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The Effect of Matric and Osmotic Potential of Soil on Phytophthora Root Disease of Persea indica. R. E. Sterne, Graduate Research Assistant, Department of Plant Pathology, University of California, Riverside, CA 92521, Present address of senior author: Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701; G. A. Zentmyer(2), and M. R. Kaufmann(3). (2)Professor, Department of Plant Pathology, University of California, Riverside, CA 92521; (3)Associate Professor, Department of Plant Sciences, University of California, Riverside, CA 92521. Phytopathology 67:1491-1494. Accepted for publication 24 May 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1491.

The influence of the matric and osmotic potential of soil on root disease caused by chlamydospores of Phytophthora cinnamomi was examined by growing seedlings of Persea indica (a species related to commercial avocado) in infested soil with different combinations of matric and osmotic potential. Matric potential in a range from 0 to 0.25 bar was controlled by ceramic tension plates. Osmotic potential of soil was adjusted with NaCl or CaCl2 and was in a range (0.37 to 2.37 bar) found in the saline soils in avocado groves in California where root rot caused by P. cinnamomi is a problem. Disease was rated by determining the portion of the root system of P. indica seedlings with black lesions, and infection was verified by plating roots on P10 VP medium. Over the range of potentials tested, disease severity was a function of matric potential but not of osmotic potential. The percentage of diseased roots averaged 80 to 90%, 50 to 90%, and 10 to 50% at matric potentials of zero, 0.05, and 0.10 bar, respectively. At 0.25 bar, only a few lesions occurred. Disease was not significantly different over the range of osmotic potentials imposed with these respective matric potentials.