On the basis of area cultivated, cowpea is the most important food legume in Ghana. The bulk of production occurs in the savannah regions of Northern Ghana, although cowpea can be grown in all ecological zones of Ghana. Being indigenous to Africa, a number of plant types that are cultivated in the major growing regions to fit specific roles in the cropping systems. Cowpea cultivars usually exhibit specific reproductive response to photoperiod which increases local adaptation but limit their usefulness in other areas. Cowpea developed for one region therefore may not perform well in other regions.
A number of landrace types are cultivated. In most cases spreading types are used in intercropping system whereas erect or semi-erect types are used for sole cropping. Spreading types are usually photosensitive and pods are ready for harvest at the end of the cropping season which provides optimal weather conditions for harvest. Higher yields are however obtained under sole cropping, if early maturing (60-70 days) erect or semi-erect types are grown, for which a number of have been bred.
Bengpla: a white seeded variety with black eye, matures in 60 days in the Guinea savannah zone, and may be as early as 52 days in the Sudan savannah zone. It produces good yields in a disease-free environment. The potential yields is 1.5 t/ha. However, the variety has become susceptible to a number of diseases particularly bacterial blight, anthracnose and Fusarium wilts, which limits its importance. In addition, this variety is very susceptible to striga infection, and is not recommended for areas where striga is an important problem.
Vallenga: is a red-seeded that matures in about 70 days. It was released in 1986 after testing with farmers. It produces stable high yields, with a yield potential of 2.0 t/ha. Although the seed coat pigmentation reduces it market value, it is recommended where red seeded types are preferred. Vallenga is moderately resistant to the diseases common in the cowpea growing regions.
Apagbaala: this variety has white seed coat with small brown eye. The seeds are small in size. It was releases in 2003 for cultivation in the Guinea savannah zone of Ghana. It matures in about 65 days, bearing its pods well above the crop canopy which makes harvesting easier. Under good management and favourable weather conditions, yields as high as 1.8 t/ha can be obtained. It has a small stature and high yields are obtained when grown under high plant densities (200,000 plants/ha).
Because of synchronous pod maturity and long peduncles that carry the pods above the canopy, this variety usually have less damage form the Maruca pod borer. The grains have a short cooking time compared with other varieties. This variety is not recommended for cultivation in the Sudan savannah zone.
Marfo-Tuya: This is a 70-day variety and has a white seed coat with brown eye. It was released in 2003 for general cultivation in Northern Ghana. The yield potentials is 2.0 t/ha. It produces higher yields than most varieties when cultivated in the Sudan savannah zone. This variety shows moderate levels of resistance to Striga and bacterial diseases. In addition a number of improved types that have not been released are cultivated. An example is IT81D-1137, a medium maturing white-seeded line with yield potential of 1.8 t/ha.
CLIMATE AND SOIL
Cowpea is warm weather and requires less rainfall than most crops. It is particularly tolerant of drought during vegetative growth. Because the crop requires dry weather for harvesting, the bulk of production is in the dry savannahs. Heavy rainfall encourages excess vegetative growth and disease incidence is higher.
Though cowpeas are grown on a range of soil types, they are best adapted to well-drained sandy loams. Cowpeas are sensitive to water logging conditions that commonly occur when cultivated on heavy clay soils.
For soils with poor structure, high run-off and low water infiltration, the physical properties can be improved markedly and cowpea yields increased if farmers hoe the land or the land is ploughed. Zero tillage (for example using Roundup spray prior to planting) may be used only where drainage is good.
On fairly fertile soils cowpeas do not need nitrogen fertilizer. Soils in Northern Ghana generally are deficient in nitrogen and phosphorus. Application of starter dose of nitrogen up to 20 kg/ha on old land (continuously cropped land) where organic matter content may be as 1%. Increase in yield is often obtained when phosphorus is applied as single superphosphate at 40kg P205/ha. Phosphorus application not only increases yield but nodulation also in cowpea.
PLANTING AND SEED RATES
For good plant stand and high yields, seeds must be free of diseases and insects. Ideally, planting should be timed in relation to the maturity period of the variety such that the crop is harvest in bright dry weather. Harvesting under humid cloudy weather favours pod rots. Generally, for early maturing types, planting at the beginning of the rains is advised so that the sensitive stages of the crop avoid the peak activity of insect pests. Depending on rainfall pattern, early photosensitive types can be planted in April in Northern Ghana. Prostrate, photosensitive types may be planted towards the end of July. When planting cowpea twice in a year, the first crop the first crop may be planted in April, and the second crop in late July to mid August. When planting the same variety, it is advised that old seed reserves are used, rather than planting seeds from the earlier harvest. Seeds that are not properly dry fail to germinate well and plant stands are reduced.
