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Age-Related Changes in Specificity and Glyceollin Production in the Hypocotyl Reaction of Soybeans to Phytophthora megasperma var. sojae. G. Lazarovits, Research Station, Agriculture Canada, University Sub P.O. London, Ontario N6A 5B7; R. Stössel(2), and E. W. B. Ward(3). (2)(3)Research Station, Agriculture Canada, University Sub P.O. London, Ontario N6A 5B7. Phytopathology 71:94-97. Accepted for publication 19 June 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-94.

Intact, 6-day-old soybean hypocotyls (Glycine max [L.] Merr.) increased in resistance to inoculation with zoospores of Phytophthora megasperma Drechs. var. sojae Hildeb. (Pms) from the top (youngest part) to the bottom (oldest part). The interaction with the compatible race at the top of the hypocotyl, was characterized by watersoaking and unrestricted tissue colonization with low glyceollin production whereas at the middle and bottom of the hypocotyl, the interaction was necrotic and glyceollin production was increased. This change in host response corresponded to a difference in tissue age of 1–2 days. The incompatible interaction, which typically was necrotic with high glyceollin production at the top of the hypocotyl, became even more incompatible lower down, where there was a reduction in both necrosis and glyceollin production. With cultivar Altona and Pms races 4 (incompatible) and 6 (compatible) the numbers of appressoria formed and penetrations were similar regardless of race or inoculation site. Similarities in symptoms and in the time-course of glyceollin production suggested that similar mechanisms govern the incompatible response whether it occurred on the incompatible host in the normal way, or on the compatible host at resistant-responding sites on the lower parts of the hypocotyl.