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Ecology and Epidemiology

Influence of Soil Texture and Temperature on the Motility of Phytophthora cryptogea and P. megasperma Zoospores. J. D. MacDonald, Assistant, Department of Plant Pathology, University of California, Davis, CA 95616; J. M. Duniway, associate professors, Department of Plant Pathology, University of California, Davis, CA 95616. Phytopathology 68:1627-1630. Accepted for publication 9 July 1978. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1627.

Mycelial disks from agar plate cultures of Phytophthora cryptogea and P. megasperma incubated in soil at –150 millibars (mb) matric potential (φ _m) on tension plates formed abundant sporangia in 3–4 days. When soil containing sporangia was wetted to saturation (φ _m = 0) or sporangia from soil were placed in water, equally large numbers of motile zoospores were released. The period of time that P. cryptogea zoospores remained motile before encysting in a saturated coarse sand fraction (>250 μm) of Yolo fine sandy loam was only somewhat less than their period of motility in water. However, the period that they remained motile in a fine sand fraction (38–60 μm) of the same soil was much less than that in the coarse sand or water. The limiting effect of the fine sand on the motile period was not altered when zoospores were released in soil at 12 C rather than 27 C. Compared with zoospores of P. cryptogea, those of P. megasperma appeared to encyst quickly after release in either of the two soil fractions, in distilled water at 12–27 C temperatures, and in solutions of sucrose or polyethylene glycol 300. Thus, these two species appear to have inherent differences in duration of zoospore motility. The rapid encystment of zoospores in fine-textured soils may contribute to the inability of the spores to swim effectively through such soils.