|Traditional Field Crops (Peace Corps, 1981, 283 p.)|
|Pest and disease control|
Maize Fungal Diseases
Seed Rots and Seedling Blights
These are often referred to as pre-emergence and post-emergence "damping off" and are caused by soilor seed-borne fungi. Seeds may be killed before germination or seedlings may be destroyed before or after they emerge from the ground. Damping off is most prevalent in cold, poorly-drained soils and with damaged seed (cracked seedcoat, etc.). Problems are less likely where conditions favor rapid germination and emergence (i.e. warm weather, adequate soil moisture). Symptoms: Above-ground signs are yellowing, wilting, and death of the seedling leaves, but this can be confused easily with injury by wind, wind-blown sand, fertilizers, herbicides, and insects. Examine the below-ground portion of the plants and look for rotted seeds, soft rot of the stems near the soil surface, and rotted, discolored roots. Control: Use good quality seed, free of molds and damage, that has been treated with a fungicide like Captan or Arasan (thiram) for protection during germination. Seed treatment is mainly effective against seed rot.
Helminthosporium Leaf Blights
Several species of Helminthosporium fungi attack maize leaves, but the two most important are H. Maydis (Southern leaf blight) and H. turcicum (Northern leaf blight). Helminthosporium maydis is more prevalent in hot, humid areas, but both species can occur on the same plant. Symptoms of H. Maydis: There are two major races of H. maydis and they have different symptoms. Race "O" leaf spots are small and diamond~shaped when young and then elongate to about 2-3 cm and may grow together, killing large areas of leaf. Race "T" leaf spots are oval and larger than those of race "O" and attack the husks and leaf sheaths, unlike race "O". Maize hybrids utilizing "Texas" male sterile cytoplasm (genetic material) in their production are very susceptible to race "T". This became evident during the severe and unexpected outbreak of H. maydis race "T" in the U.S. Corn Belt in 1970. Most hybrids now utilize "N" male sterile cytoplasm in their production to overcome this problem. Symptoms of H. turcicum: Northern leaf blight prefers high humidity and low temperatures. Small, slightly oval, water-soaked spots first appear on the lower leaves and eventually become rectangular in shape and grow to a length of 2.5-15 cm. These lesions are grayish-green to tan and can cause severe defoliation. Control: Resistant varieties offer the best protection. Seed treatment with a fungicide is of no help. Foliar fungicides give fair to good control but are seldom economical since they must be applied every 7-10 days,
Three types of rust attack maize: common rust (Puccinia sorghi), Southern rust (Puccinia polysora), and tropical rust (Physopella zeae).
Common rust occurs more frequently in cool, moist weather and produces small, powdery, cinnamonbrown pustules on both surfaces of the leaves. Southern rust is more common in warm humid regions and produces smaller, lighter-colored pustules than common rust. Tropical rust is confined to the tropical regions of Latin America and the Caribbean. The pustules vary in shape from oval to round and occur beneath the leaf epidermis (outer layer). They are cream colored and very small and are sometimes surrounded by a black area. Control: Resistant varieties are the best protection. Fungicide sprays are seldom economical.
Maize Downy Mildews
At least nine species of Sclerospora (Sclerophthora) fungi attack maize. At present, they are mainly confined to parts of Asia and Africa, but also appear to be spreading throughout the Americas. Symptoms vary with the species, age of plants when infected, and the climate, but usually include chlorotic striping of the leaves and leaf sheaths, stunting, excessive tillering, and deformities of the ears and tassels. A downy growth (mildew) may occur on the undersides of the leaves in later stages. Some of these symptoms may be confused with viruses.
Some of the more common downy dildews are listed below with their control measures: Crazy Top (S. macrospora): Rare in the true tropics but of world-wide distribution in temperate and warm-temperate climates. Crazy top causes the tassel to mutate into a mass of leafy bunches and is provoked by one or more days of flooding before seedlings have reached the four to five leaf stage. Adequate soil drainage is the only control.
