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Limiting Influence of Low Water Potential on the Formation of Sporangia by Phytophthora drechsleri in Soil. J. M. Duniway, Department of Plant Pathology, University of California, Davis 95616; Phytopathology 65:1089-1093. Accepted for publication 26 April 1975. DOI: 10.1094/Phyto-65-1089.

The formation of sporangia by Phytophthora drechsleri was examined in soil at water potentials (ψ) between 0 and 150 bars. Soil and mycelial disks were air-dried separately to the desired ψ values and placed together in the chambers of thermocouple psychrometers. The psychrometers were subsequently used to measure and maintain ψ at nearly constant values. Crocks of soil equipped with thermocouple psychrometers were also dried to various ψ values before mycelia were buried in the soil. Sporangia were counted 4-12 days after mycelial disks were placed in soil. Large numbers of sporangia formed at ψ values between 0.3 and 4 bars, whereas few or no sporangia formed at ψ *0 or at ψ values lower than 4 bars. Variation in the period of time mycelial disks were in soil or the use of sterilized as opposed to unsterilized soil did not alter the influence of ψ on the formation of sporangia. Exposure of mycelial disks to ψ values less than 8 bars in soil inhibited subsequent formation of sporangia in water. P. drechsleri remained viable in soil at all ψ values down to 40 bars.