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Host and Nonhost Effects on Soil Populations of Phytophthora parasitica var. nicotianae. R. A. Flowers, Department of Plant Pathology, University of Kentucky, Lexington 40506, Present address: Assistant Plant Pathologist, University of Georgia College of Agriculture Experiment Stations, Coastal Plain Station, Tifton 31794; James W. Hendrix, Department of Plant Pathology, University of Kentucky, Lexington 40506. Phytopathology 64:718-720. Accepted for publication 11 December 1973. DOI: 10.1094/Phyto-64-718.

Population increases of Phytophthora parasitica var. nicotianae in soil were dependent upon pathogenesis. Populations increased rapidly when susceptible cultivars were transplanted into artificially or naturally infested soils. With a moderately resistant cultivar, populations increased more slowly. Race 0 increased to a slight extent with the highly resistant cultivar, Burley 21 × L8. Nonhost plants (tomato, pinto bean, cowpea, wheat, and fescue) had no effect on populations, and none of the plants evaluated had any effect on populations of Pythium spp. indigenous to the naturally infested soil. Freezing and thawing conditions of winter weather had no influence on natural populations of P. parasitica var.nicotianae or Pythium spp.