Sorghum Downy Mildew (S. Sorghi): Widespread. Controls: Using resistant varieties, removing and destroying infected plants, and avoiding maizesorghum rotations in infected fields. Green Ear Disease or Graminicola Downy Mildew (S. graminicola): Occurs on various grasses but is usually not important in maize. Sugarcane Downy Mildew (S. sacchari): Mainly confined to Asia and the South Pacific. Controls: Eliminating the disease by using healthy planting material, growing maize in areas free of the disease and where sugarcane is not grown extensively, removing and destroying infected plants, and using resistant varieties. Fungicide sprays are used in some areas. Philippine Downy Mildew (S. philippinensis): This is the most important maize disease in the Philippines and also occurs in Nepal, India, and Indonesia. Controls: Removing and destroying infected plants, using resistant varieties and fungicide sprays where economical.
Common Smut and Head Smut
Common smut (Ustilago maydis): A fungus that causes galls (swollen areas on plant tissue) 1520 cm in size which form on any above-ground part of the plant. When young, the galls are shiny and whitish with soft interiors, but later turn into a mass of black, powdery spores. Early infection can kill young plants, but common smut is seldom a serious problem. Controls: Using resistant varieties and avoiding mechanical injury to plants. Good soil fertility is helpful. Galls should be removed from plants and burned before they rupture.
Head smut (Sphacelotheca) reiliana): Can seriously affect yields in dry, hot regions. This is a systemic fungus that enters seedlings without showing symptoms until the tasseling stage. Tassels and ears become deformed and develop masses of black, powdery spores. Head smut is mainly a soil-borne disease. Controls: Most varieties are resistant. Crop rotation and general sanitation also provide some control. Soil applied fungicides in the seed row give fair to good control, but are usually not economical. Seed treatment with a fungicide is ineffective.
Fungal Stalk Rots
Five of the more common fungal stalk rots are discussed below. They attack plants between tasseling and maturity, although Pythium stalk rot may also infect younger plants. Diplodia stalk rot: Most likely to occur several weeks after pollination. The leaves suddenly wilt and die, turning a dull grayish-green, and the stalk dies 7-10 days later. Numerous small,raised,black dots can be seen on the lower internodes of the stalk. Infected portions break readily and can be easily crushed. Diplodia infected stalks usually break between the joints (nodes). Controls: Using resistant varieties, avoiding high rates of N fertilizer without adequate K, and lower plant populations. Gibberella stalk rot: Similar to Diplodia except that the stalks tend to break at the joints, and the inside of the stalk is pinkish-red. The small black dots found on the lower portion of the stalk can be scraped off with a fingernail, unlike those of Diplodia, Controls: See Diplodia, Fusarium stalk rot: Similar to Gibberella and difficult to distinguish from it. Controls: See Diplodia.
Pythium stalk rot: Most likely to occur during long periods of hot, humid weather. Usually attacks a single internode near the soil surface and causes a brown, soft, water-soaked rot that collapses the stem. Stems do not break off but fall over,and plants may remain green for several weeks afterwards. Pythium usually occurs around tasseling time but may also affect younger plants. It is easily confused with Erwinia bacterial stalk rot. Controls: Using resistant varieties.
Charcoal rot (Macrophomina phaseoli) Attacks maize, sorghum, soybeans, beans, cotton, and others, It is most prevalent under very hot, dry conditions and first attacks seedling roots where it produces brown, water-soaked lesions which eventually turn black. The fungus usually does not invade the stalk until well after pollination when it causes the lower internodes to ripen prematurely and shred, causing breakage at the base of the plant, The inner stalk has a charred appearance due to the presence of numerous black dots (sclerotia). Controls: Charcoal rot can be reduced in irrigated fields by maintaining a good soil moisture content during dry spells after tasseling; see also Diplodia.
Fungal Ear and Kernel Rots
Maize can be attacked by a number of ear and kernel rots, especially when very wet weather occurs from silking to harvest. Insect and bird damage of stalks and ears also increases susceptibility.
