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Temperature-Induced Changes in Specificity in the Interaction of Soybeans with Phytophthora megasperma f. sp. glycinea. E. W. B. Ward, Principal, Research Centre, Agriculture Canada, University Sub Post Office, London, Ontario N6A 5B7; G. Lazarovits, assistant plant pathologists, Research Centre, Agriculture Canada, University Sub Post Office, London, Ontario N6A 5B7. Phytopathology 72:826-830. Accepted for publication 2 November 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-826.

The optimum temperature for growth of Phytophthora megasperma f. sp. glycinea races 4 and 6 in vitro was 27.5 C; there was little growth at 35 C and none at 37.5 C. Growth of cultures preincubated at 25 C was slowed or arrested by exposure to 37.5 C but resumed at 25 C after a lag, except when the period of exposure was extended to 48 hr. Etiolated soybean hypocotyls (cultivar Altona) inoculated with zoospores of race 4 (incompatible) or race 6 (compatible) were incubated at temperatures ranging from 15 to 37.5 C. At temperatures above 27.5 C, seedlings became increasingly susceptible to race 4 and at 32.5 C the responses (lesion size, necrosis, and glyceollin production) were indistinguishable from the responses to the compatible race 6. Susceptibility to race 4 could be induced by temperature elevation following initial periods of incubation at 25 C for up to 8 hr, but not 12 hr, after inoculation. A period of 812 hr at the elevated temperature was required for the induction of susceptibility. With continuous incubation above 35 C, hypocotyls appeared resistant to both races and developed restricted necrotic lesions with only light brown flecking at 37.5 C. Inoculated seedlings incubated first for 28 hr at 25 C and then exposed to 37.5 C for 412 hr developed intense brown necrotic lesions and high glyceollin levels in response to both races. After returning to 25 C, plants inoculated with race 6 became susceptible, despite theoretically inhibitory glyceollin concentrations, and developed spreading lesions. Plants inoculated with race 4 remained resistant. The results indicate that even high glyceollin concentrations may not always be inhibitory in vivo and suggest that further study is required to determine the significance of glyceollin in disease resistance.