Longevity ofPythium ultimumin Moist Soils

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Abstract
The initial rate of decline in natural soil populations of P. ultimum in the field was exponential. Following population growth in crop residues, an initial rapid population decline occurred over 2-3 mo. with an average half-life (t1/2) of .apprx. 30 days. The average rate of decline after this initial phase was slower with a t1/2 of 125 days. Reproductive bodies formed in cotton leaves or from culture showed initial population declines (t1/2 = 25-30 days) for 2-3 mo. in sandy loam or clay soils held at -70 mbar matric potential and 19 .+-. 2.degree. C. These declines were followed by stable or increased populations of P. ultimum. Population increases coincided with oospore ripening (conversion from endogenously to exogenously dormant spores). The decline in inoculum densities over the 1st 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.

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