Diplodia ear rot: Causes early-infected ears to have bleached husks, while normal husks are still green. Ears are shrunken, and the husks seem to be glued together due to the fungus growing in-between. Ears infected later in the season seem normal from the outside but have a white mold that usually starts at the base of the kernels. In severe cases, black fruiting bodies can be seen on the husks and on the sides of the kernels.
Controls: Ears that mature with the tips pointed downward are less susceptible. Good husk covering is also helpful as is an early harvest and proper storage at a safe moisture content. Gibberella ear rot (G. zeae): More prevalent in cool, humid areas and causes a pink to bright red rot starting at the ear tips. G. fujikuroi is the most common ear rot worldwide and is similar in appearance. Both types also produce a cotton-like pink growth over the kernels, and infected grain is toxic to humans, pigs, and birds. Controls: See Diplodia. Fusarium ear rot: Favored by dry, warm weather end similar to Gibberella. Nigrospora ear rot: Causes the cob to be discolored and easily shredded. The interior is gray instead of white. Kernels are poorly filled and can be easily pushed into the partially rotted cob. Spore masses in the form of black spots are found at the base of the kernels. Controls: Balanced soil fertility; see Diplodia.
Maize Bacterial Diseases
Erwinia stalk rot: Causes symptoms similar to Pythium (see fungal stalk rots). Controls: Using resistant varieties and good drainage. Bacterial leaf blight (Stewart's wilt): Transmitted by certain types of maize beetles and by the seed. Sweet maize is more susceptible. Symptoms are pale green to yellow streaks on the leaves, usually appearing after tasseling. The streaks die and may kill the leaf. The stem may also become infected, leading to wilting of the plant. Controls: Using resistant varieties, early use of insecticides to control insect vectors.
Maize Viral Diseases
Maize is attacked by some 25 virus or virus-like diseases which are transmitted mainly by aphids and leafhoppers. Alternate host plants like Johnsongrass, sorghum, and sugarcane play an important role in the spread of most of them, Symptoms can be confusing and may often be caused by other problems such as nutrient deficiencies. Some of the more prevalent viruses are dealt with below:
Maize streak virus: A major problem in many areas of Africa and transmitted by several species of leafhopper (Cicadullina spp.). Early signs are tiny round scattered spots on the youngest leaves which enlarge parallel to the leaf veins. Broken yellow streaks later appear and run along the veins. Controls: Resistant varieties; leafhopper control. Maize dwarf mosaic: Spread by several types of aphids and a wide range of alternate hosts, including Johnson-grass (a sorghum relative) and sorghum. Leaves of infected plants develop a yellowgreen mosaic pattern, mainly on the bases of the younger leaves. Foliage becomes purple or purple-red as plants mature, severe stunting may occur, and few plants produce normal ears. Controls: Using resistant varieties. Destruction of alternate hosts and insect control. Maize stunt virus: Spread by several types of leafhopper (Dalbulus, Baldulus, Graminella) and known as "achaparramiento" in Latin America. Now thought to be a viruslike organism. The Mesa Central strain causes yellowing of the young leaves which later turn red. The Rio Grand e strain produces spots at the bases of young leaves, followed by a yellow striping. Controls: Resistant varieties; insect control. Sugarcane mosaic: Occurs where maize is grown next to sugarcane and causes yellow spots and streaks. Controls: Using resistant varieties of sugarcane.
Seed rots and seedling blights: See maize.
Downy mildews: Sorghum is attacked by three species of downy mildew (S. macrospora, S sorghi, S. graminicola). (Refer to maize for details). Controls: Using resistant varieties and rotation with broad-leaf crops. Many forage-type sorghums are very susceptible to sorghum downy mildew (S. sorghi) and should not be planted on ground where grain sorghum will be sown if the disease is present. Covered kernel smut (Sphacelotheca sorghi): Carried by the seed and penetrates the young seedlings. Plants appear normal until heading time when the kernels are replaced by light-gray or brown, cone-shaped smut galls full of black spores. Controls: Seed treatment with a fungicide is very effective since the spores are carried on the surface. Resistant varieties have been developed.
