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Factors Affecting Production and Germination of Oospores of Three Pythium Species. W. A. Ayers, Microbiologist, Soilborne Diseases Laboratory, Plant Protection Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705; R. D. Lumsden, Research Plant Pathologist, Soilborne Diseases Laboratory, Plant Protection Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705. Phytopathology 65:1094-1100. Accepted for publication 26 April 1975. DOI: 10.1094/Phyto-65-1094.

Cholesterol stimulated production and maturation of oospores of Pythium aphanidermatum, P. ultimum, and P. myriotylum. P. myriotylum, especially, produced abnormal sexual structures in the absence of sterol. Temperatures of 15-35 C were satisfactory for oospore production by P. aphanidermatum and P. myriotylum, whereas 10-20 C were more favorable for P. ultimum. Nonsterile soil extract induced lysis of mycelial mats of these fungi within 1-2 weeks, leaving oospores free of hyphae. Freshly harvested oospores of P. aphanidermatum germinated (27%) on a nutrient medium within 18 hours. Air-drying or aging for 1-2 weeks in nonsterile soil extract increased germination (80-90%). Oospores of P. ultimum were dormant, at first, but became increasingly germinable with time of exposure to nonsterile soil extract. Germination exceeded 90% after about 6 weeks in soil extract. Oogonia or immature oospores of P. myriotylum, as well as mature oospores, germinated in low numbers (approximately 12%) in 18 hours from fresh cultures. Air-drying reduced germination to < 1%. Isolated single oospores germinated a few at a time over a period of 20 days, reaching a total of about 6%. Germination of dried oospores of P. myriotylum was increased slightly by preincubation of the spores on nonsterile soil agar for 7 weeks. Freezing and thawing, heat shock, and exposure to light and to bean tissue leachates and root exudates or fresh plant tissue did not break the dormancy or stimulate germination of oospores of P. myriotylum.