Natural biological suppression of soilborne diseases in southern Australian agricultural fields

Natural biological disease suppressiveness of soil is the ability of a soil to reduce disease severity even in the presence of a pathogen, its host plant and favourable climatic conditions. Disease suppression is considered as a function of both the activity and composition and of a diverse microbiota (microflora and microfauna) community. Although it is an inherent property of all biologically active soils, the level of suppression ability varies with edaphic and environmental variables. We measured the disease suppression potential (DSP) of surface (0-10 cm) soils collected from over 50 agricultural fields under continuous cropping and experimental sites in the Mediterranean region of southern Australia (Eyre peninsula and Mallee regions in South Australia, Western Australia and southern New South Wales) using a controlled environment based bioassay. The DSP index value for each soil was calculated (on a 0-1 scale where 1 represents higher DSP) based on reduction in disease when the soils were incubated with added carbon (RsSuc) relative to disease incidence with added inoculum only. Disease incidence observations were also made in field experiments evaluating the effect of crop management practices and linked to estimates of DSP. Results indicated a wide range of suppressive activity and DSP values ranged from 0.2 to 0.95 and less than 10 soils showed DSP levels high enough (>0.55) to suppress disease to agronomically valuable levels. The majority of soils from Eyre peninsula region in SA and WA showed lower DSP values (<0.3). Both soil type and management had significant effect on the level of disease suppression potential. Results suggest that a multi-level bioassay (multiple C and pathogen levels) is required to measure DSP for soils varying in soil texture and chemical properties. Overall, in the rainfed cropping regions of southern Australia it is proposed that management practices that increased C inputs over a number of years would cause a change in the composition of the microbial community and enhance activities of specific microorganisms resulting in increased suppression of soilborne diseases such as Rhizoctonia bare patch