Loose kernel smut (S cruenta):
Very common in Asia and Africa. As with covered smut, the spores are carried on the planting seed and invade young seedlings. Long, pointed smut galls are formed on the grain heads, and infected plants may be stunted and show increased tillering Unlike covered smut, loose smut spores may cause infections of late emerging grain heads on otherwise healthy plants. Controls: Same as for covered smut. Head smut (S. reiliana): The most damaging of the smuts. Destroys the entire head and replaces it with a mass of dark brown, powdery spores A large gall covered with a whitish membrane bulges out of the boot at heading time. The gall ruptures and spores are scattered by wind and rain over the soil where they survive to infect the next crop. Controls: Seed treatment will prevent the spread from field to field, but will not stop infection from spores already in the field. Resistant varieties should be used and infected plants removed and burned.
Grain (Head) Molds
These are caused by several species of fungi that are most prevalent when sorghum matures during wet weather. Seed becomes heavily molded and will germinate poorly if planted. Controls: Photo-sensitive varieties escape head mold by maturing during drier weather. Other types can be sown to mature during drier weather. Open-headed varieties are somewhat less susceptible than those with compact heads. Work in India has shown that head molds can be reduced by spraying the heads with Captan or Benlate (benomyl) plus a sticker immediately after a heavy rain, but this may not be cost effective. Sorghum Rust
This is caused by the fungus Puccinia purpurea which produces raised brownish pustules on both sides of the leaves. This disease is most common under high humidity but is usually confined to the older, mature leaves. Controls: Using resistant varieties. Fungicides are not usually economical.
This disease is caused by the fungus Collectotrichum graminicola which attacks the leaves, producing tan to reddish lesions that are round to oval and have soft, sunken centers. It may also cause a stalk rot called red rot. Controls: Using resistant varieties. Other Fungal Leaf Spots
Sooty stripe (Ramulispora sorghi), zonate leaf spot (Gloesocercospora sorghi), and oval leaf spot (Ramulispora sorghicola) are the main fungal leaf spots in West Africa, along with anthracnose. Controls: Resistant varieties offer the best means of control. Removal of host plants like Guinea-grass, Bermudagrass, and Paragrass helps.
Fungal Stalk Rots
Charcoal rot (Macrophomina phaseoli see maize): A serious disease of dryland sorghum. Losses are increasing in India, Ethiopia, Tanzania, and Upper Volta. It is the most serious sorghum disease in Nicaragua and also causes serious losses in Mexico and Colombia. Charcoal rot is especially severe when grain filling takes place during high soil temperatures and drought. Controls: See maize. Milo disease (Periconia circinata): Presently confined to the U.S. and attacks the roots as well as the stalks. Even young plants may be affected. The first symptoms are stunting and slight leaf rolling. The tips and margins of older leaves turn light yellow, and all the leaves eventually become affected. Splitting the base of the stalk lengthwise reveals a dark red discoloration in the center. Roots are also dark red. Controls: Resistant varieties. Red stalk rot (Collectotrichum graminicola): The stalk rot phase of anthracnose. The outside basal portion of the stalk becomes red or purple. If the stalk is split lengthwise, the inner pith shows a reddish discoloration which may be continuous or blotchy. The flower stem may be similarly affected. Controls: See anthracnose.
Several bacterial leaf diseases attack sorghum and are favored by warm, humid weather. Yield losses usually are not serious. Seed treatment with a fungicide, crop rotation, and resistant varieties are the best controls. Sorghum Viral Diseases Maize dwarf mosaic and sugarcane mosaic produce very similar symptoms on sorghum. The mottled light and dark green mosaic pattern is usually most prevalent on the upper two to three leaves and often includes longitudinal white or yellow streaks. Varieties with a red pigment may show a "red leaf" symptom consisting of red stripes with dead centers. Controls: see maize. Yellow sorghum stunt: A virus-like organism that is spread by leafhoppers. Plants become dwarfed with leaves bunched together at the top. Leaves develop a yellow cream color. Controls: Resistant varieties; insect control.
