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Genotype, Race, Temperature, and Cultivar Effects on Reaction Type of Unwounded Soybean Hypocotyls Inoculated With Zoospores of Phytophthora megasperma f. sp. glycinea. R. I. Buzzell, Soybean breeder, Agriculture Canada, Research Station, Harrow, Ontario N0R 1G0; E. W. B. Ward(2), G. Lazarovits(3), and P. Stössel(4). (2)(3)(4)Principal plant pathologist, assistant plant pathologist, and visiting scientist, Agriculture Canada, Research Centre, University Sub Post Office, London, Ontario, Canada N6A 5B7. Phytopathology 72:801-804. Accepted for publication 20 October 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-801.

Near-isogenic lines of soybean (Glycine max) were inoculated with race 1 or 2 of Phytophthora megasperma f. sp. glycinea (Pmg) by placing a droplet of zoospore suspension on hypocotyls of etiolated seedlings. At 48 hr after inoculation, the Rps1 Rps1, Rps2 Rps2, and Rps5 Rps5 genotypes differed in lesion length, necrosis rating, total glyceollin production, and glyceollin concentration in dissected tissue. Race 2 caused more necrosis, more total glyceollin, and greater localized glyceollin accumulation than did race 1, indicating that it is less compatible than race 1 in the reactions studied and therefore less aggressive. Incubation at 25 C resulted in longer lesions than at 20 C in compatible reactions, but temperature did not significantly affect incompatible reactions. Genetic background had significant effects on necrosis rating and total glyceollin production in compatible reactions and on necrosis rating in incompatible reactions. Thus, the Pmg-soybean interaction is considerably more complex than previous studies have indicated.

Additional keywords: resistance, susceptibility.