Cowpea (Vigna unguiculata, L Walp) is a leguminous crop indigenous to Africa and a dicotyledonous plant belonging to the family Fabaceae (Schippers, 2002). It can be grown as sole crop, but more often planted alongside crops such as sorghum, maize or millet (Agbogidi, 2010a). More than 5.4 million tonnes of cowpeas are produced worldwide, with Africa being the highest producer with nearly 5.3 million tonnes annually (IITA, 2010). Nigeria, as the largest producer and consumer of cowpea, accounts for 61 % of production in Africa and 58 % worldwide, out of which Nigeria produced 5.1 million tonnes under cultivation area of 3.59 million / ha (NAERLS & FDAE, 2013).
Cowpea is one of the oldest crops known to man and an important food legume because of its nutritional value to man and usefulness as fodder to livestock (Agbogidi, 2010b; Davis et al., 1991). Its ability to replenish soil nitrogen makes it important in the modern crop farming system in rotation with other crops (Langyintuo et al., 2003). Also, its drought tolerance, relatively early maturity and nitrogen fixation characteristics fit very well to the tropical (humid) soils where moisture, erosion and low soil fertility is the major limiting factor in crop production (Hall, 2004). It serves as food security and at the same time can be combined with other recipe (Muoneke et al. 2012).
The average yields of cowpea are generally low (Benue State Agricultural and Rural Development Authority (BNARDA, 2009). This can be attributed to many factors amongst which are drought, low soil fertility, parasitic weeds, insect pests, and diseases (Olufajo and Singh, 2002; Niringiye et al., 2005; Fawole et al., 2006). The major insect pests of cowpea include: the legume pod borer, Maruca vitrata Fabricius; the coreid pod-bugs, Clavigralla tomentosicollis Stal; the groundnut aphid, Aphis craccivora Koch; and thrips, Megalurothrips sjostedti Trybom and Sericothrips occipitalis Hood (Jackai, 1995; Tanzubil, 2000; Malgwi and Onu, 2004). Opareke et al. (2000) estimated yield loss in cowpea due to insect pests to be above 80%. Cowpea is also attacked by parasitic flowering plants such as Striga gesnerioides which causes important yield losses of between 30-80% (Muleba et al., 1997); the nematode Meloidogyne spp causes cowpea root knot (Sikora et al., 2005). The disease induced by fungi and bacteria include cowpea scab, Elsinoe phaseoli; cowpea wilt, Fusarium oxysporium f. sp. Lycopersici; and cowpea bacterial blight, Xanthomonas vignicola among others(Emechebe and Lagoke, 2002).
Scab, induced by Elsinoe phaseoli is a seed-borne disease that affects all above ground parts of the plant (leaves, petioles, stems, peduncles and pods) at all stages of growth (Emechebe, 1980; Iceduna, 1993; Mbong et al., 2010b), with yield losses of up to 80% and even total crop destruction under severe conditions reported in Nigeria (Emechebe, 1980; Mungo et al., 1995). Low seed viability and vigour in plants have been attributed to seed-borne fungi (Elias et al., 2004); infected seeds may fail to germinate, or transmit disease to the seedling and to the growing plant (Fakir et al., 2002). Nabakka (1997) reported germination reduction of up to 59.3% on cowpea seeds which was as a result of seed-borne fungi infection; Mew et al. (1994) reported that the quality of seeds account for at least 5–20% increase in productivity.