Downy mildew (Sclerospora graminicola): Can attack millet as early as the seedling stage. The systemic fungus causes the leaves to become yellowish and under wet conditions a downy white mildew may occur on the undersides of the leaves. Affected seedlings may die within a month without producing any tillers. The symptoms may first appear on the upper leaves of the main stem or on the tillers. The first leaf affected normally shows damage only on the lower portion, but subsequent leaves suffer increasing infection. Heads may be partially or totally deformed. Control: Many local varieties have good resistance. The International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) achieved excellent control of downy mildew by treating planting seed with a newly developed systemic fungicide from Ciba Geigy known as GCA 48/988.
Grain smut (Tolypossporium penicilliriae): Fungi infect the young millet florets on the seed head and replace them with plumb galls ful of black powdery spores. Controls: Use resistant varieties and general sanitation. Seed treatment with a fungicide is probably not very effective. Ergot (Claviceps fusiformis): Common,but generally not serious. The airborne fungal spores infect the young florets before grain development and produce a sweet sticky liquid called honeydew, which is pink or red. The grain head later takes on a bottle-brush appearance due to the formation of dark-colored hard structures called sclerotia Controls:
Burn infected heads. Rust (Puccinia penniseti): Sometimes serious on late millet but usually not with early millet. Leaf spots: Several fungal leaf spots attack millet but are usually not serious.
Foliar Fungal Diseases
Foliar fungal diseases can seriously reduce yields of both nuts and hay, and the decaying fallen leaves provide organic matter for incubating soil-borne diseases like Southern stem rot. Cercospora Leafspot: Attacks peanuts worldwide, but Virginia types (see Chapter 3) are somewhat less susceptible than the Spanish-Valencia types. It is encouraged by wet conditions. Symptoms: Two species of Cercospora fungi are involved. Early leafspot (C. arachidicola) is usually the first to appear and produces round, brownish-red spots surrounded by a yellow halo. Late leafspot (C. personata) occurs later in the season and produces darker spots that may or may not have halos. Both leafspots may also occur on the stems and leaf petioles (leaf stems) as the disease progresses. Severe defoliation can result, which affects yields as well as the performance of mechanical pullers, which require bulky bushes for satisfactory operation. Controls: Crop rotation helps reduce early infections. Even though Virginia types show some resistance, foliar fungicides are usually essential in most cases and are applied as preventatives. Peanuts are a relatively high value crop, which makes use of foliar fungicides very economical. Specific recommendations are given in the next unit. Peanut Rust (Puccinia arachis): This disease is presently confined to Latin America and the Caribbean. It causes small orange to brown raised pustules on the leaves, mainly on the undersides. It can spread rapidly under hot, humid conditions, and leaf drop can be severe. The stems, petioles and pegs can also be affected. Controls: As with leafspot, fungicide sprays or dusts are the only effective control.
Ground diseases caused by fungi are sometimes hard to detect and identify and can drastically reduce yields. Southern Stem Rot: Also known as Southern Blight, wilt and white mold, it is the most serious and widespread ground disease attacking peanuts and also affects beans, soybeans, other legumes, potatoes, tomatoes, and other crops. It is favored by warm, wet conditions. Symptoms: In the early stages, some of the leaves on a few branches usually turn yellowish. Under wet conditions, a white cotton-like mold occurs on the lower stem near the soil surface and on any decaying organic debris on the soil. Fungal bodies called sclerotia appear on the affected areas and are light brown to brownish red and about the size of mustard seeds. The leaves begin a gradual wilt, but at first seem to recover at night. Eventually, the entire plant can die. The pegs are destroyed, leaving many pods imbedded in the ground. The disease can also cause pod rot. Controls: There is no way to control this disease once plants are affected, but it can be effectively suppressed through a combination of chemical and cultural controls given below:
· Crop rotations with maize, sorghum, and other grass family plants.
· Deep burial of all crop residues using a moldboard plow. Coarse trash like maize and sorghum stalks need to be chopped up manually or with a disk harrow before plowing. Residues left on the surface serve as a breeding ground for the fungus.
· Planting peanuts on a flat field or on a ridge. Seed furrows should not have depressions which cause poor drainage.
