Elutriation Procedures for Quantitative Assay of Soils for Rhizoctonia solani

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Abstract
Procedures for the use of a semiautomatic elutriator to assay soils for R. solani are described and compared with a wet-sieving technique. The standard procedure involves elutriation of 500 cm3 samples of soil for 8 min. Debris collected on a 0.425 mm sieve is suspended in 2% water agar and after 18-24 h of incubation colonies of R. solani are identified by observation at .times.10-.times.400 on the assay plates. Compared to the wet-sieving technique, the elutriation procedure is more rapid and has a lower threshold of detection. It allows identification of colony origin, and can be conducted simultaneously with certain nematode assays. R. solani populations from 29 cotton [Gossypium hirsutum] fields in the coastal plain of North Carolina [USA] range 0-35 propagules/500 cm3. Colonized segments of cotton stalks and roots were frequently observed as sources of R. solani.