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The Virulence of Interracial Heterokaryons of Phytophthora megasperma f. sp. glycinea. A. C. Layton, Graduate research assistant, Biochemistry Department, Purdue University, West Lafayette, IN 47907; D. N. Kuhn, Assistant professor, Biochemistry Department, Purdue University, West Lafayette, IN 47907. Phytopathology 78:961-966. Accepted for publication 15 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-961.

In soybean, race-specific resistance to Phytophthora megasperma f. sp. glycinea is inherited as single dominant genes. However, the inheritance of virulence (cultivar-specific pathogenicity) in the fungal pathogen is unknown. Flor’s gene-for-gene hypothesis predicts that avirulence will be phenotypically dominant to virulence. To determine if avirulence is dominant to virulence, P. m. glycinea heterokaryons containing race 1 and race 3 nuclei were tested on soybean cultivars Harasoy and Harasoy 63 for virulence phenotype. Twenty-two out of 26 heterokaryons had a race 1 phenotype, suggesting that race 1 (avirulent on Harasoy 63) is phenotypically dominant to race 3 (virulent on Harasoy 63). In addition, from two heterokaryons with race 1 phenotypes, zoospores with race 3 phenotypes were obtained. These results fit Flor’s gene-for-gene hypothesis and strengthen the utility of the P. m. glycinea-soybean disease interaction as a model system for studying race-specific resistance in soybean.