· Avoiding cultivation which throws soil into the crop row, especially when plants are young. This can cause stem injury and burial of young plants, which greatly increases susceptibility to stem rot and crown rot.
· Control of Cercospora leafspot and other foliar diseases with fungicides to minimize defoliation, since fallen leaves also serve as breeding grounds for the fungus.
· Applications of soil fungicides like PCNB (Terrachlor) and Vitavax (Carboxin) in a band over the row at planting or at early pegging stage. These give fair to good protection where stem rot problems are serious. (See the next unit for specific recommendations.)
Seed Rot and Seedling Blight (Preand Post-Emergence "Damping Off")
Pre-emergence rot: It is not unusual to find germinating peanut seeds rotting in the ground. Affected seeds break down rapidly, but early examination will show them to be covered with a growth caused by various species of fungus. Seedling blight is often referred to as Aspergillus crown rot and is caused by Aspergillus niger, a black sooty fungus. True crown rot is more accurately used to describe the disease when it attacks older plants past the seedling stage. The stem tissue just below ground level is attacked on young seedlings shortly after they emerge, and the fungus quickly spreads up the stem, covering it with a mass of black spores. The stem will then suffer a total collapse. Contributing factors: Soils that have been continually cropped to peanuts for long periods have more problems with seed rots and seedling blights. Excessively deep planting weakens the stem and increases susceptibility. Seeds may also be damaged as they are being deshelled. Controls: Seed treatment with fungicides gives good control; usually a combination of two fungicides is needed to provide control of all species. Recommendations are given in the next unit. Attention should also be given to planting depth and crop rotation.
This is somewhat similar but less common. Affected plants have a white fungal growth attached to rotted areas of the stem which may extend from below the soil surface up into some or all of the individual runners. Infected stem tissue is very shredded and contains many black fungal bodies. Pegs and nuts are also attacked. Control is usually not needed,but a fungicide called Botran (dicloran) is sometimes applied as a spray in the U.S.
Peg and Pod Rots
Several types of fungi including Sclerotium and Sclerotinia attack the pegs and pods. Soil sterilants are sometimes applied before planting in the U.S., but this would seldom be economical or feasible for small farmers. Crop rotation is helpful. Aspergillus flavus is a fungal mold that attacks stored seed but is sometimes found in the field. Under certain conditions, some strains of A. flavus produce aflatoxin, a potent carcinogen (cancer-causing agent) and toxin that can affect birds, humans, and other mammals. Harvested pods are free of aflatoxin except where they have been broken or damaged by termites, hoeing, threshing or rough handling. The development of Aspergillus and other storage molds largely can be prevented by timely harvest, separation of damaged kernels, and rapid drying of moist pods. Viral Diseases Rosette virus: The most serious disease of peanuts in Africa, especially in the wetter areas. It is spread by one species of aphid (Aphis craccivora) and has several alternate host plants, including Euphorbia hirta, a weed. Plants become severely stunted, and the younger leaves turn yellow and mottled. Emerging leaves remain small and become curled and yellow. Early planting and close spacing appear to reduce the incidence of rosette virus. Affected plants should be removed and destroyed, and aphid control should be considered.
Destruction of alternate host plants is helpful. Resistant varieties have been developed in Senegal. Spotted wilt virus: Caused by tomato wilt virus and spread by several types of thrips. Affected plants have leaves with light green and yellow patterns, often in large patches or in the form of ring spots. Leaves are usually misshapen and puckered, and the plants take on a bunched appearance. Tomatoes, potatoes, lettuce, peppers, ornamental plants, and several types of weed serve as alternate hosts. It is usually not serious.
Beans suffer heavy disease losses worldwide, and one of the major reasons is the high prevalence of seed-borne diseases. According to CIAT, more than half of the major bean diseases can be transmitted by the seed; these include anthracnose, damping off, root and stem rots, bacterial wilt, bacterial blight, and several viruses. Disease-free certified seed is very difficult to obtain in Latin America and presently makes up less than 3 percent of the bean seed planted there.
