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Association of Chemical and Biological Factors in Soils Suppressive to Pythium ultimum. F. N. Martin, Former graduate research assistant, Department of Plant Pathology, University of California, Berkeley 94720, Current address: Department of Plant Pathology, University of Florida, Gainesville 32611; J. G. Hancock, professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 76:1221-1231. Accepted for publication 29 May 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-1221.

Suppression of saprophytic increases in inoculum densities of Pythium ultimum was found in finely textured soils in the San Joaquin Valley of California. Suppressive and conducive soils showed no consistent relationship between suppressiveness and soil pH, concentrations of calcium, magnesium, or potassium, cation exchange capacity, or electrical conductivity. Whereas suppressive soils had greater mean concentrations of sodium, sulfate, and chloride than conducive soils, only chloride was inhibitory to P. ultimum. When conducive soils were amended with chloride at concentrations found in suppressive soils, colonization of leaf debris by P. ultimum was partially suppressed. Suppression of debris colonization coincided with a significant increase in the frequency of colonization by Pythium oligandrum. P. oligandrum was active as a saprophyte sooner in chloride-amended soils than in field soils and was significantly more tolerant of chloride than P. ultimum. In suppressive soils, P. oligandrum was the most commonly isolated primary colonizing fungus and tended to be found at higher propagule densities than observed in conducive soils. When propagule densities of P. oligandrum were increased artificially in conducive soils, colonization and subsequent inoculum increases of P. ultimum were reduced. Suppressiveness was overcome by successive soil amendments with dried leaf debris, which resulted in progressive reductions in the frequencies of colonization by P. oligandrum. Apparently soils with elevated chloride concentrations allowed P. oligandrum to successfully compete with P. ultimum and, thus, increase its propagule density and further suppress the saprophytic activity of P. ultimum.