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Physiology and Biochemistry

Similarities Between Age-Related and Race-Specific Resistance of Soybean Hypocotyls to Phytophthora megasperma var. sojae. E. W. B. Ward, Research Institute, Agriculture Canada, University Sub Post Office, London, Ontario N6A 5B7; P. Stössel(2), and G. Lazarovits(3). (2)(3)Research Institute, Agriculture Canada, University Sub Post Office, London, Ontario N6A 5B7. Phytopathology 71:504-508. Accepted for publication 1 October 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-504.

The youngest tissue at the tops of intact 6-day-old soybean (Glycine max ‘Altona’) hypocotyls displays typical race-specific responses to inoculation with zoospores of Phytophthora megasperma Drechs. var. sojae race 6 (compatible) and race 4 (incompatible). However, the tissue becomes increasingly resistant as it matures so that sites at the bottom of the hypocotyl are incompatible to both races. Incompatibility in all race and site combinations was overcome temporarily by heat treatment of the hypocotyl at 42.5 C or preinoculation wiping of the hypocotyl surface with a cotton swab soaked with organic solvents. Duration of the heat treatment (20, 40, or 60 min) needed to overcome resistance was proportional to the incompatibility of the race-site combination, suggesting that differences were quantitative. Partial elimination of resistance frequently was accompanied by increases in necrosis and glyceollin production, presumably due to more extensive tissue colonization. Glyceollin production correlated closely with necrosis. Sensitivity to an elicitor preparation from Pms culture filtrate and to localized freezing injury, as measured by glyceollin production and necrosis, was greater in mature tissue. Our results indicate that race-specific and age-related resistance are basically similar, that differences between resistance and susceptibility are quantitative, and that the balance between the two generally favors resistance.