A bioassay to compare disease suppressive capacity of pastoral soils

Dynamic pathogen complexes can develop under pastures, thereby substantially reducing potential productivity. Suppression of such pathogen complexes is therefore of great importance, and bioassays can quantify disease suppression in soils. This study describes the development of a pasture-relevant system: Rhizoctonia solani AG 2-1 induced damping-off (wirestem) of kale (Brassica oleracea). As kale is not a component of traditional ryegrass clover pasture swards, the assay allows assessment of general disease suppression, considered more enduring in multiple-host-multiple-pathogen systems. A pathogenic Rhizoctonia solani isolate was obtained from New Zealand pastoral soil. Inoculation of soils with this isolate resulted in a level of damping-off disease comparable to that induced by reference Rhizoctonia solani isolate Rs043-2. Significantly different levels of inoculum-induced disease incidence and progression were found in four distinct pastoral soils. In combination with soil physicochemical data and environmental DNA approaches, this bioassay can be used to further advance understanding of the influence of farm management practices on disease suppression in pasture soils.