimus longicaudatus is separated from other Belonolaimus spp. by:
- having a single lateral incisure (Figure 52, F),
- vulva with no protruding lips (Figure 51, B),
- presence of sclerotized vaginal pieces (Figure 51, B).
Figure 52 Identification of Belonolaimus longicaudatus. The esophageal glands of Belonolaimus longicaudatus (A) overlap the intestine; the esophageal glands of Dolichodorus heterocephalus Cobb 1914 (Nematoda: Tylenchida) do not overlap the intestine (B); arrow marked "zero" point to the esophageal-intestinal interface (A and B). Stylet knobs of Belonolaimus longicaudatus are rounded and oriented perpendicularly (A), those of Dolichodorus heterocephalus are oriented posteriorly (B). The anterior of both sexes of Belonolaimus longicaudatus is set off by a deep constriction (C). The tails of Belonolaimus longicaudatus juveniles and females are rounded (D); tails of Dolichodorus heterocephalus juveniles and females have a point (E). Belonolaimus longicaudatus has a single lateral incisure that runs the length of the nematode longitudinally (F).
Management. Because Belonolaimus longicaudatus is an ectoparasite that spends its entire life in soil, it is among the most responsive plant-parasitic nematodes to fumigant and nematicide treatments. Despite this, it is difficult to manage with chemicals in certain systems due to its seasonal vertical migration. In Florida strawberry production, a portion of the Belonolaimus longicaudatus population will migrate deep in the soil profile during fallow and field preparation, allowing these nematodes to escape fumigation in the upper portions. Similarly, in turf systems Belonolaimus longicaudatus will migrate deeper in the soil during the summer months, limiting the effectiveness of nematicides applied to the turf surface during that time. Because Belonolaimus longicaudatus has no long-term survival stage, its numbers will rapidly decline from starvation during extended periods of clean fallow, or rotation with a non-host crop. However, the wide range of host plants for Belonolaimus longicaudatus makes it difficult to select non-host rotation crops or cover crops. Sunn hemp has been shown to be resistant to Belonolaimus longicaudatus and may be used as a summer cover crop for suppression of Belonolaimus longicaudatus and other plant-parasitic nematodes in some situations. In strawberry it is important to kill off the plants after harvest so that they do not maintain the nematode population. Weed management is very important to eliminate all Belonolaimus longicaudatus hosts during fallow or rotation. Recent breeding efforts for strawberry and turfgrass have emphasized selection of genotypes that are resistant or tolerant to Belonolaimus longicaudatus. Hopefully these efforts will lead to less susceptible cultivars that may be used in Integrated Pest Management (IPM) for Belonolaimus longicaudatus in the future. Currently, Celebration bermudagrass has exhibited tolerance to Belonolaimus longicaudatus and is recommended for lawns, athletic fields, and golf course fairways infested by Belonolaimus longicaudatus in Florida.
- Stubby-root such as Paratrichodorus Siddiqi 1974 spp. The taxonomy of this genus is: Kingdom Animalia Linnaeus 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Eubilateria Ax 1987; Protostomia Grobben 1908; Ecdysozoa A.M.A. Aguinaldo et al., 1997; Introverta C. Nielsen 1995; Nematoida A. Schmidt-Rhaesa 1996; phylum Nematoda Cobb 1932 (round worm); class Enopleae Inglis 1983; subclass Enoplia Pearse 1942; order Dorylaimida Pearse, 1942; suborder Diphtherophorina Coomans and Loof 1970; superfamily Trichodoroidea Siddiqi 1973; family Trichodoridae Clark 1961.
A species of the genus Paratrichodorus found on sweetpotatoes is Paratrichodorus minor (Colbran 1956) Siddiqi 1974. Paratrichodorus minor is a species of nematode in the family Trichodoridae, the stubby-root nematodes. It occurs in tropical and subtropical regions of the world. It damages plants by feeding on the roots and it is a vector of plant viruses. It is a pest of some agricultural crops. Like other stubby-root nematodes, this species is microscopic, reaching up to 0.71 mm in length. Its body is rounded at both ends. It has an onchiostyle, a curved, solid stylet which it uses to puncture plant roots. It stabs the plant tissue rapidly, up to 10 times per second, to make a hole. It injects saliva, which hardens into a hollow tube, and it uses this like a drinking straw to withdraw the contents of the plant cells. It moves around the root, leaving old tubes in place and creating new ones. It is an ectoparasite, attacking the plant externally rather than entering its tissues. Damage to plants is evident when it stops the roots from growing, leaving the root system "stubby"-looking. A plant cannot obtain water and nutrients from the soil and becomes stunted and wilted. It shows signs of nutrient deficiency. An affected crop field may have patches of withered plants. The nematode also introduces viruses to plants, including tobacco rattle virus, which causes the disease corky ringspot in potatoes. A potato tuber with corky ringspot has large brown rings on its surface and discolored spots inside. Entire potato crops can be made unmarketable by the appearance of the disease.
The nematode has been observed in over 100 plant hosts, including turfgrasses such as "St. Augustine" grass and bermudagrass, vegetables such as cabbage and tomato, and other crops such as corn, sorghum, sugarcane, peanut, and soybean. It is a pest of vineyards in California. It is most abundant in coarse soils. Most individuals of the species are female and they reproduce by parthenogenesis, producing offspring without fertilization. They lay eggs in the soil and the juveniles feed on roots as they develop. The length of the life cycle varies with temperature, but it may be as short as 16 days.
- Stunt is a group that include Tylenchorhynchus Cobb 1913 spp. It is a genus of nematodes including many species of plant parasites. The classification of stunt nematodes - those including the Tylenchorhynchus genus - is unstable; many newly discovered species within this genus are reconsidered to be actually subspecies. Stunt nematodes such as Tylenchorhynchus and the closely related genera, Anguillulina and Merlinia, include more than 250 known species. Members of these genera possess similar anatomy and may be easily mistaken for one another. Some debate has led to the classification of single species under different names in two distinct genera (e.g. Tylenchorhynchus cylindricus is Anguillulina dubia).
The taxonomy of Tylenchorhynchus genus is: phylum Nematoda Cobb 1932; class Secernentea Von Linstow 1905; subclass Diplogasteria Hodda, 2003; order Rhabditida Chitwood 1933 ; suborder Tylenchina (Thorne, 1949) Chitwood 1950; superfamily Tylenchoidea Oerley 1880 (Chitwood & Chitwood 1937); family Dolichodoridae Chitwood, in Chitwood & Chitwood 1950; subfamily Telotylenchinae Siddiqi, 1960.
Tylenchorhynchus are soil dwelling stunt nematodes. They inhabit the same soil as plant root systems in which they can cause stressing or disease in plants. About 8% of the studied species are parasitic. Agricultural problems associated with Tylenchorhynchus spp. affect many species such as soybean, tobacco, tea, oat, alfalfa, sweetpotato, sorghum, rose, lettuce, grape, elms, and citrus.
Tylenchorhynchus genus have body cylindroid, tapering to rounded lip region bearing 2 annules. Lateral fields with 4 incisures. Spear 21u long with large, cupped knobs, its muscles attached to a sclerotized band about the base of lip region. Excretory pore midway between median bulb and esophagus base. Hemizonid 1 annule anterior to pore. Deirids not seen. Basal bulb slightly longer than body diameter. Cardia hemispheroid, almost as wide as bulb base. Spermatheca about half as long as body width, packed with sperms. Intestine extending into tail with rectum attached to ventral side. Male tail tapering, arcuate with minutely striated, broad bursa. Spicula arcuate, 24u long, resting on thin trough-like gubernaculum. Sporozoan parasites, Duboscquia sp., occasionally present on cuticle. Labial annules often less prominent than illustrated with occasionally 3 instead of 2 (Figure 53).
Habitat is generally distributed in cultivated and virgin soil throughout South Dakota. One collection from cultivated field near Minden, Nebraska.
Figure 53 Tylenchorhynchus spp. causing stunt, virus vector. Female body (A). Head (B and C). Male tail (D and E). Female tail (F and G).
- Sweetpotato weevil caused by Cylas formicarius (Fabricius) (Insecta: Coleoptera: Curculionidae). weevil is the most serious pest of sweet potato, not only in the United States, but around the world. It causes damage in the field, in storage, and is of quarantine significance. It is inherently of interest to entomologists due to its strikingly colorful appearance and extremely long rostrum (beak).
Distribution: sweetpotato weevil was first noted in the United States in Louisiana in 1875, and then in Florida in 1878 and Texas in 1890, probably entering by way of Cuba. It is now found throughout the coastal plain of the Southeast from North Carolina to Texas. It also is found in Hawaii and Puerto Rico, and widely around the world in tropical regions.
Life Cycle and Description: a complete life cycle requires one to two months, with 35 to 40 days being common during the summer months. The generations are indistinct, and the number of generations occurring annually is estimated to be five in Texas, and at least eight in Louisiana. Adults do not undergo a period of diapause in the winter, but seek shelter and remain inactive until the weather is favorable. All stages can be found throughout the year if suitable host material is available.
Eggs are deposited in small cavities created by the female with her mouthparts in the sweet potato root or stem. The female deposits a single egg at a time, and seals the egg within the oviposition cavity with a plug of fecal material, making it difficult to observe the egg. Most eggs tend to be deposited near the juncture of the stem and root (tuber). Sometimes the adult will crawl down cracks in the soil to access tubers for oviposition, in preference to depositing eggs in stem tissue. The egg is oval in shape and creamy white in color. Its size is reported to be about 0.7 mm in length and 0.5 mm in width. Duration of the egg stage varies from about five to six days during the summer to about 11 to 12 days during colder weather. Females apparently produce two to four eggs per day, or 75 to 90 eggs during their life span of about 30 days. Under laboratory conditions, however, mean fecundity of 122 and 50 to 250 eggs per female has been reported.
When the egg hatches the larva usually burrows directly into the tuber or stem of the plant. Those hatching in the stem usually burrow down into the tuber. The larva is legless, white in color, and displays three instars. The mean head capsule widths of the instars are 0.29 to 0.32 mm, 0.43 to 0.49 mm, and 0.75 to 0.78 mm for instars 1 to 3, respectively. Duration of each instar is 8 to 16, 12 to 21, and 35 to 56 days, respectively. Temperature is the principal factor affecting larval development rate, with larval development (not including the prepupal period) occurring in about 10 and 35 days at 30o and 24 °C, respectively (Figure 54, left). The larva creates winding tunnels packed with fecal material as it feeds and grows.
The mature larva creates a small pupal chamber in the tuber or stem. The pupa is similar to the adult in appearance, although the head and elytra are bent ventrally. The pupa measures about 6.5 mm in length. Initially the pupa is white, but with time this stage becomes grayish in color with darker eyes and legs. Duration of the pupal stage averages 7 to 10 days, but in cool weather it may be extended to up to 28 days.
Normally the adult (Figure 54, right)) emerges from the pupation site by chewing a hole through the exterior of the plant tissue, but sometimes it remains for a considerable period and feeds within the tuber. The adult is striking in form and color. The body, legs, and head are long and thin, giving it an ant-like appearance. The head is black, the antennae, thorax and legs orange to reddish brown, and the abdomen and elytra are metallic blue. The snout is slightly curved and about as long as the thorax; the antennae are attached at about the mid point on the snout. The beetle appears smooth and shiny, but close examination shows a layer of short hairs. The adult measures 5.5 to 8.0 mm in length. Under laboratory conditions at 15 C, adults can live over 200 days if provided with food and about 30 days if starved. In contrast, their longevity decreases to about three months if held at 30o C with food, and eight days without food. Adults are secretive, often feeding on the lower surface of leaves, and are not readily noticed. The adult is quick to feign death if disturbed. Adults can fly, but seem to do so rarely and in short, low flights. However, because they are active mostly at night, their dispersive abilities are probably underestimated. Females feed for a day or more before becoming sexually active, but commence oviposition shortly after mating; the average preoviposition period is seven days. A sex pheromone produced by females has been identified and synthesized.
Figure 54 Sweetpotato weevil caused by Cylas formicarius. Larvae in the soil (left); adult (centre); damage to sweet potato root-tuber caused by larval feeding (right).
Host Plants: this weevil feeds on plants in the plant family Convolvulaceae. Although it has been found associated with several genera, its primary hosts are in the genus Ipomoea. Among vegetable crops only sweet potato, Ipomoea batatas, is a suitable host. Native plants can be important hosts of sweetpotato weevil. Railroad vine, Ipomoea pes-caprae, and morning glory, Ipomoea panduratea, are among the suitable wild hosts.
