Cowpea, Vigna unguiculata, is a climbing annual in the family Fabaceae grown for its edible seeds and pods. The cowpea plant is usually erect and possess ribbed stems and smooth trifoliate leaves which are arranged alternately on the stems. The plant produces clusters of flowers at the end of a peduncle (flower stalk) and 2–3 seed pods per peduncle. The seed pods are smooth, cylindrical and curved, reaching up to 35 cm (10 in) in length, with distinctive coloration, usually green, purple or yellow.

These are some of the diseases of cowpea

Anthracnose Colletotrichum spp.

Symptoms

Tan to brown sunken lesions on leaves; Lesions merging to girdle stems and petioles; Lesions may become covered in pink spore masses during periods of wet weather

Management

The best method of controlling the fungus is to plant resistant varieties if available; plant only certified disease-free seed; Practice good field sanitation such as removing crop debris from field after harvest to reduce levels of inoculum

Asochyta blight Asochyta phaseolorum

Symptoms

Severe defoliation of plants; Extensive lesions on stems and pods; If infection is severe then plants may be killed

Management

Plant disease-free seed; Applications of appropriate foliar fungicides, where available, may help to control the disease

Brown rust Uromyces spp.

Symptoms

Raised brown to black pustules on both sides of leaves; Wilting plants; Drying leaves dropping from plant

Management

Sprays of sulphur or potassium carbonate can help to control the disease

Cercospora and Pseudocercospora leaf spot Cercospora canscens
Pseudocercospora cruenta

Symptoms

Chlorotic spots on upper surfaces of leaves; necrotic spots on leaves;

masses of spores on lesions which resemble black mats on lower leaf surface; Defoliation of plants; yellowing of leaves; circular, red lesions on leaves

Management

Remove all crop residue from field after harvest; plant disease-free seed

Charcoal rot Macrophomina phaseolina

Symptoms

Discoloration of stem at soil line; cankers on stem may spread upwards; leaves may wilt and drop from plant; numerous small black sclerota (fungal fruiting bodies) develop in affected tissues and can be used to diagnose the disease

Management

Organic soil amendments such as the addition of manure or neemcake can be used to reduce levels of inocuum in the soil

Fusarium wilt Fusarium oxysporum

Symptoms

Stunted plant growth; yellowing, necrotic basal leaves; brown-red or black streaks on roots that coalesce as they mature; lesions may spread above the soil line

Management

Control relies on cultural practices e.g. do not plant in same area more than once in any 5 year span or treating seeds with an appropriate fungicide prior to planting

Powdery mildew Erisyphe polygani
Sphaerotheca fuliginea

Symptoms

White powdery fungal growth on upper surfaces of leaves; chlorotic or brown patches on leaves; leaves dropping from plant

Management

Plant resistant varieties if available; use adequate plant spacing to avoid overcrowding and promote good air circulation around plants

Rhizoctonia seedling blight Rhizoctonia solani

Symptoms

Water-soaked sunken, red-brown lesions on hypocotyls (germinating shoot below seed leaves) and epicotyls (shoot above seed leaves); small, circular brown spots on leaves; large irregular lesions with zonate banding on leaves; lesions with water-soaked borders; leaves that look like they are covered in sand (sclerotia)

Management

Crop rotation helps to reduce the build up of the fungus in the soil; reduce soil compaction; do not plant seeds too deep.

source: https://plantvillage.psu.edu/topics/cowpea/infos