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Mechanism of Biological Control in Soil Suppressive to Rhizoctonia solani. Shan -da Liu, Former graduate research assistant, Department of Botany and Plant Pathology, Colorado State University, Fort Collins 80523, Present address of senior author: Department of Plant Protection, Pingtung Institute of Agriculture, Pingtung, Taiwan, Republic of China; Ralph Baker, professor, Department of Botany and Plant Pathology, Colorado State University, Fort Collins 80523. Phytopathology 70:404-412. Accepted for publication 3 November 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-404.

Soil suppressive to Rhizoctonia solani was generated by monoculture planting of successive crops of radishes at weekly intervals in soil infested with the pathogen. A modification of Koch’s postulates was utilized to determine the mechanism of biological control associated with suppressive soil. Numbers of Trichoderma spp. propagules in the soil increased as suppressiveness increased, whereas inoculum density of R. solani was inversely proportional to the density of these Trichoderma spp. following radish monoculture. Successive plantings of cucumber also generated suppressiveness which was associated with population of Trichoderma spp. propagules. Suppressiveness did not develop and Trichoderma was undetectable in sugarbeet, alfalfa, and wheat monoculture. Thus, increase in population and constant association of Trichoderma with suppressiveness was observed. Trichoderma was isolated with high frequency from mycelial mats of R. solani incubated in suppressive soil, but only occasionally from those incubated in conducive soil. Thus, the suspected causal agent (antagonist) was isolated in pure culture. Finally, conidia of Trichoderma added to conducive soil, at the same density found in suppressive soil, induced suppressiveness and the same species could be reisolated from mycelial mats of R. solani incubated in the soil. Among the antagonists tested, T. harzianum, isolated from Fort Collins clay loam, was most effective in inducing suppressiveness. Suppressiveness during radish monoculture developed more rapidly in acid soils than in alkaline soils. This corresponded to reports of Trichoderma being strongly favored by acid conditions. The suppressive effect persisted longer in soils with low negative matric potential. This also agreed with other research worker’s observations of a high incidence of Trichoderma propagules in moist or wet soil environments.