The legume aphid (Aphis craccivora)

It is a widespread pest of cowpeas. Aphids suck sap on stems, terminal shoots and petioles of seedlings, and on pods and flowers of mature plants.

A heavy attack can cause death of young seedlings, stunting and delay in flowering on older plants. However, it is more important as vector of virus diseases (e.g. the cowpea mosaic virus).

African bollworm (Helicoverpa armigera)

The African bollworm caterpillars are 3 to 4 cm long. Several leaf-eating caterpillars feed on cowpeas. Several species of Spodoptera are sporadic cowpea pests. The hairy caterpillar Amsacta moorei causes extensive damage to seedlings and it has been considered the most important pests of cowpeas in Senegal. African bollworm (Helicoverpa armigera) can cause extensive damage on young pods. Natural enemies are important to keep pest populations at low level.

Flower or blister beetles (Mylabris spp. and Coryna spp.)

These beetles can cause serious damage to cowpeas by feeding on flowers. The adults of the flower beetles, also known as blister beetles, feed on flowers (petals and / or pollen) reducing pod set.

The adults are medium to large sized beetles (2-5cm in length), usually black and yellow or black and red in colour. The immature stages (larvae) do not feed on plants. They live in he soil and eat grasshopper eggs.

Thrips (Megalurothrips sjostedti and Frankliniella schultzei)

Thrips are among the most widespread and important pests of cowpeas in Africa. The cowpea flower thrips or African bean flower thrips (Megalurothrips sjostedti) causes yield losses of up to 100%.

During the pre-flowering period, nymphs and adults of this thrips may damage the terminal buds. However, the main damage is on the flower buds and flowers. Attacked flower buds become brown and eventually fall off, leaving behind dark red scares. Damaged flowers are distorted, malformed and show decolouration and may fall off. Infested on pods are malformed.

Bugs

There are several pod sucking bugs. The tip wilter (Anoplocnemis curvipes), the spiny brown bug (Clavigralla tomentosicollis, Riptortus bugs (Riptortus dentipes), Mirperus jaculus, the green stink bug (Nezara viridula) and Aspavia sp.

These bugs attack cowpeas in Africa. The spiny brown bug Clavigralla tomentosicollis and Riptortus dentipes are the most important, causing serious damage. Nymphs and adults attack young, tender pods and causing shrivelling and rotting of pods and malformation of seeds, which lose viability.

Bugs are difficult to control since they usually feed on a wide range of crops and are very mobile.

Legume pod borer (Maruca vitrata)

This is the most important pod borer pest, causing severe damage to cowpeas. Losses over 80% have been reported on indigenous varieties and even on high yielding varieties.

Moths usually lay eggs on flower buds, flowers, or on terminal shoots of young plants. Young caterpillars may feed on any part of the flowers or foliage. Several young caterpillars may be found together among flowers. Older caterpillars are highly mobile, feeding continuously on flowers and newly formed pods, causing severe damage to the crop. Upon reaching maturity the caterpillars drop from flowers or pods onto the soil and pupate beneath the plant under leaf debris.

Caterpillars of the legume pod borer are dull to yellow-white and often reach a length of 1.8 cm. Each segment has dark spots that form a distinct series along the length of the body. The head is dark brown to black.

Cowpea weevils (Callosobruchus spp.)

Adults are 2.0-3.5 mm long. They are also known as the cowpea seed beetles and are the principal storage pest of cowpea. These bruchids may infest up to 100% of the stored seeds within 3 to 6 months under ordinary storage conditions. A positive relationship between pod damage by field pest (pod sucking bugs and pod borers) and bruchid infestation in storage was found in Uganda. Controlling pests infesting pods of cowpeas in the field significantly reduce bruchid carryover in storage (IPM CRSP).

Root-knot nematodes (Meloidogyne spp.)

Relatively small galls or knots develop on roots of affected plants. Do not confuse root knot galls with naturally occurring bacterial nodules that are beneficial.

Storage pests

The pod weevil (Piezotrachelus varius or Apion varium). Its a common pest of cowpeas in West Africa. Generally 13-26% of the pods are damaged. Losses of seeds up to 92% have been reported in Nigeria. The shiny black weevils bore holes in fresh green cowpea pods and lay eggs into the pods. The grubs feed on the seeds and pupate within the pods.

Cowpea mosaic diseases

These viruses produce a mosaic pattern on cowpeas. They may be found singularly or in combination with others. They cause irregular light and dark green mosaic patterns on the leaves. Some viruses cause thickened, malformed leaves. The mosaic patterns are best observed on the younger foliage. Plants may be stunted and fail to produce normal pods. If the disease attacks plants at the early growth stage, no pods should be expected.

The most common virus disease on cowpeas is cowpea aphid-borne mosaic potyvirus. It is transmitted by aphids.