Seed rate depend on the plant type and seed size. Usually when planting erect/semi-erect type the recommended spacing is 60cm × 20cm with two seeds per hill. At this spacing, up to 28 kg of seeds is required per hectre. Local prostrate varieties should be planted wider spacing of 80 cm × 40 cm.
Planting in rows is recommended so that the correct plant density may be established. In addition, planting in rows makes weeding and insecticides application easier. Line planting may be done with the aid of garden lines or sighting poles.
Weeds damage cowpea by competing for light, water and nutrients. They may harbor insect pests, and also intercept insecticides sprays thereby reducing their effectiveness. Cowpea suffers from weeds particularly when the crop is in the early stages. Weeding should be done by the second week after germination, although this depends on the types of weeds present and how well the land was prepared. If a pre-emergence herbicide (e.g. Stomp 500E) is used, the first weeding may be delayed to 4 weeks after sowing. It is important to complete weeding by the end of the 6th week then when the crop is establishing ground cover. Striga gesnerioides is an important parasitic weed, and it is quite prevalent in the Upper East Region. The cultivar, Marfo Tuya is moderately resistant to this parasite.
COMMON DISEASES OF COWPEA
The commonest diseases of cowpea in Northern Ghana include the following:
- Bacterial diseases- these include bacterial blight and bacterial pustule. Bacterial blight is seed borne and using high quality seed may reduce incidence of this disease. The disease also survives on diseased crop residues. Growing cowpea after cowpea may therefore increase diseases prevalence. The initial symptoms appear as tiny dots on the leaves. The area surrounding the spot dies and develops a yellow coloration. The disease spread rapidly during heavy rainfall. Under such conditions, the dead spots merge and large areas of the leaves are affected. The disease may affect the stem, causing the stem to crack. Affected pods appear water-soaked and from this point, the pathogen enters the seed. First symptoms of the bacterial pustule are tiny dark water-soaked spots on the undersurface of the leaves. Under severe infestation, the spots enlarge becoming dry and sunken in the center, and water-soaked around the margin. The leave turn yellow and fall. Like bacterial blight, use of the clean seed and rotation may reduce the disease incidence.
- Fungal disease- A number of fungal diseases are prevalent. Some of the important ones are described below. Anthracnose: it is a stem disease, but may affect all aboveground parts. The lesions that are brown to tan in colour appear on affected plant parts. The lesions enlarge to girdle the stem, petioles and peduncles. The disease is seed borne and can be controlled by using clean seed. Although fungicides are not typically used on cowpea, benomyl or mancozeb (0.2% a.i.) is recommended under severe infection. Another disease called blotch show similar symptoms but mainly attacks the pods. Pods become distorted and black spot appear on them.
Web blight: Small, circular reddish-brown sports appear on leaves which under humid conditions enlarge into irregular-shaped areas. Leaves become dry. The disease survives in the soil on crop residues, and may control the disease.
Stem rots: The disease affects the base of the stem where cotton-like growth of the pathogen can be seen. Infected plants wilt and die.
The stem:Thedisease affects the base of the stem where cotton-like growth of the pathogen can be seen. Infected plants wilt and die. The stem rots are probably not seed borne. Good field hygiene may control the disease.
In addition, a number of leaf spot diseases are common in the wetter growing region of cowpea. Using seed from an approved source, rotation and observing field hygiene will generally reduce disease prevalence.
9. COMMON INSECT PESTS ON COWPEA
Insect pests are the most important yield reducing factors in cowpea. Farmers who do not spray their crops risk total crops failure.
- a. Pre-flowering Pests
Aphids: The cowpea aphid is a major pest common in growing areas. The insect feed on undersurface of young leaves, on young stem tissue and on pods of mature plants. Under severe infestation, there is premature defoliation and death of young seedlings. A more harmful effect is that the insect transmits the aphid-borne mosaic virus. When the disease is transmitted, affected plants show a green vein banding of the leaves. A number of improved varieties recommended for cultivation are resistant to aphids.