Natural enemies. Several natural enemies are known. Wasps such as Bracon mellitor Say, B. punctatus (Muesebeck), Metapelma spectabile Westwood (all Hymenoptera: Braconidae) and Euderus purpureas Yoshimoto (Hymenoptera: Eulophidae) have been reared from sweetpotato weevil larvae in the southeastern United States. There have been no studies of parasitoid effectiveness, but these species seem to be infrequent. Among predators, ants (Hymenoptera: Formicidae) seem to be most important. Diseases, especially the fungus Beauveria bassiana, have been observed to inflict high levels of mortality under conditions of high humidity and high insect density, but field conditions are rarely conducive for disease epizootics.
Damage (Figure 54, right). Sweetpotato weevil is often considered to be the most serious pest of sweet potato, with reports of losses ranging from 5% to 97% in areas where the weevil occurs. There is a positive relationship between vine damage or weevil density, and tuber damage. However, the plants exhibited some compensatory ability, with the relationship between vine damage and yield non-linear, and sometimes not significant.
A symptom of infestation by sweetpotato weevil is yellowing of the vines, but a heavy infestation is usually necessary before this is apparent. Thus, incipient problems are easily overlooked, and damage not apparent until tubers are harvested. The principal form of damage to sweet potato is mining of the tubers by larvae. The infested tuber is often riddled with cavities, spongy in appearance, and dark in color. In addition to damage caused directly by tunneling, larvae cause damage indirectly by facilitating entry of soil-borne pathogens. Even low levels of feeding induce a chemical reaction that imparts a bitter taste and terpene odor to the tubers. Larvae also mine the vine of the plant, causing it to darken, crack, or collapse. The adult may feed on the tubers, creating numerous small holes that measure about the length of its head. The adult generally has limited access to the tubers, however, so damage by this stage is less severe than by larvae (Figure 54, right). Adult feeding on the foliage seldom is of consequence.
sampling: over 90% of larvae are found in the upper 15 cm of the tubers and basal 10 cm of the vine. Early in the season larvae are found about equally in the vine and tuber, but later in the season most occur in the tubers. Distribution of sweetpotato weevil in fields is aggregated. Pheromone traps show great promise for monitoring of adult population density. Weevils respond to low concentrations of pheromone, and apparently will move up to 280 m to a pheromone source. The sex pheromone also shows great potential for mating disruption and mass trapping.
Insecticides: planting time applications of insecticides are commonly made to the soil to prevent injury to the slips or cuttings. Either granular or liquid formulations are used, and systemic insecticides are preferred. Postplant applications are sometimes made to the foliage for adult control, especially if fields are likely to be invaded from adjacent areas, but if systemic insecticide is applied some suppression of larvae developing in the vine may also occur. Due to the long duration of the plant growth period, it is not uncommon for preplant or planting time applications to be followed by one or more insecticide applications to the plant or soil at mid season. Insecticides are also applied to tubers being placed into storage to prevent reinfestation and inoculation of nearby fields.
Cultural practices. Cultural practices are sometimes recommended to alleviate weevil problem. Isolation is frequently recommended, and it is advisable to locate new fields away from previous crops and distant from sweet potato storage facilities, because both can be a source of new infestations. However, despite the infrequency of flight by adults, dispersal can occur over considerable distances. Dispersal rates of 150 m per day have been observed, with dispersal more rapid in the absence of suitable hosts.
Sanitation: it is particularly important for weevil population management. Discarded tubers and unharvested tubers can support large population, and every effort should be made to remove such host material. Related to this, of course, is the destruction of alternate hosts; control of Ipomoea weeds is recommended.
Biological control: entomopathogenic nematodes seem to be the organisms with the greatest potential for practical biological suppression of sweetpotato weevil. Several strains of Steinernema carpocapsae (Nematoda: Steinernematidae) and Heterorhabditis bacteriophora (Nematoda: Heterorhabditidae) penetrate the soil and tubers, killing weevil larvae. At least in the soils of southern Florida, the infective nematodes are persistent, remaining active for up to four months. In some cases nematodes are more effective than insecticides at reducing damage.
Other methods are:
- Suppression are sometimes used, especially for postharvest treatment of tubers.
- Postharvest treatment not only prevents damage in storage, but allows shipment of tubers to areas where sweetpotato weevil is not found but might survive. Traditionally, postharvest treatment has been accomplished with chemical fumigants, but they have fallen from favor.
- Irradiation is potentially effective, although older stages of insects are less susceptible to destruction.
- Storage in controlled atmospheres, principally low oxygen and high carbon dioxide, is very effective for destruction of weevils, but requires good storage conditions.
- Dendrothripoides innoxius. The taxonomy of this phytophagous is: Natura; Mundus Plinius; Naturalia; Biota; domain Eukaryota Chatton 1925; Amorphea Adl et al. 2012; Opisthokonta Cavalier-Smith 1987; Holozoa; kingdom Animalia Linnaeus 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Eubilateria Ax 1987; Protostomia Grobben 1908; Ecdysozoa A.M.A. Aguinaldo et al. 1997; superphylum Panarthropoda; phylum Arthropoda von Siebold 1848; Euarthropoda; Mandibulata; Crustaceomorpha Chernyshev 1960; Labrophora Siveter, Waloszek & Williams; subphylum Pancrustacea Zrzavý et al. 1997; Altocrustacea; Miracrustacea; superclass Hexapoda Latreille 1825; class Insecta Linnaeus 1758; subclass Dicondylia; infraclass Pterygota; Metapterygota; Neoptera; Eumetabola; Paraneoptera; superorder Condylognatha; order Thysanoptera Haliday 1836 (thrips); suborder Terebrantia Haliday 1836; family Thripidae Stevens 1829; subfamily Thripinae; genus Dendrothripoides Bagnall, 1923.
Recognition data and distinguishing features. Female macroptera: body golden yellow with brown shadings laterally; antennal segments IV-VI brown in apical half or more; fore wings pale with brown marking sub-basally and medially. Antennae 8-segmented; segments III- IV with slender forked sensorium. Head with cheeks bulging behind eyes, constricted to basal neck; dorsal surface reticulate, ocellar setae III on anterior margins of ocellar triangle. Pronotum without long setae, discal setal bases prominent. Metanotum irregularly reticulate, median setae well behind anterior margin. Fore wing unusually slender, major setae minute. Tergites II-VIII laterally with numerous broadly based stout microtrichia; VI-VIII with 1-2 pairs of stout setae medially pointing toward midline; posterior margin of VIII with dentate craspedum laterally but smooth craspedum medially; IX-X with setae stout, X fully divided medially. Sternites without sculpture, median pair of setae on VII arising at margin.
Female microptera: similar to macroptera but wing lobe shorter than thorax width.
Male microptera: similar to female; tergite IX with pair of stout thorn-like setae arising from large median tubercle, and 6 small tubercles posterior to this; antecostal ridge of sternites IV-VII with small transverse ore plate.
Related and similar species. Apart from Dendrothripoides innoxius, there are three further species in the genus Dendrothripoides; one from South Africa, one from Thailand, and one from Philippines and Sulawesi (Kudo, 1992). The genus is closely related to two further oriental genera, Indusiothrips Priesner with two species, and Isunidothrips Kudo with one species, but the tergal microtrichia are much weaker in these than in Dendrothripoides.
Life history. Feeding and breeding on leaves.
- General distribution:
- Pacific: Australia, New Caledonia, New Guinea, Vanuatu, Philippines, Guam, Marquesas, Cook Islands, Fiji, Tonga; Hawaii.
- New World: Brazil, Panama, Barbados, Bermuda, Cuba, Jamaica, Dominican Republic, Grenada, Guadeloupe, St. Croix, Trinidad, USA-Florida.
- Old World: Nigeria, Réunion; India, Burma, Nepal, Indonesia, Malaysia, Korea, China, Hong Kong, Taiwan, Japan.
- Australian distribution:
- Northern Territory, Queensland, Western Australia.
Host plants. Ipomoea sp. (Convolvulaceae).
Synonyms. Euthrips innoxius Karny 1914; Dendrothripoides ipomoeae Bagnall 1923; Tryphactothrips mediosignatus Karny 1925; Tryphactothrips mundus Karny 1927; Heliothrips ipomeae Bondar 1930; Scirtothrips gladiiseta Girault 1933.
Figure 55 Dendrothripoides innoxius. Adult (A); head and torax (B); tergite VI-X (C); male sternite (D); Antenna (E); Fore wing (F).
- Sweet potato leaf miner. The major insect pest in Western Australia is the sweet potato leaf miner. This is a small, dark reddish to black grub up to 10 mm long. The adult is a small moth. The grub eats plant tissue between the upper and lower leaf surfaces, leaving a transparent papery window in the leaf. The grub can be seen inside this window. Heavy infestations can develop quickly and severely defoliate the crop, reducing yield.
The genus Bedellia Stainton 1849 is represented ill these islands
by seven species, the larvae in all cases being leaf miners. Bedellia somnulentella (Zeller, 1847)
and Bedellia minor Busck 1900 mine the leaves of species of Ipomea. The taxonomy of genus Bedellia is: Natura; Mundus Plinius; Naturalia; Biota; domain Eukaryota Chatton 1925; Amorphea Adl et al. 2012; Opisthokonta Cavalier-Smith 1987; Holozoa; kingdom Animalia Linnaeus 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Eubilateria Ax 1987; Protostomia Grobben 1908; Ecdysozoa A.M.A. Aguinaldo et al. 1997; superphylum Panarthropoda; phylum Arthropoda von Siebold 1848; Euarthropoda; Mandibulata; Crustaceomorpha Chernyshev 1960; Labrophora Siveter, Waloszek & Williams; subphylum Pancrustacea Zrzavý et al. 1997; Altocrustacea; Miracrustacea; superclass Hexapoda Latreille 1825; class Insecta Linnaeus 1758; subclass Dicondylia; infraclass Pterygota; Metapterygota; Neoptera; Eumetabola; Holometabola; superorder Panorpida; Amphiesmenoptera; order Lepidoptera Linnaeus 1758 (butterflies, moth); suborder Glossata Fabricius 1775; Coelolepida Nielsen & Kristensen 1996; Myoglossata Kristensen & Nielsen 1981; Neolepidoptera Packard 1895; infraorder Heteroneura Tillyard 1918; Eulepidoptera Kiriakoff 1948; Ditrysia Börner 1925; superfamily Yponomeutoidea Stephens, 1829 (sedis mutabilis); family Bedelliidae Meyrick 1880.
The sweetpotatoleaf miner (Bedellia somnulentella) has a nearly cosmopolitan distribution and has been recorded from Russia, Ukraine, Georgia, southern Kazakhstan, Kirgizia, Uzbekistan, nearly all of Europe, the Middle East, Africa, India, Japan, North America, Australia, New Zealand and Oceania. Bedellia minor, the Florida Morning-glory Leafminer Moth, is a moth in the Bedelliidae family. It is found in Florida and on Cuba.
The larvae feed on Calystegia pubescens, Calystegia sepium, Convolvulus althaeoides, Convolvulus arvensis, Convolvulus siculus, Convolvulus tricolour, Ipomoea batatas and Ipomoea purpurea. They mine the leaves of their host plant. The mine starts as a narrow tortuous corridor with a central frass line, that often cuts off part of the leaf. Later, larvae leave the mine and begin to make a series of full depth fleck mines. Pupation takes place outside of the leaf. The pupa is attached to a leaf without a cocoon.
Damage. The larvae are small caterpillars which feed on the green tissue inside the leaf, leaving the transparent upper and lower membranes (epidermis) intact.The young larvae enter the leaf and form serpentine mines (narrow, grey-brown or silvery tracks). As the larva matures, it consumes a broader patch of the leaf, forming blotch mines. Later holes are produced as the mined tissues are destroyed. The lower surface of the infested leaves become dirty with small grains of blackish frass and show silken webbings containing the small pupae. During high infestation, the leaves become brown. A serious outbreak can cut down the effective leaf surface for plant food production resulting in reduced storage root yield.
Morphology. The eggs are oval, flattened against the leaf surface; translucent, greenish white with granulate surface which turns yellowish when about to hatch.
The emerging larvae are distinctly segmented with a rather pointed heads and abdomens. A mature larva measures 5.5 mm long. The larva has a yellowish body with paired pink spots on the dorsolateral sides of the thorax which later disappear and are replaced by red tubercles in all segments.
