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Electron Microscope Study of Race-Specific and Age-Related Resistant and Susceptible Reactions of Soybeans to Phytophthora megasperma var. sojae. P. Stössel, Agriculture Canada, Research Centre, University Sub Post Office, London, Ontario, Canada, N6A 5B7; G. Lazarovits(2), and E. W. B. Ward(3). (2)(3)Agriculture Canada, Research Centre, University Sub Post Office, London, Ontario, Canada, N6A 5B7. Phytopathology 71:617-623. Accepted for publication 19 November 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-617.

Six-day-old soybean (Glycine max ‘Altona’) hypocotyls were inoculated with zoospores of race 6 or race 4 of Phytophthora megasperma var. sojae either at the top (youngest tissue), which is suceptible to race 6 but resistant to race 4, or at the bottom (older tissue), which is resistant to both races. In all cases a range of host cell responses was observed. These included necrosis of host cells without evidence of fungal invasion and the production of wall appositions with and without fungal invasion and necrosis. In a small number (<5%) of cells in tissue inoculated with race 6, cell invasion occurred without wall apposition formation or necrosis. These were the only instances in which the interaction between individual host cells and hyphae was compatible. Thus, compatibility is either a rare event or one of short duration. Differences between the susceptible reaction to race 6 at the top of the hypocotyl and the resistant reaction to this race at the bottom appeared to lie chiefly in the greater proportion of uninvaded, necrotic cells at the bottom, especially in the epidermis and cell layers immediately beneath. The interactions with hyphae of race 4 were similar to those with race 6 except that no instances of invasion without necrosis were observed. At the bottom of the hypocotyl, hyphae of this race were restricted to the surface cell layers. It is concluded that the majority of hypocotyl cells react in an incompatible manner to hyphae of both races; only at the top of the hypocotyl following inoculation with race 6 is the frequency of incompatible reactions sufficiently reduced or delayed that visibly susceptible symptoms develop.