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Influence of Matric and Osmotic Water Potentials and Soil pH on the Activity of Giant Vampyrellid Amoebae. Y. Homma, Plant pathologist, Shikoku National Agricultural Experiment Station, Zentsuji, Kagawa-Ken, 765 Japan; R. J. Cook, research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Washington State University, Pullman 99164. Phytopathology 75:243-246. Accepted for publication 4 September 1984. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/Phyto-75-243.

Perforation of conidia of Cochliobolus sativus by giant vampyrellid amoebae did not occur at soil matric water potentials higher (wetter) than - 10 to - 50 mbars or lower (drier) than - 200 to - 250 mbars. Perforation activity commenced with air entry into the soil, which occurred at about - 50 mbars (or slightly drier) in both a silt and clay loam and at about - 10 and - 25 mbars in the same two soils, respectively, mixed (1:1, v/v) with coarse sand. In liquid culture, amoebae were highly active in 10% soil (Palouse silt loam) extract and in distilled water (0 bars), indicating that the flooding is not directly limiting to their activity. Perforation activity also tended to cease at lower matric potentials in the loams (- 200 to - 250 mbars) than in the loam-sand blends (- 150 mbars), either because too few water-filled pores remained in the soils and blends at these matric potentials, or because the amoebae can no longer maintain adequate turgor at these matric potentials. In soils from 183 fields in Japan, perforation of conidia of C. miyabeanus was maximal in soils at pH 6.5- 7.0 and at electrical conductivity (EC) values for salinity in the range from 400 to 800 μmho/cm, and it was nil in soils at pH 4.0 and having EC values >1,200 μmho/cm (= - 426 mbars osmotic potential). In liquid culture, the trophozoites remained active in distilled water adjusted to about - 450 mbars osmotic potential with KCl, and in 10% soil extract solution adjusted to - 750 to - 800 mbars osmotic potential with either KCl or NaNO3. The different lower limits of osmotic potential for perforation activity in 10% soil extract than in distilled water probably indicate the lower limits at which the trophozoites can maintain turgor in a saline environment. Turgor may be lost and the trophozoites may therefore cease activity at even higher water potentials in nonsaline soils.

Additional keywords: antagonists, biological control, soilborne plant pathogens.