Control of seed-borne fungi: Many fungi are carried on or in the seed coat, and seed treatment with conventional fungicides like Arasan (thiram) and Captan (Orthocide) will control them. Others like anthracnose are carried deeper in the seed and are usually unaffected by seed treatment. Systemic fungicides like Benlate (benomyl) have shown some promise in these cases. Foliar applications of systemic fungicides during the latter half of the growing season have significantly reduced the incidence of seed-borne anthracnose in the harvested seed, but are expensive. Delayed harvesting and pod contact with the soil surface during growth can increase seed-borne disease problems.
Control of seed-borne bacteria: Seed treatments will not control internally-borne bacterial diseases on beans. Seed produced in drier areas using strict sanitary and cultural practices such as crop rotation and inspection is less likely to be contaminated.
Control of seed-borne viruses: Current seed treatments are ineffective against seed-borne viruses. Control involves the production of disease-free seed in areas where vectors and hosts can be controlled.
PreEmergence Rot: Seed treatment with fungicides is very effective. (See maize and peanuts.) Root Rots: Beans are very susceptible to root rots caused by Rhizoctonia, Fusarium, Sclerotium, and other fungi. Symptoms include reddish or brown lesions on the hypocotyls (belowground portion of the stem) and rotting of the lateral roots from one to several weeks after emergence. Wilting and leaf yellowing may or may not occur.
· In temperate areas, planting only after soils have warmed up
· Good drainage
· Crop rotation
· Avoiding contamination of virgin ground with unclean tools, animal or green manure containing bean residue or dirty irrigation water.
· Treating seed with Arasan (thiram), Zineb, Demosan, PCNB, Vitavax (carboxin) or Benlate at 1-3 active ingredient per kg to give partial control.
· Applying Benlate or PCNB over the seed furrow after planting to give good control.
Anthracnose (Colleotrichum lindemuthianum): Anthracnose is of worldwide importance in cool to moderate temperatures and wet conditions and is spread by seed, soil, crop debris, rain, and tools. It produces elongated reddish-brown to purple cankers on stems and leaf veins. Pods have sunken spots with pink centers and darker borders. Infected seeds mav be discolored and have dark brown to black cankers. Anthracnose is seldom a problem in hot, dry areas.
· Use disease-free seed.
· Do not grow beans more than once every two or three years on the same field (includes cowpeas, lima beans).
· Avoid working in fields when the plants are wet.
· Plow under bean residues.
Seed treatment with fungicides is only partially effective. Preventative applications of foliar fungicides have variable results. Rust (Uromyces phaseoli): Rust is of worldwide distribution and also attacks cowpeas and lima beans. Losses are heaviest when plants are infected at or before flowering. The disease is favored by damp weather and cool nights and can infect both the leaves and the pods. First symptoms usually appear on the lower leaf surface as whitish, slightly raised spots. The spots grow into reddish-brown pustules which may reach 1-2 mm in diameter within a week. The entire leaf begins to yellow, then turns brown and dies. Rust is not carried on the seed, but the spores persist in bean residues. There are many races of rust, and bean varieties vary in their resistance to them.
· Crop rotation.
· Sulfur dust or fungicide sprays (see next section).
Angular Leafspot (Isariopsis griseola): This disease causes gray or brown angular lesions on the leaves which eventually lead to premature defoliation. Pods may be affected with oval to round spots with reddish-brown centers and seeds may be shrivelled. The disease is carried by the seed, but contaminated plant debris is a much more common source of infection. Control: Using disease-free seed, crop rotation, and removing previously infected crop debris from the field before planting. Seed treatment with a fungicide (Benlate has given good results) and fungicide sprays may help. Sclerotinia Blight (white mold): Causes water-soaked lesions and a white mold on leaves and pods (see also peanuts). It can be controlled by crop rotation and foliar sprays of Benlate, Dichlone, Dicloran, PCNB or Thiabendazole around early to mid-bloom. Irrigation intensifies this disease.
