Abstract
The effect of separately inoculating cowpea cultivars, ‘Ife brown” (IF) and “Owode” (OW) with Cowpea aphid-borne mosaic virus (CABMV), Cowpea yellow mosaic virus (CYMV) and Rhizobium IRc 284(RH-284) on nodulation was investigated. Also, the effect of inoculating the cowpea cultivars with RH-284 on the severity of infection caused by the viruses was studied. The interactive effects of inoculating cowpea with RH- 284 and each of CABMV, CYMV, Southern bean mosaic virus (SBMV) and Blackeye cowpea mosaic virus (BICMV) on nodulation, growth and yield of IF and OW were also investigated. The results showed that infection by CYMV and CABMV significantly reduced nodulation by about 20-30% and 40-45% in IF and OW, respectively. Inoculating with RH-284 alone significantly increased nodulation by about 20% in both cowpea cultivars. In the interactive study involving virus-RH 284 inocula, slight but non-significant increases of 22, 2 and 9% in nodule number were observed in IF inoculated with RH-284 and SBMV, CYMV and CABMV, respectively. The differences observed in the nodule, shoot and seed weights were not significantly different from those of the control. There was a negative correlation between nodule number and severity of symptom. BICMV caused the most severe effect on the two cowpea cultivars. It reduced the number of nodules by 55-66% with or without RH-284. It also caused significant reductions of over 80% in nodule and seed weights of OW. In conclusion, increase in nodulation reduced viral disease severity, the slight but non-significant increases observed in the growth and yield parameters suggest that improved nodulation can be advantageous to cowpea. Key words: Blackeye cowpea mosaic virus, cowpea aphid-borne mosaic virus, cowpea yellow mosaic virus, southern bean mosaic virus.  
Highlights
Cowpea is an important food and fodder legume in the sub-humid tropics of Africa
Nine viruses are reported on cowpeas in sub-Saharan Africa (Taiwo, 2003), only Cowpea aphid- borne mosaic virus (CABMV) genus Potyvirus and Cowpea yellow mosaic virus (CYMV) genus Comovirus are considered to be very important as far as geographical distribution, pathogenic variability and yield losses are concerned (Thottappilly and Rossel, 1992)
Cowpea mottle virus (CMeV) genus Carmovirus, Southern bean mosaic (SBMV) genus Sobemovirus, Cowpea golden mosaic (CGMV) genus Begomovirus, Blackeye cowpea mosaic virus (BICMV) genus Potyvirus and Cucumber mosaic viruses (CMV) genus Cucumovirus are considered to be of localized importance (Taiwo, 2003)
Summary
Cowpea is an important food and fodder legume in the sub-humid tropics of Africa. As a food, the grain is an important source of dietary protein especially for the West. Nine viruses are reported on cowpeas in sub-Saharan Africa (Taiwo, 2003), only Cowpea aphid- borne mosaic virus (CABMV) genus Potyvirus and Cowpea yellow mosaic virus (CYMV) genus Comovirus are considered to be very important as far as geographical distribution, pathogenic variability and yield losses are concerned (Thottappilly and Rossel, 1992). Effective cowpea-Rhizobium symbiosis fixes more than 150 kg/ha of N2 and supply a considerable amount of the N2 requirement of plants (Summerfield et al., 1977) This attribute allows healthy growth and optimum yields. If the process is efficient, the plant grows healthily, thereby minimizing the impact of diseases It is expected that the extraneous application of Rhizobium to cowpea at planting should increase its population in the soil, ensure optimal and rapid growth of the crop, and enhance its resistance to phytopathogenic agents, especially viruses. This study was carried out to assess the effect of rhizobial inoculant on growth and seed yield of cowpea, the impact of viral infection on nodulation, vegetative and seed yield of the crop and determine the extent of amelioration of the impact of viral infection on cowpea by rhizobial inoculant
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