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Modeling of Phenomena Associated with Soil Suppressive to Rhizoctonia solani. Chandrani Wijetunga, Former research associate, Colorado State University, Fort Collins 80523; Ralph Baker, professor of Botany and Plant Pathology, Colorado State University, Fort Collins 80523. Phytopathology 69:1287-1293. Accepted for publication 3 July 1979. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-1287.

Soils became suppressive to Rhizoctonia solani after being planted to successive crops of radishes at weekly intervals when small propagules (mycelial fragments) (<250 μm) but not large propagules (>589 μm) of the fungus were used as inoculum. Disease incidence increased more rapidly when host tissue was reincorporated into the soil during three replantings than when host tissue was removed, which suggested that no chemical entity present in radish was suppressing the activity of R. solani. Lack-of-fit tests applied to data describing increase of disease within a single crop suggested a better performance of data modified by the simple interest transformation than of either the plot of nontransformed points or the compound interest model. None of these approaches to modeling, however, demonstrated better degree of fit than did regression plots of nontransformed data on disease observed from crop to crop in successive plantings. The ED50 values indicated by the inoculum density-disease incidence (ID-DI) curve was changed from approximately 8.0 in conducive soil to 21.5 propagules per gram in the suppressive soil. Thus, change in ED50 value indicated the substantial impact of biological control achieved through the use of suppressive soil. As the concentration of conducive soil in mixtures was increased relative to suppressive soil, the conducive index (CI) increased. The curve obtained when concentrations of conducive soil were plotted against the values of CI resembled an ID-DI relationship. The curve, modeled according to the log-log transformation, had a slope value of 1.222. If the suppressive mechanism in the soil resulted from microparasitism, this could be interpreted as a “rhizosphere” effect adjacent to the thallus of R. solani.