The pupae measuring 3.5 mm appear green at first with mottled red markings. Later the red markings disappear and they turn dark brown with lateral projections on the abdomen.
The adults are very small moths, 3.5 - 4.0 mm long with grayish to brown bodies and light brown scales.
Biology and ecology. The eggs are laid singly or in groups usually on the lower surface of the leaf near the midrib, veins or at the base of the leaf blade. Incubation lasts 5-6 days. The insect undergoes five larval instars. During the fifth instar, the larva undergoes a short pre-pupal period, comes out of the mine and produces numerous silken threads which fix and support the pupa on the lower surface of the leaf. Pupation lasts 3-6 days. A female adult is capable of laying 1-67 eggs during the 1-2-day oviposition period.
Biological control. Leaf miners are generally controlled by predators and parasites like Apanteles sp.
Chemical control. The insecticides recommended for leaf miner control include carbaryl, chlorfenvinphos, diazinon, dimethoate and trichlorphon.
Figure 56 Sweet potato leaf miner (Bedellia spp.). Adult (A and B) with antennae brownish fuscous, with whitish annulations. Palpi, head and thorax greyish fuscous; face paler. Fore wings greyish fuscous, with some pale cinereous speckling throughout; the only indication of markings is in the absence of the pale speckling at the base of the fold, in a slight spot on the outer half of the fold, and in a short dark streak on the dorsum, but these markings are very obscure; cilia pale greyish fuscous. Hind-wings dark grey; cilia fuscous. Legs greyish fuscous with whitish tarsal speckling. Plants severely damaged by leaf miner (C). Apanteles Forster 1862 sp. (Hymenoptera: Braconidae) parasite of Bedellia spp. (D).
- Silverleaf whitefly. The silverleaf whitefly - Bemisia tabaci (Gennadius 1889), which is also informally referred to as the sweetpotato whitefly - is one of several whiteflies that are currently important agricultural pests. The silverleaf whitefly is classified in the family Aleyrodidae, and is included in the large sub-order of insects, Sternorrhyncha.
The silverleaf whitefly is a complex that include, among others, also Aleurodicus dispersus Russell 1965 (Sternorrhyncha: Aleyrodidae). A review in 2011 concluded that the silverleaf whitefly is actually a species complex containing at least 24 morphologically indistinguishable species.
The silverleaf whitefly thrives worldwide in tropical, subtropical, and less predominately in temperate habitats. Cold temperatures kill both the adults and the larvae of the species. The silverleaf whitefly can be confused with other insects such as the common fruitfly, but with close inspection, the whitefly is slightly smaller and has a distinct wing color that helps to differentiate it from other insects.
While the silverleaf whitefly had been known in the United States since 1896, in the mid-1980s a virulent strain appeared in poinsettia crops in Florida. For convenience that strain was referred to as strain B (biotype B), to distinguish it from the milder infestation of the earlier known strain A. Less than a year after its identification, strain B was found to have moved to tomatoes, and other fruit and vegetable crops. Within five years, the silverleaf whitefly had caused over $100 million in damage to Texas and California agriculture industries.
Bemisia tabaci have six life stages: the egg (Figure 57, D), four nymphal stages (Figure 57, B and C), and the adult (Figure 57, A).
Eggs (Figure 57, D) are oval in shape and somewhat tapered towards the distal end. The broader end has a short stalk, 0.024 mm, that is inserted by the ovipositing female into the leaf. The egg obtains moisture through this stalk. The egg is approximately 0.21 mm in length and 0.096 mm in width. The egg is pearly white when first laid but darkens over time. The distal end of the egg becomes dark brown just before the first nymphal instar ecloses (Figure 57, D).
It stands upright on the leaf, being anchored at the larger end by a tail-like appendage inserted into a stoma. Eggs are generally laid on the undersurface of younger leaves. They are white when first laid but later turn brown. There can be as many as 190 eggs per cm2 (Figure 57, D). About the The nymphal stage (Figure 57, B and C), the first is called "crawlers", when nymphs only move a very short distance before settling down again and starting to feed. Once settled they do not move again. All the nymphal instars are greenish white, oval in outline, scale-like and somewhat spiny. The last nymphal instar (the so-called pupa) is about 0.7 mm long and the red eyes of the adult can be seen through its transparent integument. the first nymphal instar is capable of limited movement and is called the crawler. It is oval in shape and measures approximately 0.27 mm in length and 0.14 mm in width. The dorsal surface of the crawler is convex while the ventral surface, appressed to the leaf surface, is flat. The crawler has three pairs of well-developed four-segmented legs, three-segmented antennae, and small eyes. It is whitish-green in color and has two yellow spots, the mycetomes, visible in the abdomen through the integument (skin). The mycetomes house several species of endosymbiotic bacteria that may play an important role in whitefly nutrition. The crawlers usually move only a few centimeters in search of a feeding site but can move to another leaf on the same plant. They initiate feeding on the lower surface of a leaf, also feeding in the phloem. After they have begun feeding, they will molt to the second nymphal instar, usually two to three days after eclosion from the egg. The second, third and fourth nymphal instars are immobile with atrophied legs and antennae, and small eyes. The nymphs secrete a waxy material at the margins of their body that helps adhere them to the leaf surface. Nymphs are flattened and oval in shape, greenish-yellow in color, and range from 0.365 mm (second instar) to 0.662 mm (fourth instar) in length. The mycetomes are yellow. The second and third nymphal instars each last about two to three days (Figure 57, C). The adult is minute (Figure 57, A), about 1 mm long and emerges through a slit in the pupal skin and is covered with a white, waxy bloom. it has solid white wings and pale yellow body. Adult Bemisia are soft and whitish-yellow when they first emerge from their nymphal exuvia. Within a few hours, their two pairs of wings become iridescent white due to the deposition of a powdery wax. The body remains light yellow with a light dusting of wax. The body of the female measures 0.96 mm from the tip of the vertex (head) to the tip of the abdomen, while the male is somewhat smaller at 0.82 mm. The snow-white colour is due to the secretion of wax on its body and wings.
Figure 57 Silverleaf whitefly Bemisia tabaci. Adult of Bemisia is soft and whitish-yellow when they first emerge from their nymphal exuvia (A).
Red-eyed nymphal or "pupal" stage (B) of Bemisia = sweetpotato whitefly B biotype, Bemisia tabaci (Gennadius 1889), or silverleaf whitefly = Bemisia argentifolii Bellows & Perring 1994 (B).
Nymphal instars (C). The egg is pearly white when first laid but darkens over time. The distal end of the egg becomes dark brown just before the first nymphal instar ecloses (D).
Detection and inspection. The insects appear as small white scale-like objects on the undersurface of the leaves. If the plant is shaken, a cloud of tiny moth-like insects flutter out but rapidly resettle.
Cultural control. 1)Field sanitation and use of insect-free planting material. 2) Movement of clean, healthy germplasm. 3) Planting non-host crops after sweetpotato.
Biological control. Natural enemies were introduced to control whiteflies. The parasitic wasp Encarsia formosa Gahan 1924 (Hymenoptera: Chalcidoidea, Aphelinidae) and Encarsia pergandiella Howard 1907 (Figure 58, A), Encarsia transvena Timberlake, 1926 (Figure 58, B), Encarsia nigricephala (Figure 58 C, left), and Eretmocerus Haldeman 1850 sp. - Hymenoptera: Aphelinidae - (Figure 58 C, left) are effective biological control agents against both Aleurodicus dispersus and Bemisia tabaci. In particular, Encarsia transvena give the highest percentages of parasitization occurred in third instar and fourth instar Bemisia tabaci and the lowest percentages in first and fourth instars). Other species of coccinellid beetles such as Delphastus catalinae Casey 1899, Coleoptera: Coccinellidae Latreille, 1807 (Figure 58, I) , true bugs Hemiptera: especially Anthocoridae such as Orius Wolff 1811 sp., Linnaeus 1758 (Figure 58, G) and Lygaeidae such as Geocoris Fallen, 1814 (Figure 58, H), and predatory Miridae), lacewings Neuroptera: Chrysopidae (Figure 58, F), Hemerobiidae, Coniopterygidae), flies (Diptera: Dolichopodidae, Syrphidae, Anthomyiidae), ants (Hymenoptera: Formicidae), spiders (Araneida) and mites (Acarina: Phytoseiidae, Stigmaeidae) also prey on them. The entomopathogenic fungus, Paecilomyces fumosoroseus is able to infected Bemisia nymphs (Figure 58, L).
Figure 58 Biological control of silverleaf whitefly. Pupa of Encarsia pergandiella (A), and black pupal case of Encarsia transvena (B), and pupa of Encarsia nigricephala (C, left) and Eretmocerus sp. (C, right, respectively) within body of Bemisia. Female of an Eretmocerus species host-feeding on Bemisia nymph (D and E). Lacewing larva Chrysoperla sp. (F), and adult minute pirate bug Orius (G), feeding on Bemisia nymphs. Adult bigeyed bug Geocoris sp. (H). Coccinellidae Delphastus catalinae predator of Bemisia nymphs (I). Bemisia nymph infected with the entomopathogenic fungus, Paecilomyces fumosoroseus (L).
Chemical control. Although control measures are not usually needed, insecticides like trichlorphon and carbaryl can be recommended during high infestation. Sugar esters from Petunia spp. are also found effective against sweetpotato whitefly. However, controlling whiteflies is not usually an effective means of limiting the incidence of the viruses they transmit.
- White-fringed weevil. The white-fringed weevil (Naupactus leucoloma Boheman 1840; Coleoptera: Curculionidae) is a major pest of several crops in Australia.
Originally from South America, they were first reported in New South Wales in 1932 but are now found throughout cropping areas of Australia, including Tasmania where they appear to be increasing their range. Infestations in pasture and lucerne often go unnoticed until crops are planted. Once established, they are extremely difficult to eradicate. The weevil grub (larva) is the pest stage. They live in the soil and feed on the roots of a wide range of plants. In potatoes they burrow into tubers, leaving small, shallow holes which can become infected with other organisms. Adult weevils feed on a wide variety of plants (over 380 plant species) at the base of leaf margins, leaving characteristic notching, but this seldom causes economic damage.
Geographical distribution. The centre of origin of Naupactus species is South America, with Naupactus leucoloma ranging from 12°S to 42°S, west of the Andes and from 15°S to 38°S, east of the Andes.
Description. Adults are slate-grey with distinctive white stripe on each side of the wing cover (resembling a sunflower seed on legs), 10-13 mm in length. Wing covers fused together, so they cannot fly but can walk long distances (Figure 59, A).
- EPPO region: Absent.
- Africa: South Africa (Northern Cape Province).
- North America: USA first recorded in Florida in 1936, it has since spread to Alabama, Arkansas, Georgia, Kentucky, Louisiana, Mississippi, New Mexico (Sites & Thorvilson, 1988), North Carolina, South Carolina, Tennessee and Texas and Virginia.
- South America: Argentina (Buenos Aires, Catamarca, Chaco, Córdoba, Corrientes, Entre Rios, Formosa, Jujuy, La Pampa, La Rioja, Mendoza, Rio Negro, San Juan, San Luis, Santa Fe, Santiago del Estero, Salta and Tucumán), southern Brazil (Rio Grande do Sul) and Uruguay. It is considered as introduced in Chile and Peru.
- Oceania: Australia, first reported in New South Wales in 1932, it is now quite widespread within the state; it is also in Victoria, Western Australia and Queensland. New Zealand, first discovered in the Auckland area in 1944, now well established in several parts of North Island; also occurs in South Island.
Eggs are Very small (<1 mm diam.), oval, laid in clusters of about 12-60 in the soil on roots, in ground litter beneath plants or on stems and lower leaves of plants. Milky-white when first laid, changing to dull light-yellow. Fixed together with a sticky, gelatinous mass which hardens into a protective film allowing them to withstand drought for several months. Soil sometimes sticks to an egg mass making detection impossible. Hatch in about 2 weeks after rain or irrigation.
Larvae are grey-white body with yellow-brown heads, large black mandibles (jaws) and no legs. Head is retracted into the body so only the jaws are clearly visible. Eleven growth stages (instars) with the first being a non-feeding stage that can survive for several months in the soil before moulting to the second instar to commence feeding on roots. Fully grown grubs are about 13 mm x 6 mm and cause the most damage. Found in the soil mostly at depths of 5-15 cm (Figure 59, B).
