VIEW ARTICLE

Ecology and Epidemiology

Competitive Colonization of Organic Matter in Soil by Phytophthora megasperma f. sp. medicaginis. J. P. Stack, Former graduate research assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853; R. L. Millar, professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. Phytopathology 75:1020-1025. Accepted for publication 3 May 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1020.

The ability of Phytophthora megasperma f. sp. medicaginis to colonize root segments (RS) of various plants in unsterile silt loam soil was investigated. A metalaxyl-insensitive isolate (Pm20) of P. m. f. sp. medicaginis was added to soil at different temperatures and moistures together with RS of various plants. Over time, the RS were retrieved and plated on a selective medium containing metalaxyl. Pm20 grew 2.0 cm through unsterile soil and colonized alfalfa RS at low soil temperatures (4 and 15 C) and high soil moistures (0 and - 10 mb matric potential). At 24 C and - 100 mb matric potential, Pm20 grew only 0.5 cm to colonize RS. Root tissues of black medic, birdsfoot trefoil, and corn also were colonized in soil. Colonization occurred in sieved, reconstituted soil, as well as in soil with an intact field structure. RS colonized by Pm20 in soil served as sources of inoculum for alfalfa plants; RS colonized at 4 C were more effective than those colonized at 15 C. In soil columns, zoospores of Pm20 could not move through the silt loam soil. Therefore, it was concluded that hyphae of Pm20 grew through the soil to effect colonization. Based upon these studies, it appears that P. m. f. sp. medicaginis may have an active nonparasitic phase in its life history.