DEVELOPMENT OF PURE LINES OF PHYTOPHTHORA-SOJAE RACES

Single-oospore isolates were obtained from field isolates of Phytophthora sojae and from successive selfed generations and were evaluated using a hypocotyl-inoculation method for race phenotype on soybean (Glycine max) cultivars having Rps, Rps1, Rps1-c, Rps1-k, or Rps6 Phytophthora-resistance genes. Several races were found among S1 (first generation) progeny of race 1 and 4; race 3 bred true. Single-oospore isolates unable to kill soybean seedlings also were found and were designated universally avirulent. Among S1 progeny of race 1, the ratio of race 1 to avirulent was 3:1. There were no avirulent isolates among the progeny of race 3 and 4. Single-oospore lines with desired phenotypes were selected for further selfing from among the progeny having the least segregation. One race 1 line bred true for race 1 phenotype in the second generation. A second race 1 line segregated for race 1 and 4 and avirulent in early generations but bred true for the race 1 phenotype by the seventh generation. Avirulent isolates, which did not cause disease on the universally susceptible cultivar Williams, bred true by the third selfed generation. True-breeding race 4 was obtained after the third generation of selfing. This study demonstrated that some field isolates of P. sojae are heterokaryotic and/or heterozygous for virulence genotypes, and pure-breeding races could be developed by repeated selfing of oospore progeny.