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Influence of the Matric and Osmotic Components of Water Potential on Zoospore Discharge in Phytophthora. 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 68:751-757. Accepted for publication 24 October 1977. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-751.

Mycelial disks from agar plate cultures of Phytophthora cryptogea and P. megasperma incubated in soil at 150 millibars (mb) matric potential (φm) on tension plates formed abundant sporangia within 3-4 days. The effect of φm on zoospore discharge was then determined by changing φm from 150 mb, where sporangia failed to release zoospores, to 0, 1, 5, 10, or 25 mb φm. Sporangia typically discharged large numbers of zoospores within 60 to 90 min in completely saturated soil (φm = 0) and at 1 mb φm. Discharge was impaired at 5 mb, greatly restricted at 10 mb, and fully prevented at 25 mb φm. Similar results were obtained with sporangia formed at 50, 150, or 300 mb. Use of different textured fractions of soil revealed that discharge is governed more by φm than by soil water content. Shifts in temperature between 16 and 24 C failed to induce zoospore discharge at limiting φm values. The influence of osmotica on zoospore discharge was evaluated by removing mycelial disks bearing sporangia from soil at φm = 150 mb and placing them in solutions of known solute potential (φs). Zoospores were discharged in solutions of KCl and MgSO4 at φs > 4.5 bars and in solutions of sucrose, NaCl, sea salts, and polyethylene glycol (PEG) 300 with φs values as low as 6 to 9 bars. Discharge in PEG 6000 occurred only in solutions of φs > 1.3 bars. These results indicate that zoospore discharge is extremely sensitive to the φm component relative to the φs component of soil water potential. Possible mechanisms of zoospore discharge are discussed.