Sources of resistance to seed transmission and variation in responses of cowpea varieties to infection by four seed- borne viruses

 S.A. Shoyinka, M.A. Ittah, I. Fawole & J.A. Hughes

Cowpea (Vigna unguiculata L. (Walp)) a major food legume and a versatile crop grown in many regions of the world, is susceptible to more than 20 viral diseases world-wide.  Some of most destructive of these are transmitted through the seed and are thereby disseminated to most cowpea–producing regions of the world.  The most effective control of cowpea viral diseases has been the development of improved genotypes with resistance to viral infection.  A field and screenhouse investigation was conducted at IITA, Ibadan, Nigeria to identify sources of resistance to seed transmission and variation in responses of 15 cowpea varieties to infection by four seed-borne viruses: bean common mosaic potyvirus (blackeye cowpea strain: BCMV-BIC), cowpea aphid-borne mosaic potyvirus (CABMV), cowpea mottle carmovirus (CmeV) and southern bean mosaic sobemovirus (SBMV).  The varieties were mechanically (sap) inoculated at the field at the seedling stage while seeds harvested from such plants were sown in seed trays in an insect-free screenhouse to determine percentage seed transmission of each virus.  Protein – A sandwich Enzyme–linked immunosorbent assay (PAS-ELISA) was used to detect the presence of virus in test seedlings.  Disease severity was scored on a 5-point visual estimation scale.  Infection incidence was the proportion of infected plants to the total number of plants in a plot.  There was considerable variation in rates of seed transmission and disease severity caused by each of the four viruses. CMeV infected significantly more varieties (14) and induced more severe symptoms in them than did BCMV-BIC (13), CABMV (12) or SBMV (7) respectively. Rates of seed transmission also varied depending on the variety/virus combination.  In the susceptible varieties the severity of infection at 6 weeks after inoculation (WAI) was significantly higher than at two WAI (p<0.05).  Conversely, resistant varieties showed significantly less severe disease at 6 WAI than at 2 (p< 0.05).  Disease severity index was not correlated with seed transmissibility of a virus through seeds of an infected variety.  There was significant variation among the varieties in loss of germinability due to infection by the viruses. CMeV reduced percentage germin ability in six varieties, BCMV-BIC in four, CABMV and SBMV in one variety each.  BCMV-BIC induced more significant effects on many of the yield parameters measured such as number of pods per plant, plant height, number of seeds per pod, weight of 100 seeds pod growth and rate and total grain yield compared to CMeV, CABMV and SBMV in that order.  None of the 15 varieties was resistant to all the four seed-borne viruses.  Those identified as being resistant to one or more of the viruses can be used in developing elite lines that are less susceptible to seed–transmission thus encouraging germplasm exchange across international boundaries.