VIEW ARTICLE

Influence of Soil Environment on the Germinability of Constitutively Dormant Oospores of Pythium ultimum. R. D. Lumsden, Research Plant Pathologist, Soilborne Diseases Laboratory, Plant Protection Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705; W. A. Ayers, Microbiologist, Soilborne Diseases Laboratory, Plant Protection Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705. Phytopathology 65:1101-1107. Accepted for publication 26 April 1975. DOI: 10.1094/Phyto-65-1101.

Dormant, thick-walled oospores of Pythium ultimum were converted to thin-walled oospores after an incubation period of 1-10 weeks on nonsterile soil agar, in soil extract, or on soil saturated with water. Thin-walled, but not thick-walled, oospores germinated within 2 hours when placed on nutrient media or in fresh water, but not in situ. Thin-walled germinable oospores (average wall thickness, 0.53 µm) were readily differentiated from dormant thick-walled oospores (average wall thickness, 2.09 µm) by differences in thickness of the oospore walls and by their stainability with lactofuchsin. Thin-walled oospores, sporangia, and vegetative hyphae lost their viability when rapidly dried, but thick-walled dormant oospores survived rapid drying and remained viable for a least 8 months. Conversion of dormant to germinable oospores was maximum at pH 7.0 in a soil saturated with water at 25 C. Damping-off of snapbean seedlings occurred in soil infested with thin-walled, germinable oospores, but not in soil infested with thick-walled, dormant oospores, until enough time had elapsed for conversion to germinable oospores. The resistant, thick-walled, constitutively dormant oospores and their conversion to germinable oospores in appropriate environments are significant to the survival of the species in soil and the etiology of plant diseases caused by this pathogen.