Soil factors affecting carbon loss and pathogenecity of oospores of Pythium aphanidermatum

Using a membrane filter system, oospores of Pythium aphanidermatum were exposed for 80 d to soil of pH 6.9 at 0, −1 or −10 kPa, and 25°C or to soil (−1 kPa) of pH 5 or 8 at 15, 25 or 35°C. Carbon loss and pathogenicity of oospores were recorded during the 80-d period. Oospores lost most endogenous carbon to soil of pH 6.9 at −1 kPa or to soil of pH 8 at 35°C. These soil conditions induced a rapid decline of pathogenicity of oospores for cucumber seedlings. Carbon loss was negatively correlated with the pathogenicity of oospores to cucumbers. In contrast, oospores lost least carbon to soil of pH 6.9 at −10 kPa, or to soil at 15°C regardless of pH. Pathogenicity of oospores to cucumbers under these conditions did not decline, even after 80 d.