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Ecology and Epidemiology

Effect of Soil Matric Potential on the Formation and Indirect Germination of Sporangia of Phytophthora parasitica, P. capsici, and P. cryptogea. E. A. Bernhardt, Graduate research assistant, Department of Plant Pathology, University of California, Davis 95616; R. G. Grogan, professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 72:507-511. Accepted for publication 27 July 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-507.

Mycelial disks of Phytophthora parasitica and P. capsici incubated in soil at a constant matric potential (φm) of 300 millibars (mb) formed abundant sporangia within 24 hr, but P. cryptogea required 4 days. Both P. capsici and P. parasitica did not form sporangia in saturated soil, unless first incubated for 2 days at 200 to 300 mb φm. The effect of φm on zoospore discharge (indirect germination) by sporangia of P. parasitica was determined by changing φm from 300 mb, at which 13% of the sporangia had germinated indirectly after 6 days, to 0, 10, 25, 50, and 100 mb. Within 24 hr, a significant increase to 40% indirect germination occurred, but only if φm was adjusted to 0 mb. Zoospore discharge by sporangia formed in soils of different textures was more closely related to changes in φm than to soil water content. Sporangia of P. parasitica consistently germinated indirectly less frequently in saturated soil than did those of either P. cryptogea or P. capsici. Indirect germination by sporangia of P. parasitica was increased by longer periods of incubation in soil at 300 mb φm prior to saturation. Sporangia were formed and zoospores were released by P. parasitica at temperatures from 15 to 33 C, and in PEG 6000 solutions at 4.6 bars solute potential.

Additional keywords: buckeye rot, tomato (Lycopersicon esculentum).