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Bacterial Population Dynamics and Interactions with Pythium aphanidermatum in Intact Rhizosphere Soil. T. Tedla, Research associate, Department of Plant Pathology, University of Arizona, Tucson 85721; M. E. Stanghellini, Professor, Department of Plant Pathology, University of Arizona, Tucson 85721. Phytopathology 82:652-656. Accepted for publication 9 March 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-652.

In tripartite rhizosphere (host, fungus, and bacteria) interactions, bacterial antagonism was found to be responsible for the general suppression of pathogen activity at low soil temperature. Host colonization by Pythium aphanidermatum was less than 25% at 20 C, in contrast to greater than 90% colonization at 27 C after 72 h. When bacterial activity was reduced or inhibited by the addition of vancomycin to soil, host colonization by the fungus at 20 C increased to 83%. Generation time of bacteria in intact rhizosphere soil was estimated to be about 8 h at both 20 and 27 C. Bacterial antagonism was shown to occur, however, before any significant increase in bacterial populations. Oospores germinated within 4 h in rhizosphere soil at 27 C but required 812 h at 20 C. Percent oospore germination increased in soils amended with asparagine and/or vancomycin at 20 C. These results indicated that competition for nutrients may be involved in suppression of P. aphanidermatum at low soil temperatures.