The experiment was conducted under greenhouse conditions at the Federal University of Uberlândia, State of Minas Gerais, Brazil, from March to August 1995. The purpose of this study was to determine if suppression of seven soil classes could inhibit infection of soybeans by R. solani, and how suppression might relate to the physical, chemical and biological characteristics of these soils. After infesting soils with R. solani, grown on autoclaved sorghum grains, disease intensity increased in soybean seedlings grown in all soils infested. This intensity was always greater in the 0-20 cm layer, which was associated with a higher organic matter content. However, one exception was found for the organic soil (SOe), which presented a similar disease intensity of the fungus in two layers from 0-20 and 20-40 cm due to high organic matter content in both layers. The suppressive effect on R. solani was observed with the Typic Plinthaquox (PTd) and Typic Acrustox (Dark-Red Latosol - LEa) soils, probably due to the clay texture, high aluminum saturation and vegetation (cerrado phase), even in the absence of Trichoderma spp. The organic soil, Acric Rhodustox (LRd) and Udic Argiustoll (TRe) was more conducive to soybean infection by R. solani, possibly due to its eutrophic character and the content of its organic matter originating from plant cover type (vegetation phase). The Typic Haplustox soil (LEam) and Typic Acrustox (Red-Yellow Latosol - LVa) materials had an intermediate reaction. The disease intensity was negatively correlated with aluminum saturation level and clay content of the soil and positively correlated with the base saturation and pH. Mineralogical characteristics of the examined soils do not appear to influence the suppressiveness of R. solani or soil conduciveness directly or to any appreciable extent, possibly because soil mineralogy only differs in its iron oxides forms.
Rhizoctonia solani; suppressive soils; tropical soils; biotic and abiotic soil factors