Influence of Soil Texture and Temperature on the Motility of Phytophthora cryptogea and P. megasperma Zoospores

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Abstract
Mycelial disks from agar plate cultures of P. cryptogea and P. megasperma incubated in soil at -150 millibars (mb) matic potential (.psi.m) on tension plates formed abundant sporangia in 3-4 days. When soil containing sporangia was wetted to saturation (.psi. = 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 .mu.m) of Yolo fine sandy loam was only somewhat less than their period of motility in water. The period that they remained motile in a fine sand fraction (38-60 .mu.m) 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.degree. C rather than 27.degree. C. Compared with zoospores of P. cryptogea, those of P. megasperma apparently encysted quickly after release in either of the 2 soil fractions, in distilled water at 12-27.degree. C temperatures, and in solutions of sucrose or polyethylene glycol 300. These 2 spp. apparently 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.