Pupae are white, 10-12 mm long, found in oval chambers 5-15 cm deep in the soil in spring and early summer. Turn brown just before hatching (Figure 59, C).
Hosts. Naupactus leucoloma is a highly polyphagous pest, able to feed on a very wide range of plant species (with more or less damage). On crops of interest for the EPPO region, it causes most severe damage in the following: Brassica spp., Daucus carota, Fragaria, Medicago sativa, Pisum sativum, Rubus spp., Solanum tuberosum, Trifolium spp. and Zea mays. Pastures can be seriously damaged, with the legumes and not the grasses being attacked. Naupactus leucoloma has been recorded on 385 species in the USA alone, including, besides the above-mentioned, various herbaceous crops such as Arachis hypogaea, Ipomoea batatas and Vigna unguiculata, weeds, grapevine and trees such as Prunus persica (peach) and Salix (willow).
Biology. Males are rare and have only been found in South America. Outside South America, only parthenogenic females are found. Five to 25 days after emerging, mature females begin to lay up to 1500 eggs in groups of 20-60 over a two-month period. Eggs hatch in 11-30 days, and developmental threshold temperatures and thermal constants have been determined by Masaki (1998). There are 11 larval instars, the first of which does not feed. The larval stage usually overwinters, although eggs can also overwinter. It is the damage caused by larval feeding that makes Naupactus leucoloma a pest. Larvae pupate in oval chambers in the soil during early summer. Adults emerge in the summer. The elytra are fused and adults cannot fly, so high densities can build up locally. Up to 200 or 300 individuals can be found per plant.
Figure 59 White-fringed weevil (Naupactus leucoloma). Adult (A), larva (B), and pupa (C).
Means of movement and dispersal. Adults cannot fly but they actively crawl and climb. Females can crawl 0.4-1.2 km during their 2-5-month adult life (Metcalf & Metcalf, 1993). Adults cling to hay and other crops and to vehicles and agricultural equipment being transported, and can thus be carried in trade. Since eggs are laid on many parts of host plants and remain viable for more than 7 months, they can also be transported in trade. Eggs, larvae and pupae may also be transported with soil attached to plants for planting or turf. As females are parthenogenetic, the chance of small populations colonizing new regions is increased.
Pest significance and economic impact. Very low population densities of Naupactus leucoloma can cause economic damage. A density of only one larva m-1 row of potatoes (equivalent to about 1 larva 1.5 m-2) resulted in a loss of 9% of average gross return. As larvae feed on roots, the damage they cause is noticed when plants begin to show stress by becoming yellow or stunted. Larvae often sever a plants main root while feeding. In potatoes, damage is more spectacular, as larvae tunnel inside the tubers. In New Zealand, the nitrogen fixation rate of Trifolium repens was reduced by 92% by Naupactus leucoloma larval feeding. Larvae hatching from eggs in early or late summer reach sufficient size to damage sweet potato roots before the autumn harves. Adults feed on leaves, but the resulting damage is very minor except at high population densities. Metcalf & Metcalf (1993) stated that entomologists who have studied the insect feel that it may become a serious pest in many regions of the United States and hence, the USA has internal phytosanitary regulations to limit its spread.
Control of white-fringed weevil grubs with insecticides is difficult and can give variable results. Planting of potatoes in paddocks previously sown to legumes or other preferred hosts should be avoided. Already established infestations within a paddock are best reduced by having long-term rotations of unsuitable host plants such as cereals or grasses.
A sampling plan for white-fringed weevil grubs, previously devised by researchers in Western Australia and Victoria, aims to assess the risk of damage to potato crops before planting so that informed decisions can be made on whether there is a need to apply insecticides or not. Sampling is best done during winter months when grubs are large, easy to identify and readily visible in the soil:
Once established on outdoor crops, little can be done to control infestations except to grow oats and small grain cereals on infested land as these crops are not attacked to any great extent by Naupactus leucoloma. Adult Naupactus leucoloma cannot fly, so ditches about 25 cm deep and 25 cm wide, with steep, well-packed sides can be used to prevent populations from spreading. Holes in the ditches can trap the adults which can then be destroyed with kerosene. Unfavourable weather, soil conditions, parasites, predators and diseases are important factors in keeping Naupactus leucoloma in check. Carabid beetle larvae, horsefly larvae, wireworms and ants feed on Naupactus leucoloma in the field and vertebrates such as toads, mice, snakes and birds feed voraciously on adult beetles (Young et al., 1950). Adult beetles are susceptible to a wide variety of insecticides, but it is the larvae that need to be targeted as they cause the most damage. However, the soil-dwelling larvae are difficult to control; chlorpyrifos and metam-sodium are the best products to use, but they do not give entirely satisfactory results. Crop rotation is probably the best form of control. Matthiesen et al. (1997) reported research suggesting that rotation with high-glucosinolate Brassica spp. will lead to the release, during their decomposition of their residues, of methyl isothiocyanate (the active decomposition product of metam-sodium), thus providing a means of "biofumigation" against larvae of Naupactus leucoloma. Methyl isothiocyanate was found to be the most active of several fumigants against Naupactus leucoloma.
- use a spade to take a sample of soil (approximately 20 cubic cm);
- sift through the soil by hand to search for the white grubs;
- for an average sized paddock take 5 spade samples from each of 9 randomly selected and widely separated areas;
- if more than 1 grub is found in the 9 sampling areas, then consider either not planting to potatoes or treating the soil with an insecticide before planting;
- if weevils are present in only localised patches, then treatment of just these areas may be adequate.
Phytosanitary risk. Naupactus leucoloma has spread from its native South America to South Africa, Australia, New Zealand and the USA. Despite phytosanitary measures in the USA, it has spread from Florida to states further north and west. Naupactus leucoloma damages many important crop plants, particularly potato and forage plants, and can survive on a great variety of other hosts. In countries where it has been introduced, it usually becomes a pest. Given the current distribution of Naupactus leucoloma (south-eastern USA and South America), most of the southern part of the EPPO region would be climatically suitable for establishment of this pest.
Phytosanitary measures. No specific measures have yet been recommended at the European level, but the general measures recommended for soil-borne pests should apply. Plants of host species with roots from countries in which Naupactus leucoloma occurs should be grown following EPPO Standard PM 3/54 (OEPP/EPPO, 1994).
- African black beetle. African black beetle causes severe damage by chewing holes in sweet potato roots and tubers may be damaged also. It is common in crops following pasture. Heteronychus arator (Fabricius, 1775) is a species of beetle in the order Coleoptera Linnaeus 1758; family Scarabaeidae Latreille 1802; subfamily Dynastinae MacLeay 1819 (the rhinoceros beetles). It is commonly called African black beetle or black lawn beetle. It is native to Africa and it is an introduced species in Australia and on the North Island of New Zealand. It is a shiny black oval-shaped beetle 12 to 15 mm long. The beetle causes tremendous damage to lawns and other turf, especially during the summer. It also attacks many crop plants and garden flowers, as well as trees and shrubs.
- Rutherglen bug. Rutherglen bug (Nysius vinitor Bergroth 1891: order Hemiptera Linnaeus, 1758 ; family Lygaeidae Schilling 1829) can appear in large numbers in November and December. The small bugs delay plant establishment. Rutherglen bug breeds on weeds, particularly portulaca, and eradication of weeds near the crop will reduce problems.
- Aphids. The aphids constitute a large group of small, soft-bodied insects that are frequently found in large number sucking the sap from the stem or leaves of plants. Melon and green peach aphids attack a number of crops and are vectors of many viruses attacking sweetpotato and other crops. The geographical distribution is worldwide.
Aphids such as Aphis gossypii Glover 1877 and Myzus persicae (Sulzer 1776), usually attack the growing shoots and expanding leaves. They feed on the lower surface of the leaves and injure the plants by sucking the sap. The leaves become deformed as they expand. They may curl down at the edges, and become wrinkled or puckered. Feeding on expanded leaves (more common with green peach aphid) may result in pale stippled areas of feeding damage between the veins. During heavy infestation, the vigour of the plant is greatly reduced, stunting growth of the plants. Leaves of such stunted plants are pale and may have yellow interveinal areas. Both species of aphid transmit several virus diseases in sweetpotato and in other crops. Infestation of the two aphid species could be differentiated by the production of the honeydew. Myzus persicae produces less honeydew than Aphis gossypii.
Aphis gossypii belongs to class Insecta Linnaeus 1758; subclass Dicondylia; infraclass Pterygota; Metapterygota; Neoptera Martynov, 1923; Eumetabola; Paraneoptera; superorder Condylognatha; order Hemiptera Linnaeus 1758; suborder Sternorrhyncha; infraorder Aphidomorpha; superfamily Aphidoidea; family Aphididae; subfamily Aphidinae; genus Aphis Linnaeus 1758.
Myzus persicae belongs to class Insecta Linnaeus 1758; subclass Dicondylia; infraclass Pterygota; Metapterygota; Neoptera Martynov, 1923; Eumetabola; Paraneoptera; superorder Condylognatha; order Hemiptera Linnaeus 1758; suborder Sternorrhyncha; infraorder Aphidomorpha; superfamily Aphidoidea; family Aphididae; subfamily Aphidinae; genus Myzus Passerini 1860 .
Morphology. These insects can be recognized by its pear-like shape, a pair of cornicles at the posterior end of the abdomen and fairly long antennae; winged forms can usually be recognized by the venation and relative size of the front and hind wings. The cornicles of aphids are tube-like structures arising from the dorsal side of the fifth or sixth abdominal segment.
Melon aphid: the nymphs are green to brown and moult four times before reaching the adult stage. The nymphs look like the wingless adults except for their small sizes and softer body. The adults are yellowish to green or black about 1.5 mm long.
Green peach aphid: the nymphs are similar to wingless adults but are smaller in size. The adults are small to medium sized, 1.2-2.5 mm long. They are usually green with darker thorax. The antennae are two-thirds as long as the body. The cornicles are clavate and fairly long. The face when viewed dorsally has a characteristic shape. This species generally produces little honeydew.
Biology and ecology. The life cycle of aphids is rather unusual and complex. The females reproduce parthenogenetically. Several generations may be produced in short period of time. The first generation usually consists of wingless individuals, however, when a colony becomes too crowded, winged individuals appear. The winged forms migrate to a different host plant and begin new colonies, a generation consisting of both males and females is produced, and the reproductive process continues. Aphids secrete honeydew which is emitted from the anus; the honeydew consists mainly of excess sap ingested by the insect, to which are added excess sugars and waste materials. This honeydew may be produced in sufficient quantities to cause the surface of leaves to become sticky.
Host range. Aphis gossypii, apart from sweetpotato, this species also damages citrus, cocoa, coffee, cotton, cucurbits, eggplant, okra, pepper, potato and also ornamentals like Hibiscus. Mysus persicae - can occur on bitter gourd, cabbage, cauliflower, condol, chayote, eggplant, lemon, lettuce, loofah, melon, mustard, pechay, pomelo, potato, raddish, squash, tomato, tobacco, watermelon, and on weeds like Prunus persica, Prunus nigra, Prunus tanella, and Prunus serotina.
Management. About the biological control, the aphids are attacked by a number of ladybird beetles (Menochilus sexmaculatus, Coelophora inaequalis and Scymnus sp.) a chrysopid predator (Chysopa oculata), larva of syrphid fly (Ischiodon scutellaris), spiders (Oxyopes javanus and Thomisus sp.), a brachonid wasp (Opius sp.) and entomopathogenic fungi.
About the chemical control insecticides applicable for aphids include organophosphorous, pyrethroids, carbamates and neonicotinoids. Apply insecticides only when necessary. Aphids have been found to develop resistance to insecticides in areas where they are regularly used. Often, predator insects are more severely affected than the aphid, with the result that aphid populations increase after insecticide use.
