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Inoculum Densities of Pythium aphanidermatum in Soils of Irrigated Sugar Beet Fields in Arizona. M. E. Stanghellini, Professor, Department of Plant Pathology, University of Arizona, Tucson 85721; P. von Bretzel(2), W. C. Kronland(3), and A. D. Jenkins(4). (2)(3)Research associate, and research assistant, respectively, Department of Plant Pathology, University of Arizona, Tucson 85721; (4)Research agronomist, Amstar Corporation–Spreckles Sugar Division, Chandler, AZ 85224. Phytopathology 72:1481-1485. Accepted for publication 22 April 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1481.

Inoculum densities of Pythium aphanidermatum in commercial sugar beet fields were estimated by using a species-specific isolation medium. Fields were not uniformly infested with P. aphanidermatum; intrafield inoculum densities exhibited a moderately clustered pattern of distribution (for the negative binomial probability distribution, k = 1.15) and interfield inoculum densities exhibited a highly clustered pattern of distribution (k = 0.28). No fluctuations in inoculum densities were detected in soil samples collected periodically from infested fields prior to the onset of root infection, which occurred about 9 mo after planting. Subsequent to root infection, high population densities of the fungus (from 232 to 5,120 propagules per gram of soil) were detected in rhizosphere soil immediately adjacent to lesions on infected sugar beet tap roots.