Biological Suppression of Crop Diseases: Role of Microbial Communities and Microflora-Faunal Interactions.

Biological suppression of soilborne plant diseases such rhizoctonia root rot and Take-All has been found in in experimental plots and farmer fields in southern Australia and Pacific North West USA. Disease suppression is a function of activity and composition of a diverse microbiota and interactions between microflora and micro- and meso-fauna. A diverse array of microbial communities can be involved in the continued effective expression of disease suppression in the field environment. Comparative analysis of suppressive and non-suppressive soils in Southern Australia showed that beneficial fungal genera/groups were dominant in suppressive soils whereas pathogenic fungi dominated diseased field soils. Similarly, specific bacterial taxa from alpha and delta proteobacteria, actinobacteria, firmicutes and acidobacteria GP6 groups contributed to the majority of differences in bacterial diversity based on suppression. Fungal community network in suppressive soils exhibited greater modularity whereas non-suppressive community network was much more centralised. Metagenomic sequencing data showed significant differences in a number functional attributes related to pathogen-microbe-plant interactions, nutrient turnover and induced resistance.

Soil fauna can affect disease incidence by influencing the survival and growth of the pathogen and through their effect on populations of microbial communities.  For example, protozoan predation and subsequent decomposition of damaged pathogenic fungal hyphae reduces the survival of pathogen from one season to next whereas fungal feeding nematodes and meso-fauna influence the survival and ability of pathogen to reach the host root. Additionally, improvements in nutrient availability from the micro-faunal predation on microflora also contributes to plant’s ability to withstand the impact of disease.

Although it is an inherent property of all biologically active soils, the level of suppression ability varies with edaphic and environmental variables. An improved understanding of the genetic mechanisms and metabolic pathways involved with reduced disease incidence will greatly assist in the development of cropping practices with higher levels of natural disease suppression.