Rhizoctonia root rot caused by Rhizoctonia solani AG8, is the major fungal root pathogen of cereals of dry land cropping systems in southern Australia. To find potential biocontrol agents to be developed as commercial inoculants for Rhizoctonia control, 2322 bacterial and fungal strains isolated from a variety of sources were assessed in a high throughput (HT) in planta screening system and 185 isolates which reduced Rhizoctonia disease were selected for further characterisation (Zhao et al., 2013). Candidate strains were assessed in pot bioassays to first confirm efficacy, determine cell number required for effective disease suppression and characterised for properties important for a commercial inoculant. Bioassays consisted of 300 g field soil, an aggressive R. solani strain and 5 wheat seedlings grown for 4 weeks with microbes applied as spore suspensions to seeds prior to planting to form a seed coat. Strains were identified by 16SrRNA gene sequencing and characterised for growth, survival on seed and in storage, resistance to chemical seed treatments, stability and efficacy against other root diseases.
Of the 185 strains that passed the HT screen, 43 were shown to provide greater, reproducible disease control in comparison to the current best biocontrol strains from Flinders University and SARDI. These 43 strains included a diversity of taxa: the fungi Trichoderma, Aspergillus and Cylindrocarpon and bacteria from 4 phyla, encompassing the genera Streptomyces, Actinomadura, Microbacterium (Actinobacteria), Bacillus, Brevibacterium, Paenibacillus (Firmicutes), Pseudomonas, Pandoraea, Phyllobacterium (Proteobacteria) and Chryseobacterium (Bacteroidetes). Strains that had poor ‘commercial’ traits such as poor growth or sporulation, poor seed survival or sensitive to agrochemicals or high temperature were rejected. From the 2322 strains assessed, 20 strains increased plant dry weight of diseased plants by more than 20%, and had good growth and survival characteristics making them suitable for further development as biocontrol inoculants.