Mosaic diseases include:

• Cowpea mosaic comovirus (CpMV)
• Blackeye cowpea mosaic potyvirus (BICMV)
• Cowpea severe mosaic virus (CPSMV)
• Cowpea aphid-borne mosaic potyvirus (CAMV)
• Cowpea mottle carmovirus (CPMoV)
• Cowpea golden mosaic bigeminivirus.

Damping-off diseases (Rhizoctonia sp., Phythium sp., Fusarium sp.)

Seeds may rot before emergence from the soil and young seedlings may die. The condition is most common on early plantings or when soil contains a large amount of undecomposed plant residue.

Damping-off diseases are favoured by cool, wet soil conditions.

Bacterial blight (Xanthomonas axonopodis pv. vignicola)

This disease appears as tan to brown angular leaf spots with yellow margins on leaves, pods, and stems. It may cause severe defoliation during periods of high humidity. It is seed-borne.

Anthracnose (Colletotrichum lindemuthanium)

This is a major cowpea disease, which can be very severe in areas where cowpeas are grown as the sole crop. Stems affected by anthracnose exhibit dark brown areas that later join up to cover the entire stem as well as branches, peduncles and petioles. The disease also attacks the pods.

Lesions on pods are sunken and brownish and under wet conditions they are covered with a pink fungal spore mass. Under severe infection, stems die. The disease attacks all legumes. Anthracnose is most prevalent during warm, humid weather. It is transmitted through infected seeds and survives in crop debris.

Powdery mildew (Erysiphe polygoni)

Symptoms consist of a light, greyish, powdery growth on the leaves, pods and occasionally the stems. This powdery growth is easily rubbed off. When the disease is severe, plants turn yellow and defoliate. Generally, powdery mildew does not damage early-planted cowpeas. It can, however, be quite destructive on a late-planted crop. A fairly dry soil and heavy application of nitrogen-based fertiliser tend to increase disease severity.

Leaf spots (Cercospora sp., Aristastoma sp., Ascochyta sp., Colletotrichum sp., Stagnospora sp.)

Leaf spots are various sized often yellowish in colour or with a yellow halo, others brown to purplish; These normally develop first on lower leaves. With Cercospora leaf spot a dark, mouldy growth develops on the lower leaf surface corresponding to the spot. Leafspot diseases are most serious during periods of prolonged moist weather and on late plantings. Severe leaf spotting results in defoliation with subsequent yield reductions.

Brown blotch (Colletotrichum truncatum)

The fungus causes pre-emergence and post-emergence damping off when infected seeds are planted. The former rots the seed before emergence from the soil while post-emergence kills the seedlings after emergence. The disease also attacks the foliage, stems and pods. Sunken, oval spots may be seen on stems; circular spots on leaves. Lesions are reddish-brown. Under prolonged wet weather heavy defoliation occurs. During late reproductive stages, infected tissues are covered with black fungal fruiting bodies, which produce minute black spines (setae) that can be seen with the unaided eye. It is transmitted through infected seeds and survives in crop debris.

Rust (Uromyces vignae)

Small, reddish-brown pustules (blisters) appear on both upper and lower leaf surfaces. Rust can develop rapidly, resulting in severe leaf damage and defoliation.

Cowpea wilt (Fusarium oxysporum f. sp. tracheiphilum)

Cowpea wilt (Fusarium oxysporum f. sp. tracheiphilum)
Fusarium wilt usually causes the lower leaves on one side of the plant to turn yellow. Infected plants usually are stunted and wilted as the organism develops in the food and water conducting tissues. Brick red tissue can be observed in the stem when it is split lengthwise.

Southern blight (Sclerotium rolfsii)

It attacks roots and stems of cowpeas. The first visible symptom of southern blight is a progressive, yellowing and wilting of the foliage beginning on the lower leaves. The plant dies within a few days after the first symptoms appear. During warm, moist conditions, the coarse, white mycelium of the fungus makes characteristic fan-shaped patterns of growth on the stem at the soil line. In this white mat of the fungus, numerous smooth, round, light-tan to dark-brown mustard seed-like bodies called sclerotia are formed.

Charcoal rot (Macrophomina phaseolina)

Charcoal Rot (fungus - Macrophomina phaseolina). Many plants are susceptible to this soil borne fungus and symptoms vary according to type. Infected stem tissue shows evidence of shredding with tiny black dots (sclerotia) between the remaining tissues. This gives those plant parts an ashy-grey appearance. This can be observed by splitting the stalk and noting the deteriorated soft pith tissue leaving the tougher vascular strands. Fungal structures (sclerotia) can be observed in the affected tissue which appears as though it has been dusted with black pepper

Charcoal rot occurs most consistently when plants are experiencing moisture stress due to drought. The fungus is widely distributed and builds up in soil when susceptible host plants are present and conditions favour its development.

Avoiding moisture stress, proper management of crop residue, crop rotation, avoiding excessive plant populations, balancing nitrogen and potassium fertility levels, and growing drought-tolerant, lodging-resistant hybrids represent the best means of control.

Witchweed (Striga hermonthica)

The parasitic weed witchweed (Striga gesnerioides) is also a problem in cowpea.