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Ecology and Epidemiology

Longevity of Pythium ultimum in Moist Soils. J. G. Hancock, Professor, Department of Plant Pathology, University of California, Berkeley 94720; Phytopathology 71:1033-1037. Accepted for publication 25 January 1981. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-1033.

The initial rate of decline in natural soil populations of Pythium ultimum in the field was exponential. Following population growth in crop residues, an initial rapid population decline occurred over 23 mo with an average half-life (t) of approximately 30 days. The average rate of decline after this initial phase was slower with a t of 125 days. Reproductive bodies formed in cotton leaves or from culture showed initial population declines (t = 25 to 30 days) for 2 to 3 mo in sandy loam or clay soils held at 70 mbar matric potential and 19 2 C. These declines were followed by stable or increased populations of P. ultimum. Population increases coincided with oosprore ripening (conversion from endogenously to exogenously dormant spores). The decline in inoculum densities over the first few months (sporangia) was less evident and the subsequent increase in densities (oospore ripening) was more prominent if soils were sterilized prior to amendment with P. ultimum reproductive bodies. Microscopic observations of propagules from soil indicated that parasitic microorganisms aided the decline in soil populations of P. ultimum.