Web Blight (Thanatephorus cucumeris) This disease can be a major limiting factor to bean production under high temperature and humidity. Many other crops are also affected. The fungus causes small round water-soaked spots on the leaves which are much lighter than the surrounding healthy tissue and look like they have been scalded. Young pods show light tan spots that are irregular in shape but become darker and sunken with age--they can be confused with anthracnose. The stems, pods and leaves become covered with a spider web-type growth that is imbedded with brown fungal bodies. Web blight can be carried by the seed but is more commonly transmitted by wind, rain, tools, and the movement of humans and draft animals through the field.
· Disease-free seed.
· Crop rotation with maize, grasses, tobacco, and other non-hosts.
· Planting beans in rows, not by broadcasting, to maximize air circulation.
· Fungicide sprays give fair to good control. Systemics like Benlate are recommended under high rainfall.
Common Blight (Xanthomonas phaseoli) and Fuscous Blight (Xanthomonas phaseoli var. fuscans): Both diseases produce the same symptoms on leaves, stems, pods and seeds. The first leaf symptoms are water-soaked spots on the undersides which grow irregularly and are surrounded by a narrow zone of lemon yellow tissue. These spots eventually become brown and dead. The stem may become girdled near the soil and break. Water-soaked spots form on the pods, gradually enlarge and become dark, red and somewhat sunken. Infected seed may rot and shrivel.
· Disease-free seed.
· Crop rotation and deep plowing.
· Copper-base fungicides have controlled leaf symptoms well, but have not given good yield increases. Antibiotics should not be used due to the danger of causing mutations.
· Seed treatment is not very effective.
· Some varietal resistance is available.
Halo Blight (Pseudomonas phaseoli-cola): This bacterial disease prefers cooler temperatures than common and fuscous blights. The initial symptoms are small,water-soaked spots on the undersides of the leaves, which eventually become infected with greasy spots if the attack is severe. Stem girdle or joint rot occurs at the nodes above the seed leaves when the disease results from contaminated seed. However, leaf yellowing and malformation may occur without many other external signs.
· Deep plowing and crop rotation
· Removing infected plant debris from the field
· Avoiding work in the fields when the foliage is wet
· Disease-free seed
· Varieties that have some resistance
· Seed treatment with Streptomycin (2.5 g active ingredient per kilogram of seed) or Kasugamycin (0.25 g active ingredient per kilogram), using the slurry (liquid) method.
· Copper-base fungicides applied to the leaves gives poor to fair control.
Beans are attacked by a number of viruses, many of which also attack cowpeas. Bean common mosaic, bean yellow mosaic and cucumber mosaic viruses are spread by aphids. Bean rugose mosaic and several others are spread by beetles. Bean golden mosaic and chlorotic mottle viruses are spread by white-flies, and curly top virus by the beet leafhopper. Symptoms include one or more of the following: green-yellow leaf mottling, leaf malformation, puckering, curling, plant stunting, and yellowing. Control consists largely of using resistant varieties and disease-free seed, and controlling insects.
Seed injury: Bean seed is very susceptible to seedcoat damage and internal injury by improper threshing and mechanical harvesting or by rough handling. Damage may be invisible or produce cracks in the seedcoat, both of which can cause the following seed abnormalities:
· Reduced germination and seedling vigor: This can also be caused by bacteria, fungi, insects, fertilizer burn, and herbicide injury.
· "Baldhead": The seedling lacks a growing point. There is only a bare stump above the cotyledons, so no further leaf growth can occur.
· Detached cotyledons: Young bean seedlings need at least one complete cotyledon or two broken ones with more than half attached to provide adequate nutrition for emergence and early growth.
Dry bean seed (14 percent moisture or below) is the most easily damaged. Bagged seed should not be dropped or thrown onto hard surfaces.
Sunscald: Intense sunlight, especially following cloudy and humid weather, can produce small water-soaked spots on the exposed sides of leaves, stems, branches and pods. These spots turn reddish or brown and may grow together into large necrotic lesions. Air pollutants and tropical spider mites can produce similar symptoms.
Heat Injury: High daytime temperatures may cause lesions that form a constriction around the stem at the soil line, especially on light-colored sandy soils. Temperatures above 35.5°C cause blossom drop if they occur during flowering.