Distribution ofPythium aphanidermatumin Rhizosphere Soil and Factors Affecting Expression of the Absolute Inoculum Potential

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Abstract
The surface area of a mature sugar beet [Beta vulgaris] tap root commonly exceeds 500 cm2. With a direct isolation method, an average of 32% of the 1 cm2 soil samples from the root-soil interface (RSI) of 10 rhizosphere sections of soil from sugar beet tap roots contained P. aphanidermatum. A bioassay with potato tuber tissue also showed that an average of 36% of the surface area of the RSI from 19 rhizosphere sections of soil contained an infective population of P. aphanidermatum. Inoculum densities within infested areas ranged 1-5 oospores/0.1 cm3 of RSI soil. The distribution of inoculum densities within the RSI of individual rhizosphere sections ranged from clustered to random to uniform. Inoculum densities effective for infection, as bioassayed with potato tuber tissue, were located primarily within 1 mm of the host surface. Maximum infection occurred at soil temperatures of 27.degree. C or greater under wet soil conditions (0 to -0.1 bar) and time period of 24-48 h.