- Armyworm (also named cluster caterpillar), common cutworm (also named beet army worm) and african cotton leafworm (also named egyptian cotton leafworm). They are three sweetpotato phytophagous belonging to genus Spodoptera: Spodoptera litura (Fabricius, 1775), the first, Spodoptera exigua (Hubner 1808), the second, and Spodoptera littoralis Boisduval 1833, the third. Their taxonomy is: Natura; Mundus Plinius; Naturalia; Biota; domain Eukaryota Chatton, 1925; Amorphea Adl et al. 2012; Opisthokonta Cavalier-Smith 1987; Holozoa; kingdom Animalia Linnaeus 1758; Epitheliozoa Ax 1996; Eumetazoa Bütschli 1910; Bilateria Hatschek 1888; Eubilateria Ax 1987; Protostomia Grobben 1908; Ecdysozoa A.M.A. Aguinaldo et al. 1997; superphylum Panarthropoda; phylum Arthropoda von Siebold 1848; Euarthropoda; Mandibulata; Crustaceomorpha Chernyshev 1960; Labrophora Siveter, Waloszek & Williams; subphylum Pancrustacea Zrzavý et al. 1997; Altocrustacea; Miracrustacea; superclass Hexapoda Latreille 1825; class Insecta Linnaeus 1758; subclass Dicondylia; infraclass Pterygota; Metapterygota; Neoptera; Eumetabola; Holometabola; superorder Panorpida; Amphiesmenoptera; order Lepidoptera Linnaeus 1758; suborder Glossata Fabricius 1775; Coelolepida Nielsen & Kristensen, 1996; Myoglossata Kristensen & Nielsen 1981; Neolepidoptera Packard 1895; infraorder Heteroneura Tillyard 1918; Eulepidoptera Kiriakoff 1948; Ditrysia Börner 1925; Obtectomera Minet 1986; Macroheterocera Chapman 1893; superfamily Noctuoidea Latreille 1809; family Noctuidae Latreille, 1809; subfamily Noctuinae Latreille, 1809; genus Spodoptera Guenée, 1852.
Cutworms, armyworms and leafworms attack a number of crops and may cause significant damage depending on severity of infestation. The geographical distribution is worldwide. In particular, Spodoptera littoralis is reported to be cosmopolitan in distribution. The overall range of the moth distribution is given by the C.I.E. map A232 (1967). This species is normally confined to tropical and subtropical and warm temperature regions, where the mean temperature does not fall below 10 °C. They noted that the moth is distributed chiefly in areas between the latitudes 35°N and South and within the annual isotherm of 20 °C. The following African states which fall in the distribution area of Spodoptera littoralis: Morocco, Algeria, Tunisia, Lybia, Egypt, Mauritania, Senegal, Gambia, Mali, Niger, Chad, Sudan, Guinea, Sierra Leone, Liberia, Ivory Coast, Upper Volta, Ghana, Togo, Nigeria, Cameroun, Central African Republic, Gabon, Zaire, Rwanda, Burundi, Ethiopia, Eritrea, Somali Republic, Kenya, Uganda, Tanzania, Angola, Rhodesia, Malawi, Mozambique, Southwest Africa, Botswana, Swatziland and South Africa. The Spodoptera littoralis moth is also distributed in the islands of the Indian Ocean (e.g. Madagascar) and in the Atlantic Islands (e.g. Canary Islands).
Damage. On hatching, clusters of young larvae feed gregariously by initially scraping the surface of the leaf (Figure 60, A). Later instars disburse and move on to other leaves and feed voraciously, producing large irregular holes and may leave only the veins. High infestation causes severe defoliation. Army worms quickly skeletonise leaves as they attack in clusters. Leafworms
Biology and ecology. The eggs hatch after 3 days from egg deposition and take about two weeks to reach the pupal stage. The larvae prefer moist sites. The larvae hide during the day in the crevices found in the soil and plant residues and become active during dusk to dawn. Pupation takes place in the soil in an earthen cell. Pupation lasts for a week. The development of the armyworm/cutworm from egg to adult takes about 3.5 to 4 weeks. The female Spodoptera exigua lays up to 1,000 eggs while Spodoptera litura lays as many as 2,000-2,600.
- Armyworm (Spodoptera litura). The eggs are laid in mass on the surface of the leaves and covered with whitish scales. The egg stage lasts for 3 days.
The larvae undergo several colour phases from green to almost black (Figure 60, B). The larval stage lasts for about 9-14 days and usually has six instars.
Pupation occurs in the soil and lasts for about 10 days.
Adults are smaller than other members of the cutworm-armyworm group, about 12 mm long with a wing spread of 25 to 40 mm. Body and wings range from silvery-grey to greyish-brown. Forewings have a lighter spot near the centre. Hindwings are paler with darker borders, with a light band at the wing edges.
- Cutworm (Spodoptera exigua). The eggs are round, pearl-white, laid in mass on the ground or on the surface of the leaves and covered with yellowish brown hairs. An egg mass contains about 100 to 300 hundred eggs. Egg hatches in 3-6 days.
About larva, the newly hatched larvae are greenish with a dark longitudinal band on each side. The larvae are pale greenish brown with dark markings and the body may have rows of dark spots or may have transverse and longitudinal grey and yellow bands. They gradually turn brownish black as they mature. The fully grown larva is stout and cylindrical measuring 30-50 mm in length (Figure 60, D).
The pupae are reddish dark brown. Their legs and appendages are not capable of free movement. Pupal stage lasts for about 12 days.
The adults (Figure 60, C) are brown moths with greyish brown forewings patterned with wavy markings and the hindwings are transparent with a brown narrow band along the outer margins.
- Leafworm (Spodoptera littoralis). The eggs are creamy white when freshly laid or pale metallic if they are infertile and turn to blue-black dorsally and pale below as the embryo develops. The eggs are laid in clusters and are covered with scales from the female body. The color of the scales is golden brown or buff and is a diagnostic characteristic for distinguishing eggs of Spodoptera littoralis from those of other Spodoptera species. The egg is as spherical and slightly flat. The dimension of eggs may range from 1/3 mm to 0.6 mm.
The full grown larva is about 35-45 mm in length and its head width is 2.9 mm with some dorsal and dorso-lateral stripes running along the body and black spots above the spiracle of the last segment. Color of the larva is green during early instars, but is brown, grey brown or black during later instars, but never green (Figure 60, G). A larval polymorphism is brought about by crowding conditions and expressed in differences of color, behavior, and developmental rates between crowded and isolated individual larvae.
The pupa is 16-20 mm in length and its cremaster bears one pair of terminal spines. The color of the pupa is brown or red-brown. The pupa is generally found in moist soil at a depth of 3-4 cm.
Adult Spodoptera littoralis (Figure 60, F) have a body length of 15-20 mm with a wing span of about 30-41 mm. Its general body color is brownish grey. The forewing is strongly variegated with light and dark markings; the hind wings are white with veins not or scarcely infuscate reported that the forewings are brown with pale lines along the veins and the bluish areas at their tip and base distinguish the male from the female. The hind wings are pearly white. The morphometrics (length of the body, hind femur, costal and apical margins of the wings of forewing and cleavimum) of adults are greater in moths which develop from larvae reared in isolated conditions than those which are reared in crowded cultures.
Host range. Apart from sweetpotato, armyworms attack asparagus, banana, cacao, corn, citrus, garlic, jute, kenaf, mulberry, onion, passion fruit, sesame, sorghum, soybean, tobacco, rice, tomato, sugarcane, cotton, beans, peanuts, castor oil plants, taro, wheat, white potato, a number of crops under Cruciferae and Cucurbitaceae, grasses, and some broad leaf weeds.
Detection and inspection. Aside from the symptoms of damage, the plant can also be inspected for the presence of eggs and feeding larvae.
From Spodoptera littoralis various parasites are recorded: important parasites of Spodoptera littoralis in Continental
Africa and in Egypt, great numbers of parasites and predators are recorded, but their impact on this species was reported
to be negligible. Several viruses are found on Spodoptera littoralis. The following parasites are found on Spodoptera littoralis: Diptera: Tachinidae Bigot 1853:
Carcelia evolans (Wiedemann 1830), Phorcida inconspicua Meigen 1824, Actia aegyptiaca Willen; Icheumonidae: Charops Holmgren 1858 sp., Metopius Panzer 1806 sp.; Braconidae: Apanteles risbeci de Saeger 1942, Apanteles nioro Risbec 1951, Apanteles sagax Wilkinson 1929, Apanteles ruficrus (Haliday 1834); Chalcidae: Hockeria unicolor Walker, 1834;>br>
Chemical control: Azinphosethyl, carbaryl, chlorphyriphos, monocrotophous phenthoate are recommended for armyworm control. Maher and Rasmy (1969) reported in Egypt, that Spodoptera littoralis has developed resistance to Toxaphene, Trichlorfon (= Dipterex) and Endrin. They suggested to control Spodoptera littoralis that several methods should be used and different insecticides should be applied in limited quantities in different areas.
Figure 60 Armyworm (Spodoptera litura), common cutworm (Spodoptera exigua), and leafworm (Spodoptera littoralis). A skeletonized leaf eaten by armyworm (A); larva of Spodoptera litura (B); adult moth of Spodoptera exigua (C); larva of Spodoptera exigua (D); adult moth of Spodoptera litura (E); adult moth of Spodoptera littoralis (F); larva of Spodoptera littoralis (G).
- Tussock moth (also named yellow tail moth). Scientific name is Euproctis similis (Fuessly 1775). The taxonomy is: Class Insecta Linnaeus 1758; subclass Dicondylia; infraclass Pterygota; Metapterygota; Neoptera; Eumetabola; Holometabola; superorder Panorpida; Amphiesmenoptera; suborder Glossata Fabricius 1775; Coelolepida Nielsen & Kristensen 1996; Myoglossata Kristensen & Nielsen 1981; Neolepidoptera Packard 1895; infraorder Heteroneura Tillyard 1918; Eulepidoptera Kiriakoff 1948; Ditrysia Börner 1925; Apoditrysia Minet 1983; Obtectomera Minet 1986; superfamily Noctuoidea Latreille 1809; family Erebidae Leach 1815; tribe Leucomini Grote 1895; genus Euproctis Hübner 1819.
Geographical distribution. Worldwide in distribution.
Economic importance. Tussock moth is a potential defoliator. A group of tussock moth caterpillars can defoliate a whole plant overnight.
Damage. Young larvae damage sweetpotato by scraping the lower surface of the leaf leaving the epidermis intact. Older larvae chew off the leaves leaving irregular holes, with main veins intact (Figure 61, A).
Morphology. Eggs are round yellowish eggs are laid in a cluster, generally on the lower leaf surface, and protected with yellowish brown hairs (Figure 61, B). The first larval instar has brown head and translucent creamy brown body covered with brown hairs (Figure 61, C). Later instars have black body with narrow yellowish orange line in between two white lines on the dorsal side of the abdomen. The head and posterior regions are orange with black short lines. Creamy white lines traverse the lateral sides of the abdomen of the second and third instars. The lines change to yellow as the larva matures. Long black and short white irritant hairs arise from the tubercles around the abdomen, head portion and posterior tip of the larva (Figure 61, D). First instar larva measures 1.5 to 2 mm (Figure 61, C) and mature larva measures 26-31 mm (Figure 61,D). The obtect pupa is enclosed in a brown cocoon attached on the surface of the leaf. The adult is a small light brown moth with a large tuft of yellowish hairs at the tip of the abdomen which is used to cover the newly-laid eggs.
Biology and ecology. The female deposit eggs only once and lay all eggs together, covering them with yellowish brown hairs. Incubation lasts for a week. This species of tussock moth undergoes six larval instars lasting about 18-30 days. Pupation takes place in a cocoon produced by the last instar larva, and lasts about a week. Total developmental period requires 30-40 days. Adult longevity lasts for 5-7 days for the male and 6-8 days for the female.
Host range. Yellow tail tussock moth has a wide host range including forest trees, ornamentals and cultivated crops like sweetpotato.
Detection and inspection. Young larvae feed in groups, usually on the lower leaf surface. Older caterpillars are less gregarious. Surface of the leaves grazed by young larvae is littered with black small frass. Frass is present on soil under defoliated plants.
Management. Control of this pest is seldom necessary. About the biological control, a number of natural enemies including egg and larval parasitoids and entomopathogens are reported to attack immatures of yellow tail moth.
