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

Using a membrane filter system, oospores of Pythium aphanidermatum exposed for 80 d to soil of pH 6.9 at 0, -1 or -10 kPa, and 25 degrees C or to soil (-1 kPa) of pH 5 or 8 at 15, 25 or 35 degrees C. Carbon loss and pathogeni city of oospores were recorded during the 80-d period. Oospores lost most e ndogenous carbon to soil of pH 6.9 at -1 kPa or tu soil of pH 8 Lit 35 degr ees C. These soil conditions induced a rapid decline of pathogenicity of oo spores for cucumber seedlings. Carbon loss was negatively correlated with t he 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 degrees C regardless of pH. Pathogenicity of oospores to cucumbers under these conditions did n ot decline, even after 80 d. (C) 2000 Elsevier Science Ltd. All rights rese rved.