1. Introduction
Cowpea [Vignaunguiculata (L.) Walp, Fabaceae (2n = 22)] is an important food legume and an essential component of cropping systems in the drier regions of the tropics 1. At least 12.5 million hectares of cowpea are cultivated with annual production of over 3 million metric tons worldwide 1. The largest production is in Africa, with Nigeria and Niger predominating, while Brazil, Haiti, India, Myanmar, Sri Lanka, Australia, the U.S., Bosnia, and Herzegovina all have significant production. Worldwide area of production of cowpeas is approximately 10.1 million hectares. More than 5.4 million tons of dried cowpeas are produced worldwide, with Africa producing nearly 5.2 million. Nigeria, the largest producer, and consumer accounts for 61% of production in Africa and 58% worldwide. As many as a thirds of the world’s people do not meet their physical and intellectual potential because of vitamin and mineral deficiencies 2. In Africa, it is estimated that over 200 million people consume cowpea daily 3. Cowpea is consumed in many forms: the young leaves, green pods, and green seeds are used as vegetables; dry seeds are used in various food preparations, and the hulls are fed to livestock as a nutritious supplement to cereal fodder. In West and Central Africa, cowpea is of major importance to the livelihoods of millions of people providing nourishment and an opportunity to generate income. Trading fresh produce and processed food and snacks provide rural and urban women with the opportunity for earning cash income and, as a major source of protein, minerals, and vitamins in daily diets. Also, it positively impacts on the health of women and children. The bulk of the diet of rural and urban poor Africa consists of starchy food made from cassava, yam, plantain and banana, millet, sorghum, and maize. The addition of an even small amount of cowpea ensures the nutritional balance of the diet and enhances the protein quality by the synergistic effect of high protein and high lysine from cowpea and high methionine and high energy from the cereals 4. In spite of the great importance of this crop to the people where it is grown, its productivity is constrained by a variety of biotic and abiotic factors. Insects, fungi, bacteria, parasitic plants, and nematodes are the major biotic stresses, and drought, salinity, and heat are among the major environmental limitations to cowpea productivity 5, 6. Besides fungal and viral diseases, cowpea bacterial blight (CoBB) and pustules caused byXanthomonasaxonopodispv. vignicola (Xav) 7 formerly X. campestrispv. Vignicola 8 is the most important disease of cowpea. CoBB is prevalent in all major cowpea growing areas of the world 9, 10 causing severe grain yield loss of more than 64% in some areas of West Africa 11. When highly susceptible cultivars are sown the crop may even be completely destroyed 12. The symptoms of CoBB appear as tiny, water-soaked, translucent spots, which are more clearly visible from the abaxial surface of the leaves 13. The spots enlarge, coalesce and develop to big necrotic spots, usually with a yellow halo, leading to premature leaf drop. The pathogen also invades the stem causing cracking with brown stripes. Pod infection appears as dark green water-soaked areas, from where the pathogen enters the seeds and causes discoloration and shrivelling 11. Cowpea bacterial blight (CoBB) is seed-borne 11 and the pathogen can be spread by wind-driven rain and insects 14 but also crop debris and weeds can play a role as inoculum sources 15.
Bacterial blight caused by Xanthomonasaxonopodispv. Vignicolais an important and widespread disease of cowpea (VignaunguiculataL. Walp) in many tropical and subtropical countries 16, 17, 18, 19, 20, 21. The pathogen is seed-borne and seed transmitted Shekhawat & Patel, (1977) and causes discoloration of seeds and cotyledons, seedling mortality, stem cankers, bushy and stunted growth, leaf and pod blight 18. The resultant attack causes a reduction in yield and lowers the quality of seeds 19, 22, 23. Losses range from 2.66 to 92.24% according to cultivar and stage of infection 17. One of the major goals of cowpea breeding and improvement programs is to combine resistance to numerous pests and diseases and other desirable agronomic traits, such as those governing maturity, photoperiod sensitivity, plant type, and seed quality 18. There is, therefore, the need for progressive work done to eradicate or reduce the effect of this problem. Therefore the aim of the study was to investigate Cowpea genotype that are resistant to bacterial blight.