Figure 61 Tussock moth (Euproctis similis). Tussock moth larvae feeding on leaf (A); egg mass covered with hairs (B); first instar larvae (C); sweetpotato tussock moth late instar larva (D):
- Leaffolders are considered important defoliators of sweetpotato. The most important sweetpotato leaffolders are:
- Black leaffolder, Helcystogramma convolvuli (Walsingham 1908), phylum Arthropoda von Siebold 1848; class Insecta; order Lepidoptera Linnaeus 1758; family Gelechiidae Stainton 1854; genus Helcystogramma Zeller 1877. Synonyms are: Trichotaphe convolvuli Walsingham 1908; Brachmia convolvuli Lecithocera convolvuli Lecithocera emigrans Meyrick 1921; Brachmia crypsilychna Meyrick 1914; Lecithocera effera Meyrick 1918; Brachmia dryadopa Meyrick 1918. Other commun name of this moth dangerous species are: sweet potato moth, sweetpotato webworm moth, sweetpotato leaf roller or black leaf folder. It is mainly found in Asia and Africa, but there are also records from the Oceania, the Middle East, the Caribbean and Florida. The species is also found on the Canary Islands and Madeira. The wingspan is 1315 mm. The forewings are dark tawny fuscous. The hindwings are brownish grey (Figure 62, A). The larvae feed on Convolvulaceae species, including Ipomoea batatas, Convolvulus arvensis, Merremia quinquefolia and Ipomoea aquatica. First instar larvae create a tunnel of silk along the leaf vein and feeds underneath on the surface tissue. The second instar larvae move to the upper leaf surface and fold the leaf, feeding within the fold until the green tissues are consumed, when it moves to another leaf. The young larva feeds on the upper leaf surface leaving the lower epidermis intact. As the larvae mature, they eat through the leaf blade producing lace-like holes with the main veins remaining intact (Figure 62, B and C). Folds are usually single, but sometimes two folds are made, or two leaves are joined together. The holes produced by green leaffolder are generally bigger than those produced by the black leaffolder. The feeding area may turn brown and is littered with blackish frass. Frass is excreta of insects such as leaffolders, sweetpotato stem borer and sweetpotato weevils. It is brown or blackish, and may be dry and friable or moist. It is one of the best indicators of borer presence in a vine.
Morphology: the eggs are oval, yellowish white when newly laid and turn pinkish yellow when about to hatch. About the larvae, newly hatched larvae are whitish at first turning greenish yellow later without any markings; markings appear only in the second instar with distinct black and white marks appearing on the head, thorax and first and second abdominal segments; the later instars retain the black markings which become larger and more prominent as the larvae mature; full-grown larvae measure about 15 mm (Figure 62, B). The pupae of the black leaffolder are about 7 mm, yellowish brown at first turning dark golden brown later. They have a tuft of hairs at the tip of the abdomen and are enclosed in scanty cocoon. The adults are grayish black moths, 8 mm long, with scattering of white scales on the body and appendages (Figure 62, A).
Biology and ecology. The eggs are laid singly along the veins on the underside of the leaf or on the terminal shoots. Incubation period is about 3-5 days. The insect undergoes five larval instars for a period of 2-5 days for each instar. A newly hatched larva is whitish at first. The average total larval period is eleven days. The pupal period is 4-7 days. A female moth lays an average of 44 eggs and lives an average of five days.
- Brown leaffolder, Ochyrotica concursa Walsingham 1891; class Insecta Linnaeus 1758; order Lepidoptera Linnaeus 1758; family Pterophoridae Latreille 1802; genus Ochyrotica Walsingham 1891. A synonym is Steganodactyla concursa Walsingham 1891. Ochyrotica concursa is known from Sri Lanka. In the past it was also recorded from the Ryukyu Islands (Tokuno-shima, Okinawa), as well as in Minami-Daito-jima, Taiwan, China, the Philippines, India, the Moluccas and New Guinea, but research suggest these records are not related to this species. The length of the forewings is 67 mm. (Figure 62, D). The larvae feed on Ipomoea batatas. Brown leaffolder attacks the shoots and feeds inside the unopened leaves. The young larva feeds on the upper leaf surface leaving the lower epidermis intact. The damaged tissue may turn brown in response to the injury (Figure 62, F)). Blackish frass is present and a good indicator that the damage was caused by a caterpillar (Figure 62, E). As the larva matures, it eats right through the leaf blade producing irregular holes on the young expanding leaves (Figure 62, F).
Morphology: the eggs are oblong and brownish yellow. About larva, the body is green which turns brownish as it matures. The head is greenish with brownish tinge; the body is covered with short white setae, 2 pairs of slightly longer setae per segment, prothorax with anteriorly directed setae; the different instars have similar markings and colouration but differ only in size; full-grown larvae measure 5 mm (Figure 62, E). About pupa, the Ventral part is light green while the dorsal part is brown; the mid-dorsal region is sharply ridged and the anterior part of the abdomen is covered with setae; just before adult emergence, wing part turns brown with darker brown markings; length of pupa is 6.5 mm. The adults are dark brown moths (4.5-5 mm) with brown head and long light brown antenna. The wings are brown with light brown markings. The entire margin of the wings has dense long hair (Figure 62, D).
Biology and ecology. The eggs are laid singly along the veins on the shoots, especially on very young unopened leaves. Sometimes eggs are observed on the petiole of the shoots. The female lays 2-10 eggs in seven days. The eggs hatch 4-5 days after oviposition. The insect undergoes five larval instars lasting from 2-4 days per instar. The total larval period ranges from 9-17 days. Pupation lasts for 5-6 days. The duration from egg-laying to adult emergence ranges from 18-27 days. Longevity of adults lasts from 2-7 days.
- Green leaffolder, Herpetogramma hipponalis (Walker 1859), phylum Arthropoda von Siebold 1848; class Insecta Linnaeus 1758; order Lepidoptera Linnaeus 1758; family Crambidae Latreille 1810; genus Herpetogramma Lederer 1863. Herpetogramma hipponalis was described by Walker in 1859. It is found in Malaysia, China, the Keeling Islands, Guadalcanal, New Guinea and Australia, where it has been recorded from the Northern Territory and Queensland. The wingspan is about 20 mm. Adults are bright yellow with dark brown zig-zag lines across the wings (Figure 62, G). The larvae feed on Ipomoea batatas. Older larvae create a shelter in a folded leaf of their host plant which is closed with silk. The larvae are green with black marks on the thorax. Folds are usually single, but sometimes two folds are made (Figure 62, L), or two leaves are joined together (Figure 62, I). The holes produced by green leaffolder are generally bigger than those produced by the black leaffolder (Figure 62, I and L).
Morphology: the eggs are shiny green, oblong, and covered with a scale-like gelatinous material. The larvae are light yellow with dark brown head. A dark brown sclerite appears on the dorsal part of the prothorax in the second instar which becomes divided and appears circular. The body of the later instars turns darker green and the integument appears moist and waxy. Full-grown larvae measure 13 mm (Figure 62, H). The obtect pupae are yellowish white at first and turn reddish brown; the abdomen tapers anteriorly and has distinct constriction. The adults are yellowish brown with dark brown markings on the wings (Figure 62, G).
Biology and ecology. The female lays an average of 90 eggs per day for three days. The eggs are laid singly or in groups on the upper surface of the leaf, usually near the midrib. Incubation period lasts for 3-6 days. Like the black leaffolder, the first and second instar larvae do not fold the leaf margin. Only the third and later instars fold the leaf margins together using silk threads they spin. The green leaffolder undergoes five larval instars which lasts from 16-31 days. The pupal period lasts from 4-8 days.
Leaffolder host range. Beside sweetpotato, the black leaffolder can complete its development only on two Ipomoea species, Ipomoea triloba, Ipomoea aquatica Ipomoea pes-caprae and on a weed Mikania cordata (Burm. F.) B. L. Robinson (Asteraceae). Alternate hosts of the brown leaffolder include Ipomea triloba and Ipomea aquatica. The green leaffolder can only complete its development on Ipomoea triloba, Ipomoea purpurea, Ipomoea aquatica, Ipomoea pes-caprae and Mikania cordata.
Cultural control: to use of insect-free planting materials.
Biological control: a species of earwig (order Dermaptera) and an ichneumonid parasite, Macrocentrus sp. attack the larvae of black leaffolder in the field. Macrocentrus sp. attacks young larvae when they have not yet folded the leaf margins. A hymenopterous parasite, Brachymeria sp., attacks the brown leaffolder pupa. Two species of hymenopterous parasites Brachymeria sp. and a chalchid wasp attack the green leaffolder pupae and another unidentified species parasitises the larvae.
Chemical control: If level of infestation warrants the use of chemicals, then contact-systemic insecticides can be applied.
Figure 62 Leaffolders. Black leaffolder (Helcystogramma convolvuli): adult moth (A); larva (B); leaf damages (C). Brown leaffolder (Ochyrotica concursa): adult moth (D); larva (E); browning and holes on young leaves produced by brown leaffolder (F). Green leaffolder (Herpetogramma hipponalis): adult moth (G); larva (H); fold produced by green leaffolder with web visible through feeding hole (I); double fold produced by green leaffolder (L):
- Sweet potato leaf miner, Bedellia somnulentella (Zeller 1847), is a moth in the Bedelliidae Meyrick 1880 family, Bedellia Stainton 1849 genus. Synonyms are: Lyonetia somnulentella Zeller 1847; Bedellia orpheella Stainton 1849; Bedellia convolvulella Fologne 1860; Bedellia mnesileuca Meyrick 1928; Bedellia ipomoeae Bradley 1953; Bedellia staintoniella Clemens 1860; Bedellia annuligera Meyrick 1928; Bedellia autoconis Meyrick 1930.
Sweet potato leaf miner has a nearly cosmopolitan distribution and has been recorded from Russia, Ukraine, Georgia, southern Kazakhstan, Kirgizia, Uzbekistan, nearly all of Europe, the Middle East, Africa, India, Japan, North America, Australia, New Zealand and Oceania.
The wingspan is 810 mm. The larvae feed on Calystegia pubescens, Calystegia sepium, Convolvulus althaeoides, Convolvulus arvensis, Convolvulus siculus, Convolvulus tricolour, Ipomoea batatas and Ipomoea purpurea. They mine the leaves of their host plant. The mine starts as a narrow tortuous corridor with a central frass line, that often cuts off part of the leaf. Later, larvae leave the mine and begin to make a series of full depth fleck mines. Pupation takes place outside of the leaf. The pupa is attached to a leaf without a cocoon.
Damage. The larvae are small caterpillars which feed on the green tissue inside the leaf, leaving the transparent upper and lower membranes (epidermis) intact. The young larvae (Figure 63, E) enter the leaf and form serpentine mines (narrow, grey-brown or silvery tracks). As the larva matures, it consumes a broader patch of the leaf, forming blotch mines. Later holes are produced as the mined tissues are destroyed. The lower surface of the infested leaves become dirty with small grains of blackish frass and show silken webbings containing the small pupae. During high infestation, the leaves become brown. A serious outbreak can cut down the effective leaf surface for plant food production resulting in reduced storage root yield (Figure 63, F).
Morphology. The eggs are oval, flattened against the leaf surface; translucent, greenish white with granulate surface which turns yellowish when about to hatch. The emerging larvae are distinctly segmented with a rather pointed heads and abdomens; a mature larva measures 5.5 mm long; the larva has a yellowish body with paired pink spots on the dorsolateral sides of the thorax which later disappear and are replaced by red tubercles in all segments (Figure 63, C). The pupae measuring 3.5 mm appear green at first with mottled red markings. Later the red markings disappear and they turn dark brown with lateral projections on the abdomen (Figure 63, D). The adults are very small moths, 3.5-4.0 mm long with grayish to brown bodies and light brown scales (Figure 63, A and B).
Biology and ecology. The eggs are laid singly or in groups usually on the lower surface of the leaf near the midrib, veins or at the base of the leaf blade. Incubation lasts 5-6 days. The insect undergoes five larval instars. During the fifth instar, the larva undergoes a short pre-pupal period, comes out of the mine and produces numerous silken threads which fix and support the pupa on the lower surface of the leaf. Pupation lasts 3-6 days. A female adult is capable of laying 1-67 eggs during the 1-2-day oviposition period.
Biological control: leaf miners are generally controlled by predators and parasites like Apanteles sp.
Chemical control: the insecticides recommended for leaf miner control include carbaryl, chlorfenvinphos, diazinon, dimethoate and trichlorphon.
Figure 63 Sweet potato leaf miner Bedellia somnulentella. Adult (A and B); larva (C); pupa (D); leaf serpentine mines (E); fields severely damaged by leaf miner (F and G).
- Falls Acraea or small Yellow-banded Acraea (Acraea acerata Hewitson 1874) is a butterfly in the Nymphalidae Rafinesque 1815 family, Acraea Fabricius 1808 genus.
