Differential Field Infection of Cowpea Genotypes by Southern Bean Mosaic Virus

Hobbs, H. A., and Kuhn, C. W. 1987. Differential field infection of cowpea genotypes by southern bean mosaic virus. Phytopathology 77:136-139. Natural spread of the cowpea strain of southern bean mosaic virus among susceptible lines and between susceptible and resistant lines. (SBMV) from infected cowpea to several cowpea genotypes, including Inoculation by needle pricking showed similar levels of infectibility among susceptible ones and those with different levels of resistance, was studied the susceptible cultivars but a higher virus concentration in California under field conditions. Little spread (0-2%) occurred in three lines with Blackeye. Lateral spread and virus accumulation into an uninoculated leaf different levels of resistance (resistance based on virus concentration and portion were greater in California Blackeye than in Knuckle Purple Hull symptomatology). Disease incidence was about twofold higher in the and Coronet. We conclude that virus concentration and spread within susceptible cultivar California Blackeye than in two other cultivars, plants contribute to differences in field incidence of SBMV because of Knuckle Purple Hull and Coronet, believed to be similarly susceptible. effect on virus acquisition by beetles. Evidence is also presented showing Both older plant age at time of inoculation and lower inoculum that susceptible Coronet and resistant Early Pinkeye are more difficult to concentration caused decreases in virus accumulation and infectibility infect than susceptible California Blackeye and Knuckle Purple Hull. (proportion of plants becoming infected after mechanical inoculation) Resistance in cowpea (Vigna unguiculata (L.) Walp. subsp. explore possible reasons for differences in field incidence involving unguiculata) to the cowpea strain of southern bean mosaic virus these virus-host interactions. SBMV is efficiently transmitted by (SBMV) has been reported (12). In addition to the resistance the bean leaf beetle, Cerotoma trifurcata Forst., from cowpea to related to necrotic local lesions (2), resistance in hosts with cowpea (14,15). Individual beetles can transmit the virus after a nonnecrotic reactions has been associated with reduced virus 24-hr acquisition feeding and can continue for at least 5 days (14). concentration (11), and host genes controlling virus concentration Preliminary results (8) showed a very low incidence of SBMV in have been identified (7). resistant cowpeas and differential levels of incidence among The purpose of this study was to compare field transmission of cowpea cultivars that were believed to be equally susceptible. SBMV in susceptible and nonnecrotic resistant cowpea lines and to MATERIALS AND METHODS The publication costs of this article were defrayed in part by page charge payment. This Field transmission, 1982. Six cowpea genotypes were used in the article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. § experiment: California Blackeye, Coronet, and Knuckle Purple 1734 solely to indicate this fact. Hull (all susceptible to SBMV); Early Pinkeye (moderately © 1987 The American Phytopathological Society resistant); Iron (resistant); and PI 186465 (extremely resistant).