Rhizoctonia root rot caused by Rhizoctonia solani AG8 is the major fungal root disease of cereals in low to medium rainfall regions across southern Australia, causing $77 million loss of yield annually in wheat and barley with limited control options. Rhizoctonia is known to be impacted by a diversity of soil microbes and the development of microbial inoculants is a potential control option. Microbes that are both effective at reducing disease and have properties suitable for a commercial inoculant are likely to be rare and rapid in vitro screening methods are not predictive of disease control in planta. For these reasons a high throughput system was developed to screen large numbers of microbes assessed directly into an in planta system simulating conditions under which isolates will need to perform in the field. This system contained field soil, an aggressive R. solani strain, and two wheat seedlings. Test strains were applied directly to seeds and the response determined by increased shoot and root growth.
Bacterial and fungal strains from SARDI, Flinders and Murdoch Universities were isolated from a variety of sources, including native plants, cereals and legumes and included endophytes, root and rhizosphere colonising microbes. A total of 2322 isolates were assessed in the high throughput screen resulting in 185 candidate strains being selected that had improved disease control compared to uninoculated controls and our current best agents for Rhizoctonia control, Trichoderma strain TB or Streptomyces strain EN16. These strains were then assessed in a more robust bioassay to confirm efficacy, determine cell number required and characterised for properties important for a commercial inoculant. This result shows that microbes can be successfully and rapidly screened directly for disease control on plants and avoid the use of uninformative screens such as pathogen inhibition on agar plates.