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Ecology and Epidemiology

Effect of Soil Matric Potential on Phytophthora Root Rot of Alfalfa. Ta -Li Kuan, Department of Plant Pathology, University of California, Riverside 92521, Senior author is now with Asgrow Seed Co., Division of Upjohn Inc., 500 Lucy Brown Lane, P.O. Box L, San Juan Bautista, CA 95045; D. C. Erwin, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 72:543-548. Accepted for publication 14 August 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-543.

The effect of soil matric potential (φm) on Phytophthora root rot of alfalfa was studied with custom-built ceramic cups on which φm values from 0 to 500 mb were maintained. When seedlings were inoculated with mycelial fragments and oospores, the highest incidence of disease occurred at 0 mb with proportional reduction as φm was reduced. At 0 mb, hyphal tips grew from buried mycelial fragments, zoospores were released from the sporangia formed from germinated oospores, and sporangia formed on infected roots. These effects increased the inoculum density, which subsequently increased the incidence and severity of disease. The optimal φm for production of sporangia from infected roots was 0 mb and from mycelial disks was 100 mb. The progress of disease, following inoculation with mycelial fragments or with oospores, at different matric potentials was plotted against time using the logit transformation values of ln 1 / (1 x) in which x = the porportion of plants killed. The disease increase rates for soil infested with oospores at the φm of 0 mb was 0.24 per unit per day; at 10 mb, 0.06 per unit per day; and at 350 mb, 0.02 per unit per day. In soil infested with mycelial fragments the disease increase rate at 0 mb was 0.36 per unit per day; at 10 mb, 0.04 per unit per day; and at 350 mb, 0.02 per unit per day.