VIEW ARTICLE

Ecology and Epidemiology

Factors Affecting Suppressiveness to Rhizoctonia solani in Soil. Y. Henis, Professor, Department of Plant Pathology and Microbiology, Hebrew University of Jerusalem, Israel; A. Ghaffar(2), and Ralph Baker(3). (2)Associate professor and Fulbright-Hays scholar, Department of Botany, University of Karachi, Pakistan; (3)Professor, Department of Botany and Plant Pathology, Colorado State University, Fort Collins, 80523. Phytopathology 69:1164-1169. Accepted for publication 16 November 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-1164.

Pathogenicity and growth of Rhizoctonia solani was suppressed in soils initially infested with this pathogen and planted to successive weekly crops of radishes. Suppressiveness developed in 5 wk in a Fort Collins clay loam. In untreated soil, or in soil uninfested with R. solani but planted with radishes weekly for 5 wk, almost 100% damping-off occurred when inoculum was added at a density of five propagules per gram of soil. In contrast, only 46% damping-off occurred in suppressive soil at 40 propagules per gram. When aliquots of soil were amended weekly with dead mycelium of R. solani or chitin at 1 g/kg, little suppressiveness was observed even when amendments continued for 5 wk. The soil became suppressive, however, when amended weekly with viable dried yeast at 1 g/kg soil. Samples of five cultivated soils collected in California were planted every 7 days with radishes over a 9-wk period. After this treatment, two of the soils infested with R. solani, became highly suppressive, one was as conducive as at the beginning of the experiment, and two were intermediate in suppressiveness. This indicated that soils may vary in capacity to develop suppressiveness in monoculture. No correlation was detected between suppressiveness of the soil and antagonism of the soil microflora as assayed in vitro. There was, however, a greater increase in soil lytic properties and populations of Trichoderma spp. in suppressive soils than in conducive soils. Three assays are suggested to develop quantitative measurements of the degree of suppressiveness in soils: a simple comparison of disease incidence (DI) in suppressive and conducive soils; generation of inoculum density/disease incidence curves for various soils (probably the most sensitive indicator of suppressiveness); development of a value for conducive index for soilborne pathogens (eg, R. solani) capable of growing through soil.