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The Effect of Temperature on the Pathogenicity of Pythium aphanidermatum, P. debaryanum, and P. ultimum on Soybean. T. B. Thomson, Former Research Assistant, Department of Botany and Plant Pathology, Purdue University, Lafayette, Indiana 47907; K. L. Athow(2), and F. A. Laviolette(3). (2)(3)Professor, and Assistant Professor, Department of Botany and Plant Pathology, Purdue University, Lafayette, Indiana 47907. Phytopathology 61:933-935. Accepted for publication 4 March 1971. DOI: 10.1094/Phyto-61-933.

Pythium aphanidermatum, P. debaryanum, and P. ultimum were all pathogenic on soybeans. Three methods of inoculation were used: (i) placing a small piece of mycelium in an incision in the hypocotyl; (ii) pouring macerated mycelium over seed in the soil; and (iii) placing seeds on agar overgrown with the fungi. All three techniques led to infection except when inoculum of P. aphanidermatum was poured over seeds in soil. Temperature has a differential effect on severity of disease incited by these fungi. Pythium aphanidermatum infected soybeans between 24 and 36 C, while P. debaryanum and P. ultimum were more virulent at 15 to 20 C or after preconditioning at 4, 8, or 12 C more than 4 days.