Pod-borers (African bollworm, Legume pod-borer, lima pod borer)

Pod borers such as the African bollworm (Helicoverpa armiguera), the legume pod borer (Maruca vitrata), and the lima bean pod borer (Etiella zinckenella) can cause serious economic damage. Young caterpillars of the African bollworm feed on leaves by scraping tissue for short time, and then bore into the pods and feed on the seeds with their heads thrust inside and most part of the body outside. The entry hole is large and circular. They also cause significant damage to flower buds and flowers.

Caterpillars of the legume pod borer (Maruca vitrata) are dull to yellow-white and often reach a length of 18 mm. Each segment has dark spots that form a distinct series along the length of the body. The head is dark brown to black. Caterpillars web together leaves, buds and pods and feed inside the web. Flowers attacked may be discoloured and have damaged or missing reproductive parts. Damage by this caterpillar also results in flower bud shedding and reduced pod production. Damaged pods have small, darkened entry holes on the surface.

Young caterpillars of the lima bean pod borer are green, later turning red. They feed inside the pod reaching a length of 14 mm. They are generally found in maturing and dried pods. Faeces in the form of granules are found inside the damaging pods. Once the caterpillars have entered the pods they are difficult to control and by then they have already caused damage.

Anthracnose (Colletotrichum lindemuthanium)

It attacks all above ground parts of the plant. It does most serious damage on pods. Affected pods have brownish sunken spots, which under humid conditions are covered with a pink spore mass. Infected seeds become discoloured (brownish black).

Aphids

Aphids, mainly the legume aphid (Aphis craccivora) (also called groundnut aphid) are relative small. Immatures are slightly dusted with wax, adults without wax. Apterae are 1.4 to 2.2 mm long. Alatae (winged form) 1.4 to 2.1 mm. They feed on young plants, leaflets, stem and pods of green gram. Attacked young leaves become twisted. Excretion of honeydew leads to growth of sooty mould. Aphids are also vectors of virus diseases.

Bacterial blight (bean blight) (Xanthomonas pv. phaseoli)

Leaf spots first appear as small, water-soaked or light-green areas on leaflets. They later become dry and brown. The spots may join to affect much of leaf surface eventually killing the leaflet. Similar water-soaked spots develop on pods. The spot margin is a shade of red. Severely diseased pods shrivel. In humid weather, a yellowish crust of the blight bacteria covers the spot surface.

Bean flies (Ophiomyia phaseoli and related species)

Bean flies are tiny about 2mm long, shiny black-bluish in colour. They can cause serious stand reductions at the seedling stage. Bean flies lay eggs in punctures of leaves near the petiole. The small white maggots feed inside the main stem just above the soil line. Pupation occurs inside the stem. The life cycle may be completed rapidly, often in less than 2 weeks.

Seedlings attacked by bean flies may wilt or die. Leaves of older plants may be yellow and stunted. Stems are thicker than normal and cracked lengthwise just above the soil. Maggot feeding facilitates the entry of disease-causing microorganisms leading to secondary infections. In cases of heavy infestation, many plants die. Bean flies are important only during the seedling stage (up to 4 weeks after germination).

Pod sucking bugs

Pod sucking bugs such as giant coreid bugs (Anoplocnemis curvipes), spiny brown bugs (Clavigralla spp.), green stink bugs (Nezara viridula, Acrosternum acutum), and Riptortus bugs (Riptortus spp) are the most important pests of green gram at the podding stage.

They suck sap from pods and seeds and cause various levels of damage depending on the stage of growth of seeds at the time of attack. Feeding may cause necrosis, pod malformation, premature drying, shrivelling of seeds, loss of germination ability, and formation of empty pods. Bugs are difficult to control since they usually feed on a wide range of crops and are very mobile.

The cowpea weevil (Callosobruchus maculatus)

Cowpea bruchids (Callosobruchus spp.) are the most common and widespread insect pests in storage. Adults are 2 to 3.5 mm long. They are major pests of pulses (cowpeas, pigeon peas, soybean, green gram and lentils). They attack both pods in the field and seeds in storage. They attack nearly mature and dry pods. Infested stored seeds can be recognised by the round exit holes and the white eggs on the seed surface. Post-harvest losses are highly variable, but losses can be over 90%.

It is a serious storage insect, which can destroy whole seed-lots.

Damping-off diseases (Pythium spp., Fusarium spp., Rhizoctonia solani)

They are caused by a complex of fungi. They cause rotting of seeds before emergence and seedlings after emergence from the soil. Affected fields appear patchy. They are favoured by wet, cool weather.

Foliage beetles (Ootheca spp.)

Foliage beetles have been reported as pests of green grams in West Africa. They are a threat, when present in large numbers since they can defoliate young plants. They are 4-7mm long.

Powdery mildew (Erysiphe polygoni)

White powdery patches appear on leaves and other green parts, which later become dull coloured. These patches gradually increase in size and become circular covering the lower surface. When the infection is severe, both the surfaces of the leaves are completely covered by whitish powdery growth. Severely affected parts get shrivelled and distorted. In severe infections, foliage becomes yellow causing premature defoliation. The disease also creates forced maturity of the infected plants which results in heavy yield losses. The fungal agent (pathogen) has a wide host range and survives on various hosts in off-season. It is spread by wind and water splash.

Rust (Uromyces phaseoli)

The disease appears as circular reddish brown pustules (blisters) which appear more commonly on the underside of the leaves, less abundant on pods and sparingly on stems. When leaves are severely infected, both the surfaces are fully covered by rust pustules. Shrivelling of pods is followed by defoliation resulting in yield losses. Long distance spread of rust is by wind. Plant to plant spread is by farm tools, and moving bodies within the crop.

Flower thrips (Megalurothrips sjostedti)

It may feed on petioles and leaves, but prefer flowers. Attacked petioles and leaves have tiny holes surrounded by discoloured areas. Affected flowers are brown, dry or completely distorted. The flowers drop prematurely. Thrips also feed on pollen leading to decrease in pollination and seed set. Pod production is low and pods are deformed.

Whiteflies (Bemisia tabaci, Aleurodicus dispersus)

Several species of whiteflies are found on cassava in Africa. Feeding causes direct damage, which may cause considerable reduction in root yield if prolonged feeding occurs. Some whiteflies cause major damage to cassava as vectors of cassava viruses. The spiralling whitefly (Aleurodicus dispersus) was reported as a new pest of cassava in West Africa in the early 90s. The adults and nymphs of this whitefly occur in large numbers on the lower surfaces of leaves covered with large amount of white waxy material. Females lay eggs on the lower leaf surface in spiral patterns (like fingerprints) of white material secreted by the female. This whitefly sucks sap from cassava leaves. It excretes large amounts of honeydew, which supports the growth of black sooty mould on the plant, causing premature fall of older leaves.

The tobacco whitefly (Bemisia tabaci) transmits the African cassava mosaic virus, one of the most important factors limiting production in Africa. The adults and nymphs of the tobacco whitefly occur on the lower surface of young leaves. They are not covered with white material. The nymphs appear as pale yellow oval specks to the naked eye.

Mungbean yellow mosaic virus (MYMV)

MYMV causes serious loss in pulse crops such as beans, pigeonpea, mungbean and soybean. It is transmitted by the whiteflies, (Bemisia tabaci).
Symptoms: initially mild scattered yellow spots appear on young leaves. The spots gradually increase in size and ultimately some leaves turn completely yellow. Infected leaves also show necrotic symptoms. Diseased plants are stunted, mature late and produce very few flowers and pods. Pods of infected plants are reduced in size and turn yellow in colour.

The virus is not seed transmitted in mungbean or soybean.