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Ecology and Epidemiology

The Influence of Matric Potential, Soil Texture, and Soil Amendment on Root Disease Caused by Phytophthora cinnamomi. R. E. Sterne, Graduate Research Assistant, Department of Plant Pathology, University of California, Riverside, CA 92502, 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 92502; (3)Associate Professor, Department of Plant Sciences, University of California, Riverside, CA 92502. Phytopathology 67:1495-1500. 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-67-1495.

The amount of root disease caused by chlamydospores and mycelium of Phytophthora cinnamomi was examined in a sandy loam and a clay soil of matric potentials adjusted to 0, -0.05, -0.10, or -0.25 bar with ceramic tension plates. Data also were collected on the influence of glucose and asparagine amendments on root disease and chlamydospore germination in sandy loam soil at various matric potentials. Root disease was rated as the percentage of the root system of Persea indica seedlings with black lesions caused by P. cinnamomi. Infection was confirmed by plating roots on a selective medium. When 100 g of sandy loam soil was infested with chlamydospores (15 spores/g dry soil) or mycelium (four 0.5-mm-long fragments/g dry soil) disease ratings with either inoculum averaged 50-100% at -0.10 bar or less negative matric potentials but only averaged 4-8% at -0.25 bar. In soil infested with diseased roots of avocado seedlings (forty 5-mm-long segments/g dry soil), again disease ratings were much lower at -0.25 bar than at higher matric potentials. In clay soil infested with chlamydospores (15 spores/g dry soil), no sharp difference was observed between disease ratings at -0.25 bar and -0.10 bar or less negative matric potentials. Average disease rating at -0.25 bar was 50% in clay soil. At 0.25 bar in sandy loam soil amended with glucose and asparagine, 0.9 and 0.225 mg/g dry soil respectively, disease ratings averaged 86%. Chlamydospore germination and germ tube growth in sandy loam soil were reduced significantly at -0.25 bar matric potential compared to potentials approaching zero. When the soil was amended with glucose and asparagine, germination and germ tube growth at -0.25 bar were as high as in nontreated soil at -0.10 bar or less negative matric potentials. The results suggest that at -0.25 bar matric potential in sandy loam soil nutrient availability rather than matric potential per se limited disease development.