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Effect of Trichoderma harzianum Seed Treatment and Rhizoctonia solani Inoculum Concentration on Damping-Off of Snap Bean in Acidic Soils. David S. Marshall, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907. Plant Dis. 66:788-789. Accepted for publication 29 January 1982. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-788.

The efficiency of biocontrol on damping-off of bean (Phaseolus vulgaris) induced by Rhizoctonia solani depends on soil reaction and inoculum concentration of the pathogen. Soil was acidified to pH 3.5 or 5.6 and infested with R. solani inoculum at concentrations of zero, 0.1, 1.0, 5.0, or 10.0 g/kg of soil. Seeds were coated with conidia of Trichoderma harzianum and planted in the acidified, infested soils. As inoculum concentration of R. solani increased from 0.1 g/kg of soil, there was a corresponding increase in disease incidence (proportion of plants damped off). Disease incidence was reduced by 32% in soil of pH 3.5 compared with soil of pH 5.6 and by 65% when seeds were treated with conidia of T. harzianum as opposed to untreated seed. Piecewise linear regression indicated that the rate of increase of damping-off was greater when the range of R. solani inoculum was from 0 to 0.1 than from 0.1 to 10.0 g/kg of soil. The results suggested that T. harzianum seed treatment of snap bean reduces incidence of R. solani damping-off in acidic soils.