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Time Sequence for Phytoalexin Production in Harosoy and Harosoy 63 Soybeans. J. A. Frank, Research Assistant, Department of Plant Pathology, University of Illinois, Urbana 61801; J. D. Paxton, Assistant Professor, Department of Plant Pathology, University of Illinois, Urbana 61801. Phytopathology 60:315-318. Accepted for publication 15 September 1969. DOI: 10.1094/Phyto-60-315.

Examination of soybean hypocotyls inoculated with Phytophthora megasperma var. sojae indicated that both Harosoy (susceptible) and Harosoy 63 (resistant) soybean plants produced a phytoalexin in response to infection. Within 4 hr after inoculation, phytoalexin production and fungal development is similar in both varieties. Differences in the host-parasite interactions of the two varieties become apparent between 4 and 8 hr after inoculation. Phytoalexin is no longer detectable in Harosoy after 8 hr, and the disease develops, resulting in the collapse of the hypocotyls within 48 hr after inoculation. Phytoalexin production continues in the resistant variety, and after 24 hr the plant cells surrounding the fungus become discolored and fungal invasion is halted. After 72 hr the pathogen is killed, phytoalexin production ceases, and the existing phytoalexin begins to disappear. Therefore, since plant reactions to the pathogen are identical in the first 4 hr after invasion, the reactions responsible for resistance or susceptibility appear to occur between 4-8 hr, when phytoalexin production either increases or declines.