Falls Acraea is found in Guinea, Sierra Leone, Liberia, Ivory Coast, Ghana, Togo, Benin, Nigeria, Cameroon, the Republic of Congo, the Democratic Republic of Congo, Sudan, Rwanda, Kenya, Tanzania, Zambia, Angola, Mozambique, north-western Zimbabwe, northern Botswana and northern Namibia. The habitat consists of disturbed areas in the forest zone, usually near water or in riverine bush. Both sexes show colour and pattern variation. The larvae feed on Merremia hederaca, Lepistemon owariense, Solanum, Passiflora, Vernonia, Ipomoea (including Ipomoea whytei, Ipomoea repens and Ipomoea batatas) and possibly Zea species.
- Acraea (Actinote) acerata Hewitson 1874;
- Acraea vinidia Hewitson 1874;
- Acraea tenella Rogenhofer 1891;
- Telchinia tenella Rogenhofer 1891;
- Acraea abbotti Holland 1892;
- Acraea brahmsi Suffert 1904;
- Acraea vinidia diavina Suffert 1904;
- Acraea pullula Grünberg 1911;
- Acraea vinidia ab. ruandae Grünberg 1911;
- Acraea acerata vinidia ab. burigensis Strand 1913;
- Acraea acerata tenella f. alluaudi Le Cerf 1927;
- Acraea acerata hoursti Le Cerf 1927.
Damage. The caterpillars feed on the leaves of sweetpotato, generally working from the edge inward, and leaving only the midrib and main veins (Figure 64, D). They can completely defoliate plants (Figure 64, E). Young caterpillars hatch in large groups inside a nest formed of several leaves joined with web. They feed on the upper leaf surface, and the leaves of the nest become brown and shrivelled and covered with frass (Figure 64. Older larvae become solitary and feed at night, usually remaining on the soil during daylight.
Morphology. The pale yellow eggs measure about 0.5 mm wide and 0.7 mm long, and are laid on the leaf surface in groups of several hundred (Figure 64, A). The mature larvae are greenish-black and covered with fleshy, branching spikes. Caterpillars reach a size of 20-24 mm before pupating (Figure 64, B). The pupae are cream to blackish with brownish banding on the back, and measure 12-15 mm. The adult sweetpotato butterfly has orange wings with brown margins. Wingspan is 30-40 mm (Figure 64, C).
Biology and ecology. Acraea acerata is found in all zones of sweetpotato production in Eastern Africa, but is considered a constraint in relatively dry agroecological zones. Outbreaks are common in Rwanda. Females lay eggs in batches of 70 to 500 eggs on both surfaces of the leaves. Development is temperature-dependent. The egg stage takes 5 to 10 days. The larva passes through five larval stages in 16 to 26 days. The larvae are concentrated in a protective webbing for the first 2 weeks after hatching. They then become solitary and hide from the sunlight on the ground during the day. For pupation the caterpillars crawl up the plant or any convenient support, such as tall grass or a wall bordering the sweetpotato field. Here the pupa is suspended in a vertical position. The pupal stage takes 4 to 10 days.
Host range. The insects prefer sweetpotato. Observations were made that A. acerata completed all its life stages on several Ipomoea spp., which are indigenous to Africa.
Detection and inspection. The "nests" of young caterpillars can consist of several leaves and are easily spotted in the field. Pretty, orange butterflies, swarming over a sweetpotato field, are easily recognisable.
Mechanical control: the traditional method of controlling outbreaks has been to handpick and destroy nests of young caterpillars. A limiting factor might be lack of labour.
Chemical control: severe outbreaks might warrant the use of contact insecticides.
Biological control: no parasitoids were recovered from eggs or adults. Larvae have been observed to be parasitised by the hymenopterans Apanteles acraeae Wilkinson 1932, Zenillia vara Curran, 1927, Charops sp. and Meteorus sp. and one dipteran, Carcelia normula. Total parasitism could reach 25% in the dry season. Pupae have been observed to be parasitised by two species of Brachymeria (Chalicididae) while the ants Camponotus rufoglaucus Forel and Pheidole megacephala (F.) preyed on larvae. The pathogenic fungus Beauveria sp. was observed on larvae in the field during the rainy season.
Cultural control: intercropping sweetpotato with onion and/or the silverleaf desmodium, Desmodium uncinatum, might reduce the number of eggs laid by the females on sweetpotato.
Figure 64 Falls Acraea (Acraea acerata). Eggs (A); larva (B); adult butterfly (C); damage on leaves (D); complete defoliation due to feeding by older larvae (E).
- Convolvulus hornworm or Hawk moth (Agrius convolvuli L.). Taxonomy of Agrius convolvuli is: clade Natura;
clade Mundus Plinius;
clade Amorphea Adl et al. 2012;
clade Opisthokonta Cavalier-Smith 1987;
clade Holozoa Lang et al, 2002;
clade Epitheliozoa Ax 1996;
clade Eumetazoa Bütschli 1910;
kingdom Animalia Linnaeus 1758;
clade Bilateria Hatschek 1888;
clade Eubilateria Ax 1987;
clade Protostomia Grobben 1908;
clade Ecdysozoa A.M.A. Aguinaldo et al. 1997;
superphylum Panarthropoda Nielsen 1997;
phylum Arthropoda von Siebold 1848;
clade Euarthropoda Lankester 1904;
clade Mandibulata Latreille 1825;
clade Crustaceomorpha Chernyshev 1960 ;
clade Labrophora Siveter, Waloszek & Williams 2003;
Subphylum Pancrustacea Zrzavý et al. 1997;
superclass Hexapoda Latreille 1825;
class Insecta Linnaeus 1758;
infraclasse Pterygota Gegenbaur 1878;
order Lepidoptera Linnaeus 1758;
suborder Glossata Fabricius 1775;
clade Coelolepida Nielsen & Kristensen 1996;
clade Myoglossata Kristensen & Nielsen 1981;
clade Neolepidoptera Packard 1895;
infraorder Heteroneura Tillyard 1918;
clade Eulepidoptera Kiriakoff 1948;
clade Ditrysia Börner 1925;
clade Apoditrysia Minet 1983, sedis mutabilis;
clade Obtectomera Minet 1986, sedis mutabilis;
clade Macroheterocera Chapman 1893, sedis mutabilis;
superfamily Bombycoidea Latreille 1802;
family Sphingidae Samouelle 1819, sedis mutabilis;
tribe Acherontiini (Butler 1877) Janse 1932;
genus Agrius Hübner 1819;
species Agrius convolvuli Linnaeus 1758 (convolvulus haw).
Economic importance. Sweetpotato hornworm is not usually a serious pest, although severe outbreaks have been reported in Vietnam. Yield losses can occur if heavy defoliation takes place when the crop is young.
Geographical Distribution. Agrius convolvuli occurs worldwide, but is prevalent in Africa, Asia, Australia, the Pacific and Southern Europe.
Damage. The larva is a large voracious caterpillar that is capable of defoliating the plant. It feeds on the leaf blade causing large irregular holes or may start feeding on the leaf edges eventually eating the entire leaf blade, leaving only the petiole. They are initially found at the shoot tip, preferring young leaves, but will eat all leaves if population is high. They are extremely sluggish, moving only enough to reach a new leaf after one has been consumed. Frass can be found near the infested plant part.
Morphology. The adults are large greyish brown hawkmoths with black lines on the wings and pink markings on the abdomen; wingspan is 8-12 cm (Figure 65, A and B). The greenish spherical eggs measure 1 to 2 mm in diameter. The larvae are variable in colour from green to brown, occasionally yellow and are distinctly patterned; they have a distinctive posterior horn and reach 95 mm in length (Figure 65, C). The reddish brown pupae are characterized by their prominent proboscis or "trunk" which is curved downward (Figure 65, D); the body surface is glossy; pupae are found in the soil under the plants.
Biology and ecology. The female lays spherical eggs singly on either surface of the leaves or stem. Egg hatching takes place 4 days after oviposition. The larvae have five larval instars from 13 to 25 days with an average of 2 to 6 days per instar. In the early stages, larvae nibble holes through the leaf, little food is consumed hence growth is slow. In the second and third instar, the ground colour deepens further, and pale yellow lateral stripes appear. By the fourth instar larvae consume more food, both by day and night, and growth becomes more rapid (Figure 65, E and F). They change their colour to green, brown and, occasionally, yellow. When fully grown, most larvae only emerge at night to eat. Pupation takes place in the soil. Pupal period is 5 to 26 days. The total life cycle ranges from 22 to 60 days.
Host range. Apart from sweetpotato, it also attacks eggplant, grapes, legumes, pepper, tomato and taro.
Cultural Control. Plowing the field to expose the pupae reduces infestation. Handpicking of the larvae may be quite effective in small areas. Light trapping can be used to monitor the population of the adults.
Biological Control. Among its important egg parasites are Trichogramma spp. while Sycanus sp., a large reduviid, and tachinid flies feed on the larvae.
Chemical Control. Pesticide use is not recommended as it disrupts the action of the egg and larval parasites.
Figure 65 Convolvulus hornworm or Hawk moth (Agrius convolvuli L.). Male (A); female (B); larva (C); pupa showing prominent curved proboscis (D); damage, consisting in irregulare hole, on sweetpotato leaf (E); sweetpotato hornworm feeding on leaf (F)
- Eriophyiid mite (Eriophyes gastrotrichus Nalepa 1918). Taxonomy of Eriophyes gastrotrichus is: clade Natura; clade Mundus Plinius; clade Biota; clade Amorphea Adl et al. 2012; clade Opisthokonta Cavalier-Smith 1987; clade Holozoa Lang et al, 2002; clade Epitheliozoa Ax 1996; clade Eumetazoa Bütschli 1910; kingdom Animalia Linnaeus 1758; clade Bilateria Hatschek 1888; clade Eubilateria Ax 1987; clade Protostomia Grobben 1908; clade Ecdysozoa A.M.A. Aguinaldo et al. 1997; superphylum Panarthropoda Nielsen 1997; phylum Arthropoda von Siebold 1848; clade Euarthropoda Lankester 1904; subphylum Arachnomorpha Heider 1913; superclass Chelicerata Heymons 1901; epiclass Euchelicerata Weygoldt & Paulus 1979; class Arachnida Cuvier 1812; clade Micrura Hansen & Sørensen 1904; clade Acaromorpha Dubinin, 1957; subclass Acari Leach 1817; superorder Acariformes Zakhvatkin 1952; order Actinedida van der Hammen 1968; suborder Eleutherengona Oudemans 1909; section Raphignathae; superfamily Eriophyoide Nalepa 1898 (gall and rust mites); family Eriophyidae Nalepa 1898; genus Eriophyes von Siebold 1836.
Economic importance. Gall mites rarely cause serious plant damage, but are a concern among growers because of the unsightly appearance of infested leaves. There are no reports on its impact on yield, but is a major concern in Bicol Region, Philippines.
Geographical distribution. Philippines.
Damage. Galls with irregular sizes and shapes are formed on vines and leaves by injecting a chemical into plant tissues during feeding. These chemicals cause plant tissues to grow abnormally. Mites in all stages of development live inside the same gall.
Morphology. Adult are extremely small and difficult to see without magnification. The worm-like body is about 148-160 µm long and 46 µm thick. It is white, cylindrical and tapers to the rear. The entire body surface has a large number of close-set fine discontinuous lines giving the shield a wrinkled appearance. Forelegs are moderately arched and the hind legs are shaped like the foreclaw. The abdomen has about 67 rings.
Biology. Eggs are laid within the gall; nymphs mature within the gall and the emerging adults infest new foliage.
Host range. It has also been reported on other Convolvulaceae like Ipomoae staphylina.
Detection and inspection. Presence of galls on the leaves makes it easy to diagnose the problem.
Management. Control is not necessary but parts with unsightly galls may be removed.
- Two-spotted mite (Tetranychus urticae Koch 1836). Taxonomy of Tetranychus urticae is: clade Natura; clade Mundus Plinius; clade Biota; clade Amorphea Adl et al. 2012; clade Opisthokonta Cavalier-Smith 1987; clade Holozoa Lang et al, 2002; clade Epitheliozoa Ax 1996; clade Eumetazoa Bütschli 1910; kingdom Animalia Linnaeus 1758; clade Bilateria Hatschek 1888; clade Eubilateria Ax 1987; clade Protostomia Grobben 1908; clade Ecdysozoa A.M.A. Aguinaldo et al. 1997; superphylum Panarthropoda Nielsen 1997; phylum Arthropoda von Siebold 1848; clade Euarthropoda Lankester 1904; subphylum Arachnomorpha Heider 1913; superclass Chelicerata Heymons 1901 or Cheliceriformes; epiclass Euchelicerata Weygoldt & Paulus 1979; class Arachnida Cuvier 1812; clade Micrura Hansen & Sørensen 1904; clade Acaromorpha Dubinin, 1957; subclass Acari Leach 1817; superorder Acariformes Zakhvatkin 1952; order Actinedida van der Hammen 1968; suborder Eleutherengona Oudemans 1909; section Raphignathae; superfamily Tetranychoidea Donnadieu 1876;family Tetranychidae Donnadieu 1876; subfamily Tetranychinae Donnadieu 1876; genus Tetranychus Dufour 1832.
