Peas

 Various species of caterpillars feed on leaves and pods of peas. The most common are the African bollworm (Helicoverpa armiguera), the beet armyworm (Spodotera exigua) and hairy caterpillars (Arctiidae).

Caterpillars bore holes and feed on leaves, flower buds and maturing pods eating the seeds. They also attack young pods eating the pod wall and the developing seeds.

Aphids are serious pests of peas. The pea aphid (Acyrthosiphon pisum), the cowpea aphid (Acyrthosiphon pisum), and Myzus sp. (Aphis carccivora), are found on young leaves and growing points.

They suck sap from leaves, stems, blossoms and pods. Feeding by aphids causes leaf distortion and wilting. Heavily infested plants are stunted and produce fewer and smaller pods and seeds. The pea aphid may also transmit viruses such as pea enation mosaic virus, pea streak virus and pea leaf roll virus. Parasitic wasps and predators (hoverflies and ladybird beetles) are important for natural control of aphids.

Ascochyta blight attacks all parts of the plant. The infected leaves have small to large, round to irregular, dark brown to purple spots. Some of the spots may have ashy grey centres. The spots usually have purple margins and may have rings. Spots may join up to form brownish purple blotches. Severely diseased leaves shrivel and dry, starting at the base of the plant and progressing upwards. Spots on pods are sunken but have no rings. Stem symptoms consist of brownish to purple streaks. Infection from infected seeds can cause a brown to black stem and foot rot just above the soil line. Such plants may die and result in a poor stand in the field. Blossoms may be infected and drop off during extended wet weather.

The disease is favoured by cool wet weather, heavy dews in the morning and relative humidity above 89%.

Cutworms are the caterpillars of some moths. They cut-off seedlings at ground level, usually at night. They also feed on roots and leaves. They hide in the soil during the day.

The fungal growth over leaf and pod is the most striking symptom. The fungal growth occurs on the lower leaf surface as well as on pods. Initially it is white but later it changes to a shade of violet and eventually to almost black. The disease may appear on pods even when it is not apparent on leaves. Affected pods show yellowish brown areas. Inside of the affected pods a white fungal growth can be seen. Peas in such pods are small and have brown spots. The disease is favoured by cool and moist conditions.

The fungus can attack plants at any stage of development. Distinct symptoms consist of yellowing of foliage and wilting leading to death of affected plants. The disease appears in scattered areas of the field and eventually may cover bigger areas. If stem of diseased plant is split, the pith is brick-red in colour. The disease can be seed-borne.

Leafminers are major pests of peas. The maggots mine on leaves and pods. Mining of leaves affect the photosynthetic capacity of the plant. Mining of pods causes cosmetic damage leading to rejection of pods in varieties grown for their tender green pods such as snow peas and sugarsnap peas.

It is a pest of peas during the flowering and podding stages. The adult is a butterfly bright blue (males) to brown (females) in colour. They have small tails at the edge of the hind wings, and two eye-like spots near each tail. The wing undersides are sandy-brown with creamy transverse bands. The moth lays eggs singly on the shoots, on or near the young flowers. Upon hatching the small caterpillars are yellow in colour, and are very active feeding first inside the flower and then inside the developing pod eating the young seeds. On small pods the caterpillar makes a hole in the pod wall in order to reach the seeds. Older caterpillars are grub-like green or pear white in colour and reach up to 15 mm when fully-grown. Pupation takes place amongst debris and leaves on the ground, under stones, or even in a curled-up, withered leaf on the plant.

It is characterised by a white powdery growth on the leaves, stems and pods. The initial symptoms consist of tiny slightly discoloured spots on the upper surface of leaves. Theses spots enlarge and become covered with powdery fungal growth. The tissue beneath affected areas may turn purple and later brown. If infection is severe, affected plants turn brown and die. Affected seeds become brown. Water stress accelerates mildew development. Warm days and cool nights favour disease development. The fungus is seed-borne.

Root-knot nematode galls on tomato roots. Root-knot nematodes (Meloidogyne incognita / M. javanica) affected plants are normally stunted and eventually wilt and die. The most characteristic symptom is formation of root galls (knots) and these can be seen with the naked eye. Affected roots rot.

Spider mites cause small white yellow specks on leaves. If spider mites are present in large numbers the leaf may be dry and fall off. Plants grow poorly when they are heavily infested. When infestations are heavy the spider mites will go up on the supports for the plants and accumulate on the tips so that the wind can carry them to new crops.

A number of beetles feed on dry cassava causing post harvest losses. In Benin Republic the most common are Dinoderus sp., Carpophilus sp., the coffee bean weevil (Araecerus fasciculatus), the lesser grain borer (Rhizopertha dominica), and more recently, the larger grain borer (Prostephanus truncatus). (see below)

Infestation by these insects is heavier in the rainy season than in the dry season, is more prevalent in the humid zone than in the savannah, and is found more in large chips than in smaller ones. Maximum infestation was found after 6 to 8 months in storage, at which time chips would fall into dust when squeezed (Bokanga, IITA, FAO).

Thrips suck plant tissues causing withering of the plant. Infested parts show silvery white discolouration. Thrips feeding on pods cause tinny scars and blemishes. The damaged pods may not be noticed at harvest, but scars become more visible during post harvest transport to market. Thrips damage lowers the market value of snow peas and sugar snaps (which are eaten as pods) and may lead to rejection. Their feeding punctures may also be a point of entry for disease-causing fungi such as Ascochyta. Thrips are difficult to control since they often migrate into peas from surrounding vegetable crops and weeds.

Peas are susceptible to a large number of aphid- transmitted viruses, which can produce diseases individually or in combination. The main viruses infecting pea are pea enation mosaic, pea streak and bean yellow mosaic.

1) Pea enation mosaic virus (PEMV)

In addition to pea, PEMV also infects broad bean, sweet pea, and alfalfa. It probably overseasons in many common perennial legumes. The virus is spread in nature most efficiently by the pea aphid (Acyrthosiphon pisum) and to a lesser extent by the green peach aphid (Myzus persicae). The virus is transmitted in a persistent (circulative) manner. Infected pea plants develop mosaic and chlorotic vein flecking (appears as translucent windows) and veinal enations (blisterlike outgrowths), which are very characteristic for pea enation mosaic virus. Plants are stunted, and proliferation of basal branches is common. Pods are distorted, split open, and may show prominent enations.

2) Bean yellow mosaic virus (BYMV)

Bean yellow mosaic virus is transmitted by at least 20 aphid species in a non-persistent manner. The symptoms depend upon the strain of virus involved. The pea strain, for example, causes a very bright yellow mosaic whereas the more typical isolates produce a dull light and dark green mosaic. Symptoms are masked or delayed at low temperatures (below 15 °C ), but develop at 18 - 23°C . Seed transmission is absent or very rare in pea and several other legumes.