Environmental Influences on the Passive Survival ofPythium ultimumin Soil

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Abstract
Population densities of germinable propagules (GP) of P. ultimum declined exponentially (death rate) at progressively higher rates at increasingly higher temperatures (9.degree., 15.degree., 21.degree., and 27.degree. C) during the incubation of culture-produced sporangia in raw sandy loam soil. The average death rates of GP at each temperature were about 40% higher at initial matric potentials (.psi.m) of -0.4 bar than at -3 bar. When soil was infested with a mixture of culture-produced oospores and sporangia (7.3:1) the death rates of GP were lower under comparable conditions than when soil was infested with sporangia. However, death rates were higher at 9.degree. or 27.degree. C than at 15.degree. and 21.degree. C, but they were always higher in the soils initially adjusted to -0.4 bar than to -3 bar .psi.m. Differences in death rates of GP between intermediate temperatures (15.degree. and 21.degree. C) and temperature extremes (9.degree. and 27.degree. C) could reflect differences in the effect of temperature on oospore ripening, e.g., higher rates of conversion of dormant oospores to GP at 15.degree. and 21.degree. C would reduce the overall death rates of GP at these temperatures. Yet, when dormant oospores alone were incubated in soil under the same conditions, no obvious direct relationships were found between .psi.m or temperature and oospore conversion to GP. Air-drying and remoistening of soil previously infested with oospores and incubated for 6 mo. usually stimulated increases in the conversion of oospores to GP.