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Water Potential Relations of Three Root-infecting Phytophthora Species. L. E. Sommers, Graduate Student, Department of Soils, University of Wisconsin, Madison 53706; R. F. Harris(2), F. N. Dalton(3), and W. R. Gardner(4). (2)(3)(4)Associate Professor, Graduate Student, and Professor, respectively, Department of Soils, University of Wisconsin, Madison 53706. Phytopathology 60:932-934. Accepted for publication 5 January 1970. DOI: 10.1094/Phyto-60-932.

The effect of nutritional status and mode of water potential control on the susceptibility to water stress of three soil-borne Phytophthora species was investigated. The osmotic water potential relations of Phytophthora cinnamomi, P. megasperma var. sojae, and P. parasitica were relatively independent of the inorganic solute (KCl or salts mixture) used for water potential control of a low organic nutrient agar medium: Optimum growth of P. cinnamomi occurred at 10 to 15 bars; P. megasperma var. sojae growth rate declined steeply with decreasing potential, and growth ceased at about 30 bars; although P. parasitica growth also declined with decreasing potential, some growth occurred at 50 bars. Substitution of sucrose to control the water potential of the basal medium or the use of nutrient-rich media increased the growth rates at all water potential levels. The nutrient-water potential interaction was particularly pronounced for P. parasitica; on V-8 juice media, no appreciable water stress occurred before 15 bars, and vigorous growth was evident at 40 bars. Phytophthora cinnamomi showed a marked growth rate reduction and susceptibility to water stress on vapor pressure-controlled as compared to osmotic-controlled agar media; it is probable that this growth response reflected solute transport as well as water potential properties of the media.