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Temperature and Water Stress Effects on Sporangium Viability and Zoospore Discharge in Phytophthora cryptogea and P. megasperma. J. D. MacDonald, Assistant, Department of Plant Pathology, University of California, Davis, CA 95616; J. M. Duniway, Associate Professor, Department of Plant Pathology, University of California, Davis, CA 95616. Phytopathology 68:1449-1455. Accepted for publication 19 May 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1449.

Mycelial disks from agar plate cultures of Phytophthora cryptogea and P. megasperma were incubated in soil at 150 millibars (mb) matric potential (φm) on tension plates where they formed abundant sporangia in 2-4 days. When sporangia in soil were exposed to saturation (φm = 0) conditions for various lengths of time ranging from 10-60 min to initiate zoospore release, and then dried again to 150 mb φm, zoospore release was stopped short of completion. Such interruptions of the discharge process, which lasted 4 or 24 hr, did not affect sporangia adversely; when wetted again to saturation, they completed the release of zoospores and yielded a cumulative release similar to sporangia which were exposed to noninterrupted periods at saturation. Mycelial disks bearing sporangia also were removed from soil at 150 mb φm and dried to various extents by vapor exchange with salt solutions. After drying to 40 bars water potential (φ), many sporangia of P. cryptogea were still capable of releasing zoospores when placed in water and of germinating directly when plated on a selective agar medium. However, after drying to 70 bars φ or less, most sporangia were no longer capable of either direct or indirect germination. Sporangia of P. megasperma appeared to be more sensitive to drying and most were killed by drying to 50 bars φ or less. Sporangia of both species were capable of releasing zoospores in water over a wide range of temperatures, with the upper limits for release being 30 and 33 C for P. megasperma and P. cryptogea, respectively. Sporangia of P. cryptogea underwent a steady decline in capacity for indirect germination on prolonged exposure to temperatures of 33 or 36 C.

Additional keywords: water potential, Phytophthora root rot.