Organic matter input influences incidence of root rot caused by Rhizoctonia solani AG8 and microorganisms associated with plant root disease suppression in three Australian agricultural soils

Abstract

Soil-borne plant root disease caused by Rhizoctonia solani AG8 is prevalent in cereal farming systems worldwide, particularly in semiarid agricultural regions. A controlled environment study was undertaken using three Australian soils to test the hypothesis that OM input from crop roots and residues decreases infection by Rhizoctonia root rot via biologically mediated disease suppression. The specific aim was to determine the relative effect of two different OM inputs (wheat stubble or roots) on (a) abundance (DNA) of the pathogen R. solani AG8 and soil organisms putatively associated with disease suppression, and (b) incidence of Rhizoctonia root rot infection of wheat seedlings (% root infected). An increase in microbial biomass carbon (C) following OM amendment indicated a potential for enhanced general biological disease suppression in all soils. OM inputs also increased the population size (DNA) of certain bacteria and fungi putatively associated with specific suppression for Rhizoctonia root rot, suggesting a C resource-mediated change in microbial functions related to disease suppression. There were no significant changes to measured pathogens with stubble addition. However, OM inputs via root residues and rhizodeposits from living roots increased the populations of R. solani AG8 and Gaeumannomyces graminis var. tritici so that in subsequently planted wheat there was greater incidence of root disease infection and reduced plant shoot and root DM compared with that following OM input as stubble. Differences between soils in terms of plant and soil organism responses to each OM input suggest that abiotic factors modify the development of biological disease suppression and the expression of the disease.