Leafhoppers can destroy cowpea during the seedling stage. Their feeding causes yellow discoloration of leaf veins and margins, followed by cupping of leaves. Plants become stunted.
Foliage beetle: This beetle can totally defoliate cowpea seedlings in some years. The adults appear either as shiny reddish brown, brown or black. This beetle also transmits a viral disease in cowpea.
Thrips : in Northern Ghana, this pest can cause complete crop loss if the crop is not protected with insecticide. The adults appear as shiny black, small insect in flower bud and flowers. Under severe infestation, plants do not produce flowers. Open flowers may appear distorted and discolored.
Control: Most of the improved varieties recommended for cultivation are resistant to these pests, except for thrips. Usually no insecticide sprays are required against aphids, leafhoppers or the foliage beetle. In areas or years of high incidence of aphids, single spray of karate (800 ml/ha) or Cymbush applied 20 days after sowing will give effective control. Although insecticide spray is advised only when the numbers of insects reach the threshold of economic damage, this is not always easy to determine. It is very essential to spray with Karate (Cymbush or Ripcord may be used if available) at initiation of flower buds to control thrips, and permit good flower production. For most varieties, this will be between 30 to 35 days after sowing. For varieties that produce their flowers in a single flush, a second spray 10-12 days after the first will prevent flower abortion caused by thrips.
- b. Post-flowering pests
The Maruca pod borer is a pest that causes damage to pods and seeds. The adult is a nocturnal moth. Larvae feed, on tender parts of the stem, peduncles, flowers and pod. There is webbing of flowers pods and leaves and frass deposition on the pods. Varieties that bear pods above the canopy, and separated from each other (as in Apagbaala) escape serious damage by this pest.
Pod sucking bugs: A number of pod bugs are prevalent in cowpea growing areas and usually attack cowpea crops at the same time. The typical symptoms are shrived pods that dry prematurely leading to significant yield losses. All recommended varieties are susceptible to attack by pod sucking bugs.
Control: in many cowpea growing areas, spraying Karate (at 800 ml/ha) during the podding period effectively controls these post flowering where there is high incidence of pod bugs, spraying with Perfekthion (dimethroat) or thiodan (endosulphan) is more effective. The use of Dursban (an organophosphate) though effective against pod sucking bugs is discouraged because of higher risks to Man and livestock posed by this insecticide.
- c. The cowpea weevil (bruchid)
Is a serious pest of cowpea and can completely destroy the grain within six months. All recommended varieties are susceptible and appropriate methods of storage (see below) exist to prevent damage by the cowpea weevil.
Matured, dried pods should be harvested promptly, Delayed in harvesting will encourage weevil infestation in the field, seed shattering and in humid weather the grains amy deteriaote. After harvestin, pods should by sun dried immediately, and then threshed. Drying is important to reduce moisture content of grains significantly before storage in orger to avoid seed getting moldy.
Cowpea should be threshed before storage. Storage in pods makes control of cowpea weevil more difficult. For seeds: foe small quantities of seed, storage in wood ash is effective. The following points should be noted.
1.) Use equal volume of wood ash and cowpea seed.
2.) The ash and seed should be mixed thoroughly and stored in a container.
3.) Cover ash/seed mixture with up to 3 cm of ash.
4.) Close container tightly.
Fine sand may be used in place of wood ash.
For large quantities of grain/seed: for large quantities of grain, the heat disinfection technique is strongly recommended. Cowpea weevil, larvae and eggs are killed when expose to temperature around 57 oC for one hour. The following steps may be followed.
1) Spread straw or dry grass on a level ground.
2) Spread a black polyethylene sheet over the straw. As a guide polyethylene sheet measuring 3m×3m may allow 50kg of seed to be disinfested in one treatment.
3) Spread the cowpea grains uniformly on the plastic material.
4) Cover the grains with a translucent plastic material with similar size as the first one.
5) Fold the edges of the two plastic sheets under and secure with stones.
6) Leave in the sun for at least two hours.
Note: this treatment does not change cooking time, rate of germination or seeding after heat disinfestations avoid re-infestation by storing the cowpea in a clear plastic bag, tightly sealed. Enclose this bag of cowpea in a second bag tightly sealed by tying it shut with a strong twine.
THEN KEEP AWAY FROM RODENTS!!