Cowpea suffers from a wide range of fungal, viral and bacterial pathogens as well as several species of insect pests.
Major cowpea diseases which occur frequently in the high rainfall areas of Zambia are scab (Sphaceloma sp.) and ascochyta (Phoma exigna). Sometimes the two diseases occur at the same time on a cowpea crop and cause substantial yield loss. Both diseases can be prevented by use of clean crop debris and practicing crop rotation. Spraying Dithane M-45 and Benlate is effective in controlling the two diseases.
Cercospora leaf spots (Cercospora canescens and Psedocercospora cruenta) are widely prevalent in Western Province of Zambia. The pathogens are seed borne and they also survive on alternate hosts and infected crop debris. The disease can be reduced by use of clean seed, burning of crop debris and application of fungicides (Benlate or Dithane M-45).
The most destructive disease of cowpea is the cowpea aphid borne mosaic virus (CAMV). Use of virus free seed is the most important measure to prevent spread of the disease especially to new areas. Planting early and intercropping with cereals are useful cultural practices to reduce CAMV incidence.
Insect pests are a major constraint to cowpea production in Zambia. Several species of insects attack the crop, commencing from germination stage to crop harvest. Soon after germination the young seedling can be attacked by ground beetles, stem maggots and cutworms, causing plant mortality during the vegetative stage. Sucking pests including aphids, leaf hoppers and whiteflies, and defoliators such as leaf beetles, caterpillars and grasshoppers are often very damaging.
Bruchid beetles may commence the infestation even in the field by laying their eggs on mature pods and continue damage during storage. The extent of importance of the different insects may differ with regions and/or seasons. However, the more common and apparently serious pests are the aphids, flower thrips, flower beetles, maruca pod borer, apion weevil, pod sucking bugs and bruchids.
Aphids are fairly well distributed, infesting the crop right from the seedling stage, sometimes even up to maturity. The winged adults, which tend to disperse are more actively involved in the spread of CAMV disease.
To prevent aphid attack, early planting may be beneficial, especially in minimizing the vector role of aphids. If quick control is necessary, sprays of dimethoate (Rogor 40% EC at 2ml/l) or monocrotophos (Nuvacron 40% EC at 2 ml/1) may be used.
Flower beetles commonly known as blister beetles are medium to large sized insects, black in colour with dark, red, orange or yellow markings on the fore wings. The adults feed on flowers, mainly chewing out the petals, resulting in flower drop and consequent poor pod set.
Since these beetles tend to feed on flowers of a wide range of plants, it is difficult to control them by just spraying on the cowpea crop. Where possible periodical hand picking and destruction of the adults can be adopted. If however, insecticide use is necessary, sprays of endosulfan (Thiodan 35% MO at 2 ml/1) or triazophos(Hostathion 40% EC) may be used but the protection may not even be a few days as the pest is highly tolerant to such sprays.
Maruca pod borer and apion weevil are both satisfactorily controlled by spraying with endosulfan (Thiodan 35% Mo at 2 ml/litre) or synthetic pyre-throid (Fastac 10% EC at 0.5 ml/litre).
Pod sucking bugs are particulary severe in the low rainfall regions of Southern and Western Provinces. Control can be effected by use of carbarly (Sevin 10% dust at 25 kg/ha) or spraying with monocrotophos (Nuvacron 40% EC at 2 ml/litre).
Bruchids are found in all regions of the country and more than one species is known to attack cowpea, but the nature of damage is similar. Field infestation of bruchids may be minimized by harvesting pods as soon as they are mature. Storing the cowpeas unshelled is useful to reduce fresh infestation. Mixing the seed with pirimophos methyl (Actellic 2% dust at 1 g per kg seed) or malathion (Blue Cross 2% dust at Ig per kg seed) is recommended. Coating with vegetable oil (5-10 ml per kg seed) may offer some relief from bruchids.