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The Survival and Activity of Phytophthora megasperma in Naturally Infested Soils. R. G. Pratt, Former Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706, Present address of senior author: Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; J. E. Mitchell, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 65:1267-1272. Accepted for publication 10 June 1975. DOI: 10.1094/Phyto-65-1267.

Infective activity of Phytophthora megasperma in naturally infested soils was measured by a baiting technique using alfalfa seedlings. Activity decreased in three soils stored for 3-9 months at 25 C, but only in one soil stored at 4 C. Activity decreased slightly in soils stored at high moisture levels for 7 months, and it disappeared in soils stored at low moisture levels (equivalent to less than –40 bars tension). P. megasperma remained infective in 13 of 26 naturally infested soils stored 3.5 years at 25 C at undetermined moisture levels and in 24 of 46 soils stored 2.5 years. Growth of alfalfa seedlings of susceptible cultivars in naturally infested soils resulted in greater increases in infective activity of the pathogen than did growth of seedlings of resistant cultivars. Significant decreases in infective activity from initial values occurred in the greenhouse in two soils not planted to alfalfa. When corn, oats, clover, soybeans, or peas were grown in samples of two soils for 3-6 months, decreases in infective activity were comparable to, or only slightly greater than, that which occurred in fallow soil. These results indicate that P. megasperma is not a strong saprophytic competitor in the soil.