Effect of Soil Matric Potential on the Formation and Indirect Germination of Sporangia ofPhytophthora parasitica, P. capsici,andP. cryptogea

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Abstract
Mycelial disks of P. parasitica and P. capsici [root and crown rot pathogens of tomato, Lycopersicon esculentum] incubated in soil at a constant matric potential (.psi.m) of -300 millibars (mb) formed abundant sporangia within 24 h, 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 .psi.m. The effect of .psi.m on zoospore discharge (indirect germination) by sporangia of P. parasitica was determined by changing .psi.m 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 h, a significant increase to 40% indirect germination occurred, hut only if .psi.m was adjusted to 0 mb. Zoospore discharge by sporangia formed in soils of different textures was more closely related to changes in .psi.m 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 .psi.m prior to saturation. Sporangia were formed and zoospores were released by P. parasitica at temperatures from 15 to 33.degree. C, and in PEG [polyethylene glycol] 6000 solutions at -4.6 bars solute potential.