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In Planta Formation of Heterokaryons of Phytophthora megasperma f. sp. glycinea. A. C. Layton, Department of Biochemistry, Purdue University, West Lafayette, IN 47907, Current address: Department of Microbiology, Center for Environmental Biotechnology, University of Tennessee, Knoxville 37932-2567; D. N. Kuhn, Department of Biochemistry, Purdue University, West Lafayette, IN 47907. Phytopathology 80:602-606. Accepted for publication 29 December 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-602.

The generation of heterokaryons in planta could represent a mechanism to account for the high rate of occurrence of new virulent races of Phytophthora megasperma f. sp. glycinea. Zoospores of drug-resistant mutants of P. m. glycinea races 1 and 3 were inoculated on a susceptible soybean cultivar (Williams), and fungal colonies were reisolated onto plates selecting for double drug resistance. One heterokaryon was reisolated from a mixed inoculation with P. m. glycinea race 1 metalaxyl resistant and race 1 fluorotryptophan-resistant zoospores. This colony grew on plates containing both metalaxyl and fluorotryptophan and gave rise to zoospores of either metalaxyl or fluorotryptophan resistance. The drug-resistant mutants were also used to investigate the effects of simultaneous or sequential inoculation of both races (one metalaxyl resistant and one fluorotryptophan resistant) and competition between races early in infection. Two virulent races were inoculated on a susceptible cultivar (Williams), and a virulent (fluorotryptophan-resistant) and an avirulent (metalaxyl-resistant) race were inoculated on cultivar Harosoy 63. P. m. glycinea was reisolated from the inoculated roots and characterized for drug resistance. When susceptible roots were sequentially inoculated, the race that was inoculated first was reisolated more frequently. When resistant roots were inoculated with an avirulent race 2 hr before inoculation with the virulent race, the number of roots from which P. m. glycinea could be reisolated was reduced. The importance of the response of the susceptible and resistant cultivars to sequential inoculation is discussed.