Decreased germinability and virulence of oospores of Pythium aphanidermatum in relation to loss of endogenous carbon during incubation on soil

Freshly harvested oospores of Pythium aphanidermatum did not germinate in the absence of an external nutrient source (1% of Pfefferʹs salts solution) and germinated poorly in a nutrient medium (corn meal agar, CMA). Oospores became germinable in the Pfefferʹs salts solution and showed increased germinability in CMA after incubation on a membrane-soil system. However, their germinability dropped to minimum level (6 or 11%) when the incubation period was lengthened to 90 days. During this time, their virulence to cucumber declined significantly. The linear depletion of endogenous 14C significantly and negatively correlated with the germinability and virulence of oospores. Oospores of Pythium aphanidermatum lost their endogenous carbon progressively through respiration and exudation during exposure to non-sterile soil over a period of 90 days. Evolution of 14CO2 due to respiration of oospores and soil microbes, and residual 14C in soil comprised the total 14C loss which accounted to 36% of total label. Oospore respiration was the major factor contributing to endogenous C loss (61–100% of total 14C loss), whereas residual 14C in soil and soil microbial respiration contributed 7% and 4% of total label, respectively. High respiratory activity and exudation loss of endogenous C from oospores suggests that these propagules in soil are metabolically active.