DIFFERENTIAL FIELD INFECTION OF COWPEA GENOTYPES BY SOUTHERN BEAN MOSAIC-VIRUS

Natural spread of the cowpea strain of southern bean mosaic virus (SBMV) from infected cowpea [Vigna unguiculata] to several cowpea genotypes, including susceptible ones and those with different levels of resistance, was studied under field conditions. Little spread (0-2%) occurred in three lines with different levels of resistance (resistance based on virus concentration and symptomatology). Disease incidence was about twofold higher in the susceptible cultivar California Blackeye than in two other cultivars, Knuckle Purple Hull and Coronet, believed to be similarly susceptible. Both older plant age at time of inoculation and lower inoculum concentration caused decreases in virus accumulation and infectibility (proportion of plants becoming infected after mechanical inoculation) among susceptible lines and between susceptible and resistant lines. Inoculation by needle pricking showed similar levels of infectibility among the susceptible cultivars but a higher virus concentration in California Blackeye. Lateral spread and virus accumulation into an uninoculated leaf portion were greater in California Blackeye than in Knuckle Purple Hull and Coronet. We conclude that virus concentration and spread within plants contribute to differences in field incidence of SBMV because of effect on virus acquisition by beetles. Evidence is also presented showing that susceptible Coronet and resistant Early Pinkeye are more difficult to infect than susceptible California Blackeye and Knuckle Purple Hull.