Survival Dynamics of Aphanomyces cochlioides Oospores Exposed to Heat Stress

ABSTRACT To determine how exposure to heat effects their survival, oospores of Aphanomyces cochlioides isolate C22 were exposed in water to 35, 40, 45, or 50 degrees C for prescribed times and then examined for viability. The Weibull model was modified to represent the effects of temperature on survival of oospores. The final fitted model gave lethal doses for 50% of the oospores of 251, 49.8, 9.8, and 1.9 h at 35, 40, 45, and 50 degrees C, respectively. To determine if alternating high and low temperatures resulted in (i) recovery from heat damage during low temperature periods, (ii) increased susceptibility to heat damage, or (iii) if effects of heat damage were cumulative, oospores were examined after each of four 24-h cycles at 45 degrees C for 4 h and 21 degrees C for 20 h. Survival of oospores exposed to alternating high and low temperatures fit the cumulative effects model. Significant variability in heat tolerance among five isolates was observed (P< 0.001) but model parameters successfully accommodated this variability (R(2) = 0.96, P < 0.001). This research shows that under wet conditions, there are predictable patterns to mortality for A. cochlioides oospores exposed to continuous or fluctuating high temperatures.