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Ecology and Epidemiology

Effects of Soil Temperature and Moisture on Activity of Phytophthora megasperma f. sp. medicaginis and Alfalfa Root Rot in the Field. H. T. Wilkinson, Former graduate student, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Present address of senior author: Department of Plant Pathology, Washington State University, Pullman 99164; R. L. Millar, professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 72:790-793. Accepted for publication 22 October 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-790.

Activity of Phytophthora megasperma f. sp. medicaginis was determined for four soil irrigation treatments applied to first-year and to 1- and 2-yr-old stands of Iroquois alfalfa. PA was measured with an alfalfa seedling baiting assay every 10 days from May through October. Incidence and severity of Phytophthora root rot, based on root destruction and plant survival, were assessed every 10 days from the second trifoliolate leaf stage through the first-year growing season. PA was not detected below ~1215 C, but at higher temperatures it was substantial, even in soil not continually irrigated to a moisture level of ≤0.1 bars. PA activity was lower in soil maintained at ≤0.1 bars than in drier soil. Root rot was directly proportional and plant survival was inversely proportional to soil moisture. Root lesions appeared ~8 wk after seeding in soil continually maintained at ≤0.1 bars. Extensive root rot developed in soil irrigated to ≤0.1 bars either continually or for 10 days on a 20-day schedule from the first trifoliolate leaf stage. Severe root destruction also occurred in soils continually wetted to that moisture level starting 14 wk after the first trifoliolate leaf stage.