Factors Affecting Suppressiveness toRhizoctonia solaniin Soil

Abstract

Pathogenicity and growth of R. solani were suppressed in soils initially infested with this pathogen and planted to successive weekly crops of radishes [Raphanus sativus ''Early Scarlet Globe'']. 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 5 propagules/g of soil. In contrast, only 46% damping-off occurred in suppressive soil at 40 propagules/g. 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 when amended weekly with viable dried yeast at 1 g/kg soil. Samples of 5 cultivated soils collected in California [USA] were planted every 7 days with radishes over 9 wk. After this treatment, 2 of the soils infested with R. solani became highly suppressive; one was as conducive as at the beginning of the experiment and 2 were intermediate in suppressiveness, indicating 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 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); and development of a value for conducive index for soil-borne pathogens (e.g., R. solani) capable of growing through soil.