New method for screening cowpea germ plasm for resistance to Cucumber mosaic virus

Utilizing the Cucumber mosaic virus (CMV)-resistant cowpea germ plasm line, GC-86L-98, a new method of screening for resistance in the greenhouse followed by field screening was developed. A uniform source of CMV inoculum (freeze-dried infected cowpea tissue) was diluted to provide an infection rate in GC-86L-98 similar to that observed under field conditions. Plants of test lines were mechanically inoculated with this standard inoculum rate and assessed under greenhouse conditions. Lines considered equivalent in infection percentage with GC-86L-98 were then evaluated in field tests. Test line plants were exposed to virus from susceptible cultivar Coronet when plants were at the first or second trifoliolate leaf stage, and then leaf samples were assayed by direct antigen coating-enzyme-linked immunosorbent assay. Lines with infection percentages similar to or lower than the resistant control were considered resistant. A new line (PI 441917) with CMV resistance and several lines with Blackeye cowpea mosaic virus resistance were found. Newly discovered CMV-resistant lines will help to control the serious cowpea stunt disease caused by synergism of the two viruses.