Resistance-Breaking Variants of Cowpea Chlorotic Mottle Virus in Soybean. O. R. Paguio, Departments of Plant Pathology and Agronomy, University of Georgia, Athens 30602. C. W. Kuhn, and H. R. Boerma. Departments of Plant Pathology and Agronomy, University of Georgia, Athens 30602. Plant Dis. 72:768-770. Accepted for publication 22 April 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0768.

Disease reactions and virus concentration levels of seven strains of cowpea chlorotic mottle virus (CCMV) were studied in six soybean genotypes, each with a different disease reaction to the soybean strain (S). Although differences occurred among genotypes, both disease reactions and concentration levels were similar among virus strains within each genotype. In general, variable environmental conditions (time of year, constant 24 or 30 C) did affect disease reactions and levels of virus concentration, but any variations observed were similar among the virus strains. One notable exception was that the bean yellow stipple (BYS) strain produced more virus than other virus strains in some genotypes. Furthermore, at 30 C it caused systemic symptoms and frequently death in genotype Bragg, which developed no systemic symptoms when inoculated with other strains. In subsequent studies, two new CCMV isolates, designated D and N, were derived by passage of the BYS strain through Bragg. Both isolates D and N overcame the hypersensitive-type resistance in genotypes Bragg and Williams. Isolate D caused leaf distortion and severe stunting, while isolate N caused a mild mottle.