Enzyme-linked immunosorbent assay (ELISA) was used to detect viruses in 649 cowpea leaf samples collected from115 fields during a 3-year survey carried out from 1991 to 1993 throughout all agroecological zones in Nigeria. Six viruses, cowpea aphid-borne mosaic potyvirus (CAMV), blackeye cowpea mosaic potyvirus (BICMV), southern bean mosaic sobemovirus (SBMV), cowpea mottle carmovirus (CMoV), cowpea (yellow) mosaic comovirus (CpMV) and the cowpea strain of cucumber mosaic cucumovirus (CMV-CS) were detected in 390 out of the 649 samples assayed. There was considerable variation in both disease incidence and virus distribution in the different crop habitats within and between seasons. Thus, while SBMV had the highest incidence (27× 4%) and was the most prevalent (66× 7% in 1991, CAMV had the highest incidences (11× 3% and 28× 8%, respectively) and was the most prevalent 33% and 68%, respectively) in 1992 and 1993. CpMV and CMoV remained consistently moderate both in incidence and prevalence throughout the duration of the survey. The detection of the three beetletransmitted spherical viruses (SBMV, CpMV and CMoV) in the Sudan and Sahel Savanna indicates that there is no ecological restriction to the distribution of any ofthe viruses. Incidence ofmultiple infection of up to four and five viruses in individual samples was observed, but mixed infection with two viruses was more prevalent (13%). The potential for horizontal resistance breeding based on common vector transmission is discussed.
SummaryA yam potyvirus was isolated from Dioscorea alata samples collected in Nigeria. The virus was not transmissible mechanically but was transmitted by Aphis craccivora to four cowpea cultivars (Ife Brown, IT84S‐2114, IT82E‐10 and TVu2657), and from which it could be mechanically transmitted between the cowpea cultivars. In infectivity‐ tests using cowpea extracts, the virus had a dilution end point of 10‐4, a thermal inactivation point of 60–65°C and longevity in vitro of 2 days at room temperature. The virus coat protein had an estimated molecular weight of 32 100 daltons. The virus was identified as an isolate of Dioscorea alata virus (DAV; syn. yam virus 1) due to its biological characteristics and its serological reaction with antiserum raised against DAV. The virus is not related to yam mosaic virus, but distantly related to blackeye cowpea mosaic virus and cowpea aphid‐borne mosaic virus.
The responses of 24 white yam ( Dioscorea rotundata ) cultivars to mechanical and vector transmission with each of three viruses infecting yams were assessed through enzyme-linked immunosorbent assay (ELISA) and symptom development. The viruses were Dioscorea alata virus (DAV), genus Potyvirus ; Dioscorea alata bacilliform virus (DaBV), genus Badnavirus ; and Cucumber mosaic virus (CMV), genus Cucumovirus . Only TDr 95-128, a landrace cultivar from Nigeria, developed symptoms of infection with CMV and DaBV following mechanical and vector transmission, respectively. PAS-ELISA showed that nine genotypes remained uninfected by DAV and 11 were uninfected by CMV or DaBV. Genotypes TDr 747 and TDr 1640 both showed resistance to all three viruses.
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