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Epidemiology of Rhizoctonia solani Preemergence Damping-Off of Radish: Inoculum Potential and Disease Potential Interaction. D. M. Benson, Former Research Assistant, Colorado State University, Fort Collins 80521, Present address of senior author: San Joaquin Valley Agricultural Research and Extension Center, University of California, Parlier 93648; Ralph Baker, Professor of Botany and Plant Pathology, Colorado State University, Fort Collins 80521. Phytopathology 64:957-962. Accepted for publication 29 January 1974. DOI: 10.1094/Phyto-64-957.

Preemergence damping-off of radish incited by Rhizoctonia solani was most severe at 26 C, with only 0.76 propagules/g soil required for 50% disease incidence. As temp became less favorable for disease development, greater inoculum densities were required to incite the same level of disease. Comparison of the inoculum density-disease mcidence (ID-DI) curve at 15, 20, 22, 26, and 30 C by either the semilogarithmic, log-probit, or log-log transformations indicated that curves were parallel at 22, 26, and 30 C. At 15 and 20 C, slope values were significantly greater than those at the higher temp. As slope values were over 1.0 on a log-log basis, synergism between R. solani propagules was suggested. The change in slope of the ID-DI at 15 and 20 C was explained by the interaction of inoculum potential and disease potential. Inoculum potential was greatest at 26 C, and least at 15 C. Duration of radish susceptibility to preemergence damping-off was longest at 15 C, at which 100 h were required before a disease incidence of 10% or less occurred; and shortest at 26 C, where only 40 h were required. Disease potentials of radish at 22, 26, and 30 C were similar. The hypothesis that slopes of ID-DI curves give information on mechanisms of disease induction was confirmed.

Additional keywords: infection court, seed germination.