NATURAL SUPPRESSIVENESS OF SOILS FROM WEST CENTRAL BRAZIL TO Rhizoctonia solani Kühn

Abstract

Rhizoctonia solani is a highly destructive world wide soil fungus, with a large host-range, that causes important diseases in a great number of the crops. It is a complex specie whichpossesses many biotypes, differing in their pathogenicity, hosts,distribution in the nature and cultural appearance in solid media.Dry beans are susceptible to this pathogen and the susceptibility is inversely proportional to the host development. The microbial activity of some soils can prevent the establishment of phytopathogenic fungi. Soils with this property are named antagonistic, long life, resistant or supressive. The objective of this work was to evaluate the levels of natural supressiveness to R. solani of some soils previously submitted to different managements from West Central Brazil. Soil samples were collected in Itumbiara, Silvânia, Jussara and Santa Helena de Goiás counties of Goiás State, and classified as: Purple Latosol, Dark-Red Latosol, Sandy Soil and Purple Latosol, respectively. Each soil sample was collected in the layer of 0-20 cm, in contiguous areas with the following characteristics: a) soil cultivated with beans irrigated with central pivot, for more than four consecutive years; b) soil under native vegetation, and c) soil under Brachiaria decubens pasture. Sorghum grains colonized by Rhizoctonia solani, were ground and mixed to the soil samples. Six inoculum densities were used: 0, 100, 500, 1000, 5000 and 10000 propagules per gram of soil. The experiment was conducted under greenhouse conditions, using a randomized complete block design, in a factorial scheme 6 x 4 x 3, with four replications. The experimental unit was a plastic tray with 4 kg of soil and 40 plants. Fifteen days after the emergence symptoms severity were evaluated and the McKinney index estimated. The analysis of variance revealed significance of triple interaction and the degrees of freedom were unfolded in regression analyses among the inoculum doses and the disease index in percentage, in the following exponential equation: ID = A x e (-B/dose do inóculo + 1). In the soil samples from Itumbiara and Silvânia the disease index increased with the increment of the number of propagules for gram of soil, reaching values larger than 70%. However, for both soils, there were no significant differences among the natural vegetation, pasture and bean cultivated soils. On the other hand, in soils from Jussara and Santa Helena, in spite of the disease index increment with the increase of the inoculum doses, the natural and pasture soils showed similar disease indexes for all the inoculum doses used. In soil from irrigated beans, the increment in the diseases index was smaller, not surpassing 60%.

KEY-WORDS: Rhizoctonia root rot; biological control; soil microbial population.