VIEW ARTICLE

Population Density of Phytophthora parasitica var. nicotianae in Relation to Pathogenesis and Season. R. A. Flowers, Department of Plant Pathology, University of Kentucky, Lexington 40506, Senior author now Assistant Plant Pathologist, University of Georgia, College of Agriculture Experiment Stations, Coastal Plain Station, Tifton, Georgia 31794; James W. Hendrix, Department of Plant Pathology, University of Kentucky, Lexington 40506. Phytopathology 62:474-477. Accepted for publication 22 November 1971. DOI: 10.1094/Phyto-62-474.

Numbers and location of soil-borne propagules of Phytophthora parasitica var. nicotianae are determined largely by pathogenesis. Population densities were almost undetectable at the time tobacco plants were transplanted into the field, but increased rapidly in the rhizospheres of plants of susceptible cultivars, and reached maxima at the time plants were killed. Plants of a moderately resistant cultivar were not killed by the pathogen, and populations of the fungus rose slowly and reached maxima near the end of the growing season. Populations failed to rise to a detectable level in the rhizosphere of plants of the cultivar M S Burley 21 X L8 in a field infested with race 0, to which this cultivar is highly resistant; but populations rose sharply in the rhizospheres of plants of this cultivar in a field infested with race 1, to which this cultivar is susceptible. Propagule populations were high directly beneath infected plants, but low 6 and 18 inches away. They were high in the upper 3 inches of soil, low in the next 3 inches, and not detected 6 to 12 inches deep. The pathogen readily overwintered in the upper 3 inches of soil. Population levels of total Pythium species were not affected by these factors.