CHARACTERIZATION OF UNIVERSALLY AVIRULENT STRAINS OF PHYTOPHTHORA-SOJAE

During the process of selfing race 1 of Phytophthora sojae M. J. Kaufmann & J. W. Gerdemann, a pathogen of soybean (Glycine max (L.) Merr.), a 3:1 segregation ratio of race 1 to avirulence was obtained. Avirulent strains were identical in appearance to races 1, 3, and 4 but induced the hypersensitive response when inoculated on the universally susceptible soybean cultivar Williams. A representative avirulent strain did not cause disease on cultivars with known Rps (resistance to P. sojae) genes or gene combinations, or on 33 plant introductions that were susceptible to race 1. The avirulent strain rotted apples in a manner similar to races 1 and 4 and did not rot soybean pods similar to avirulent races. It induced the hypersensitive response in soybean cotyledons, which then rotted after being placed in flooded conditions. Seventy-eight typical avirulent single-oospore strains of P. sojae and Phytophthora and Pythium spp. induced the hypersensitive response and elicited as much glyceollin in 'Williams' and 'Williams 79' as races 1, 3, and 4 elicited in 'Williams'. Four Phytophthora spp., which caused some seedling rot of soybean, elicited levels of glyceollin as high as races 1 and 3 in 'Williams 79'. It is concluded that the avirulent strains obtained from race 1 are either pathogenic and have host-specificity for soybean or universally avirulent strains that have lost all race-specific virulence genes.