Germinability, Viability, and Virulence of Chlamydospores ofFusarium solanif. sp.phaseolias Affected by the Loss of Endogenous Carbon

Chlamydospores of Fusarium solani f. sp. phaseoli progressively lost their endogenous 14 C during incubation on membrane filters on soil, maintained at -1 kPa for 70 days. The endogenous 14 C loss from labeled chlamydospores was comprised of residual 14 C in soil and evolved 14 CO 2 . Residual 14 C in soil ranged from 0.2 to 9.6% of the total label. 14 CO 2 evolution from the respiration of chlamydospores and soil microbes accounted for 53.4% of the total label. Soil microbial respiration of exudates of chlamydospores accounted for 15 to 38% of the total residual 14 C (2 to 5% of the total label). Chlamydospore respiration represented 55 to 78% of the total 14 C loss and was the major factor contributing to the loss of endogenous carbon. Chlamydospores were highly germinable (above 90%) in the absence of an external source of nutrients (Pfeffer's salts solution) when their incubation period on nonsterile soil was 10 days or less, but germinability began to decrease when their exposure to soil exceeded 10 days. Germinability of chlamydospores dropped to zero and virulence significantly declined after incubation on nonsterile soil for 70 days. The linear depletion of 14 C was closely associated with the decline in germinability and virulence of chlamydospores. These results suggest that endogenous nutrient reserves are of importance in establishing a pathogenic relationship between a host and a soil-inhabiting pathogen.