The two-spotted spider mite, Tetranychus urticae, has been controversial in its taxonomic placement. About 60 synonyms included under this species have compounded the controversy. The body of a spider mite is separated into two distinct parts:
The gnathosoma includes only the mouthparts. The idiosoma is the remainder of the body and parallels the head, thorax and abdomen of insects. After hatching from the egg, the first immature stage (larva) has three pair of legs. The following nymphal stages and the adult have four pairs of legs.
- the gnathosoma and
- the idiosoma.
Distribution. The twospotted spider mite was originally described from European specimens. It is considered to be a temperate zone species, but it is also found in the subtropical regions. It is found throughout the USA in greenhouses where it survives the winters beyond its natural limits. Tuttle and Baker (1968) report this species to be found on deciduous fruit trees in northern regions of the U.S. and Europe.
Damage. Adults and nymphs of spider mites suck the sap from the leaves, causing the area around the feeding punctures to become chlorotic and appear as conspicuous whitish to yellowish stippling on the upper surface of the leaf. Under heavy infestation, photosynthesis is greatly reduced and the chlorotic areas may coalesce forming mottled yellowish interveinal patches. The leaves eventually turn yellow and may become brown and scorched and drop prematurely (Figure 66, A).
Description. The two-spotted spider mite is oval in shape, about 0,51 mm long and may be brown or orange-red, but a green, greenish-yellow or an almost translucent color is the most common. The female is about 0.4 mm in length with an elliptical body that bears 12 pairs of dorsal setae. Overwintering females are orange to orange-red. The body contents (large dark spots) are often visible through the transparent body wall. Since the spots are accumulation of body wastes, newly molted mites may lack the spots. The male is elliptical with the caudal end tapering and smaller than the female. The axis of knob of aedeagus is parallel or forming a small angle with axis of shaft (figure 66, B).
Life Cycle. Spider mite development differs somewhat between species, but a typical life cycle is as follows. The eggs (figure 66, E) are attached to fine silk webbing and hatch in approximately three days. The life cycle is composed of the egg, the larva, two nymphal stages (protonymph and deutonymph) and the adult. The length of time from egg to adult varies greatly depending on temperature. Under optimum conditions (approximately 27 ºC), spider mites complete their development in five to twenty days. There are many overlapping generations per year. The adult female lives two to four weeks and is capable of laying several hundred eggs during her life.
Biology and ecology. The eggs are deposited singly and scattered on the lower surface along or near the midrib or veins. The incubation period lasts 3-5 days. Protonymphal and deutonymphal periods range from 1-2 and 1-3 days, respectively. The total developmental period takes about 7-10 days for both sexes. Females are capable of parthenogenetic reproduction with unfertilised eggs developing into males exclusively. Mated females lay more eggs than the unmated ones. The former is capable of laying an average of 134 eggs and the latter 61 eggs during their entire lifetime. The colony produces webbing located on the undersurface of the leaf.
Detection and inspection. These mites may be found on the underside of leaves, with the help of a magnifying glass. Close inspection would also show tiny spider webs on stems and leaves. Applying mist on the plants before inspection will make the webs more visible.
Host range. Sweetpotato spider mite is a polyphagous species (i.e. it feeds on a wide range of plant species). Its common hosts are Maxima cucurbita, Passiflora edulis, Centrosema pubescens, Ipomea triloba, Merremia vitifolia, Acalyphaa wilkesiana, Ricinus communis, and many more.
- Biological control. Several species of coccinellid beetles, and phytoseiid mites such a Amblyseius linearis Corpuz-Raros & Rimando 1966 and Amblyseius longispinosus Evans 1952 prey on all stages of development. Orius insidiosus Say 1832, (minute pirate bug or insidious flower bug) is a species of minute pirate bug, a predatory insect in the order Hemiptera (the true bugs) That are considered beneficial, as they feed on small pest arthropods and their eggs (figure 66, C).
- Chemical control. Several insecticides/acaricides available for mite control. Overwintering mites may be reduced in numbers by the destruction of weeds such as pokeweed, Jerusalem oak, Jimson weed, wild blackberry, wild geranium and others. Insecticidal soaps and oils should be carefully considered when a pesticide is required. They are effective against mites and the least toxic to people, other non-target organisms and the environment. The effectiveness of laundry soaps, washing detergents, and vegetable oils is less consistent than with chemical pesticides. Although some growers have been quite pleased with the results of non-insecticidal quality soap and oil use, some have been disappointed. Also, plant varieties differ in their susceptibility to burning induced by soaps and oils. Environmental conditions, as well as micronutrients, fertilizers, and other additives may affect a tendency to "burn" foliage. At higher rates of application, (2%), burning and stunting are more likely (Capinera 1992).
The two-spotted spider mite develops a resistance to most acaricides after prolonged use. Most miticides are not effective on eggs. Therefore two or more applications of the miticide will be required at five-day intervals during the summer or seven-day intervals during the winter.
Figure 66 Two-spotted mite (Tetranychus urticae. Sweetpotato leaves damaged from two-spotted spider mite (A); adult male and female sweetpotato spider mite (B); larva of the minute pirate bug, Orius insidiosus (Say), an insect predator of the twospotted spider mite (c); adult predatory mite, Phytoseiulus persimilis (D); sweetpotato spider mite eggs (E).
- False spider mite or red flat mite (Brevipalpus californicus Banks 1904). The taxonomy of Brevipalpus californicus is: clade Natura; clade Mundus Plinius; clade Biota; clade Amorphea Adl et al. 2012; clade Opisthokonta Cavalier-Smith 1987; clade Holozoa Lang et al. 2002; clade Epitheliozoa Ax 1996; clade Eumetazoa Bütschli 1910; kingdom Animalia Linnaeus 1758; clade Bilateria Hatschek 1888; clade Eubilateria Ax 1987; clade Protostomia Grobben 1908; clade Ecdysozoa A.M.A. Aguinaldo et al. 1997; superphylum Panarthropoda Nielsen 1997; phylum Arthropoda von Siebold 1848; clade Euarthropoda Lankester 1904; subphylum Arachnomorpha Heider 1913; superclass Chelicerata Heymons 1901 or Cheliceriformes; epiclass Euchelicerata Weygoldt & Paulus 1979; class Arachnida Cuvier 1812; clade Micrura Hansen & Sørensen 1904; clade Acaromorpha Dubinin, 1957; subclass Acari Leach 1817; superorder Acariformes Zakhvatkin 1952; order Actinedida van der Hammen 1968; suborder Eleutherengona Oudemans 1909; section Raphignathae; superfamily Tetranychoidea Donnadieu 1876; family Tenuipalpidae Berlese, 1913; genus Brevipalpus Donnadieu, 1876.
Brevipalpus californicus, sometimes called the "omnivorous mite" in the U.S., has Worldwide extensive host range and may cause economic damage, depending on the host. Mites in the family Tenuipalpidae are called false spider mites or flat mites. The false spider mite is an important acarine pest of sweetpotato. It attacks many species other than sweetpotato. High infestation occurs during the dry season.
Distribution. Brevipalpus californicus has been reported from Algeria, Angola, Australia (as "bunch mite"), Brazil (São Paulo), Congo, Cyprus, Egypt, European Union, French Guiana, Greece (including Crete), India, Israel, Italy (including Sicily), Japan (Ryukyu Islands), Libya, Malaysia (Peninsular), Mauritania, Mexico, Mozambique, Nepal, Papua New Guinea, Portugal, Senegal, Sri Lanka, South Africa, Thailand, United States, Zimbabwe (EPPO). In the U.S. it is reported from Arizona, California, Florida, Hawaii, Kansas, Louisiana, Maryland, and Texas. In Florida it has been reported from the following counties: Alachua, Baker, Brevard, Dade, Duval, Hillsborough, Indian River, Jefferson, Martin, Orange, Palm Beach, Pinellas, Polk, Putnam, Sarasota, Seminole, and Volusia.
- Hystripalpus californicus Mitrofanov & Strunkova 1979;
- Brevipalpus australis Baker 1949;
- Brevipalpus browningi Baker 1949;
- Brevipalpus confusis Baker 1949;
- Brevipalpus woglumi McGregor 1949;
- Tenuipalpus vitis Womersley 1940;
- Tenuipalpus australis Tucker 1926;
- Tenuipalpus californicus Banks 1904.
Description. The female is 228 microns long. It is reddish in immature specimens and rufous amber in adults. The body shape from above is ovate-sagittate with the width approximately 2/3 that of the length. The dorsal cuticular surface of body is conspicuously reticulated. The areolae on cepahlothorax laterad of mandibular plate is about 1/3 longer than wide. Dorsally, the cephalothorax bears three pairs of rather weak setae; one pair at anterior margin between coxae 1, one pair just in front of and one pair just behind eyes. Abdomen bears 20 very weak setae dorsally: 7 along each lateral margin from the main suture back to the caudal tip; 3 submedian pairs, the first near the main suture, the 2nd and 3rd pairs opposite the 2nd and 3rd marginal setae, respectively. All dorsal setae appear to be simple, unpectinate, and unserrate. A pair of dusky-bordered pores open dorsally on the abdomen a short distance behind the main suture. The legs are short, stout, and the posterior pair barely reaches beyond the tip of abdomen (figure 67, A and B).
Host Range. This species feeds on a wide variety of plants including many crops and ornamentals. Pritchard and Baker (1958) listed 43 host species known from all over the world. Important crops and weeds apart from sweetpotato as alternate hosts include the following: Ipomoea spp. such as Ipomoea purpurea, Ipomoea triloba, Ipomoea aquatica, Ipomoea pes-caprae, Moringa oleifera, Solanum melongena, Glycine max, Cucurbita maxima, Lycopersicon esculentum, Manihot esculenta, Vigna sesquipedalis, Hibiscus esculenta, Ceiba pentandra, Carica papaya, Averrhoa carambola, Citrus mitis, Citrus nobilis, Cocos nucifera, Commelina diffusa, Leucaena leucocephala, Ricinus communis, Stachytapheta jamaicensis, Passiflora edulis, Terminalia citrina, Costus speciosus, Acalypha spp, Capsicum frutescens, Desmodium gangeticum, Gmelina arborrea, Ixora longistipula, Manihot multiglandusa, Wedelia biflora and many more.
Economic Importance. Brevipalpus mites inject toxic saliva into fruits, leaves, stems, twigs, and bud tissues of numerous plants including citrus. Feeding injury symptoms on selected plants include: chlorosis, blistering, bronzing, or necrotic areas on leaves. Stunting of leaves and the development of Brevipalpus galls on terminal buds were recorded on sour orange, Citrus aurantium L., seedlings heavily infested with B. californicus in an insectary (Childers et al. 2003).
Several mites in the genus Brevipalpus may transmit the citrus leprosis virus, but only Brevipalpus phoenicis has been experimentally confirmed to transmit the virus. However, Brevipalpus californicus and Brevipalpus obovatus also are suspected transmitters. (USDA 2004). Citrus leprosis causes yield reduction and eventual death of the trees if its mite vectors are not controlled. Citrus leprosis, while not currently a problem in the U.S., substantially damaged Florida's orange crop in the early 20th century but was eradicated in the mid-1920s. However, it is slowly progressing northward from its outbreak epicenter in South America (USDA 2004).
Biological control. Several species of coccinellid beetles, and phytoseiid mites such as Amblyseius linearis Corpuz-Raros & Rimando 1966, and Amblyseius longispinosus Evans 1952 prey on all stages of development.
Chemical control. Several insecticides and acaricides are available for mite control.
Figure 67 False spider mite or red flat mite (Brevipalpus californicus). Adult false spider mite (A); dorsal view of a typical female false spider mite (B).
Table 3 - Sweet Potato Varieties and Their Reaction to Diseases
||Root Knot Nematode
R = Resistant or Tolerant I = Intermediate S = Susceptible
Return to Introduction
Return to Home page
This page has been visited times
Fiume Francesco 2005