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Population of Rhizoctonia solani in Agricultural Soils Determined by a Screening Procedure. A. R. Weinhold, Professor, Department of Plant Pathology, University of California, Berkeley, CA 94720; Phytopathology 67:566-569. Accepted for publication 11 October 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-566.

A rapid, reliable procedure for the quantitative determination of the population of Rhizoctonia solani in agricultural soils is presented. The procedure is based on wet-screening soil through a 0.35-mm mesh sieve to remove the organic particles that contain the pathogen. The material retained on the sieve is dispersed in 1% water agar and, after 18 to 24 hr, incubation, suspected R. solani colonies are transferred to potato-dextrose agar for identification. Soil from 60 fields in the San Joaquin Valley of California were assayed. Most of the fields, when sampled, were planted to either cotton or potatoes. The population of R. solani ranged from 0 to 15 propagules/100 g of soil and was distributed as follows: 22, 24, 6, and 8 fields had populations within the ranges of 0 to 0.5, 0.6 to 2.0, 2.1 to 5.0, and greater than 5.0 propagules/100 g, respectively. Therefore 77% of the soils sampled contained less than 2.0 propagules of R. solani/100 g of soil.