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Genetics

Development of Pure Lines of Phytophthora sojae Races. R. G. Bhat, former graduate research associate, Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691; B. A. McBlain(2), and A. F. Schmitthenner(3). (2)former assistant professor, Department of Agronomy, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691; (3)professor, Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691. Phytopathology 83:473-477. Accepted for publication 14 January 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-473.

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.