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Saprophytic Development of Pythium ultimum in Soil as a Function of Water Matric Potential and Temperature. R. Lifshitz, Graduate research assistant, Department of Plant Pathology, University of California, Berkeley 94720; J. G. Hancock, professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 73:257-261. Accepted for publication 12 July 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-257.

The population densities of Pythium ultimum increased markedly in natural soils amended, under laboratory conditions, with fragments (about 1 mm2) of mature (green) dried cotton leaves. The population density increases during a 5-day incubation period were related directly to initial population densities of up to 100 propagules per gram of soil at 21 C. Population density increases were also directly related to soil water matric potentials between - 5 and - 0.25 bar at 16, 21, and 27 C. Population densities of P. ultimum were not significantly increased in water-saturated soils or when the matric potential was - 10 bars or lower. In most of the natural soils, the optimal temperature for saprophytic development was lower than 27 C. When soils were previously sterilized, however, it was between 27 and 30 C, and this was similar to the range of temperatures at which maximum linear growth occurred in culture. Antagonistic microflora may play a key role in limiting the activities of P. ultimum in raw soil at temperatures between 21 and 30 C and thus may shift the optimum for growth and development to less than 27 C.