CRY, THE RESISTANCE LOCUS OF COWPEA TO CUCUMBER MOSAIC-VIRUS STRAIN-Y

Most cowpea cultivars are resistant to infection by cucumber mosaic vi rus strain Y (CMV-Y) that causes a hypersensitive reaction followed by the development of necrotic local lesions. It has been anticipated th at resistance to CMV-Y depends upon the existence of a resistance (R) gene in cowpea. To define the nature of the R gene, we screened 38 cow pea cultivars to isolate susceptible ones. The cultivar PI 189375 was identified as susceptible based on symptoms and the presence of progen y virions on noninoculated upper leaves. Protoplasts prepared from bot h 'PI 189375' and the resistant cultivar Kurodane-Sanjaku supported vi rus multiplication. This result suggested that there was no difference between the capacities for virus multiplication in both cultivars at a single-cell level, implying an expression of the specific R gene in intact 'Kurodane-Sanjaku' plants. F-1 and F-2 populations were then co nstructed by reciprocal crosses between 'Kurodane-Sanjaku' and 'PI 189 375'. Reactions of F-1 and F-2 populations to CMV-Y inoculation demons trated that the R gene was inherited as a single dominant gene. We des ignated this resistance locus as Cry (cowpea R gene to CMV-Y).