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The Effect of Osmotic Potential and Specific Ions on Growth of Phytophthora cinnamomi. R. E. Sterne, Graduate Research Assistant, Department of Plant Pathology, University of California, Riverside 92502; G. A. Zentmyer(2), and F. T. Bingham(3). (2)Professor, Department of Plant Pathology, University of California, Riverside 92502; (3)Professor, Department of Soil Science and Agricultural Engineering, University of California, Riverside 92502. Phytopathology 66:1398-1402. Accepted for publication 7 June 1976. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1398.

To establish the response of the root pathogen Phytophthora cinnamomi to osmotic potential, growth of the fungus was studied in a liquid minimal medium amended with eight osmotica. The isolate used in the experiments had a high radial growth rate on agar, 196 µm/hour, but a low specific growth rate in liquid culture, 0.027/hour. The amount of growth reduction varied with the osmoticum. Common osmotica like NaCl and KCl reduced growth 50% at approximately 9 bars. Comparable reductions by CaCl2 and MgCl2 were at 11 and 16 bars, respectively, and MgSO4 at 4 bars. Osmotic solutions of polyethylene glycol 6000 caused 50% reductions in growth at 13.3 bars. The relative activity of Na+ in NaCl that reduced growth 50% was 0.092. All other cations affected growth at lower activities; Mg2+ in MgSO4 was most toxic with 50% reduction at 0.009. Polyethylene glycol solutions acted like NaCl and KCl and these three osmotica influenced growth only as they altered osmotic potential. Collectively, the data argue for specific ion toxicities rather than a general osmotic effect of some salt solutions on growth. The osmotic potential that severely limited colonial radial growth rate of P. cinnamomi on agar media was twice as low as that that severely limited specific growth rate in liquid media. The data with solid media agree with responses previously reported and illustrate that culture methods directly affect osmotic potential responses of P. cinnamomi.