Microbial biocontrol of Sclerotinia stem rot of canola

Stem rot caused by Sclerotinia sclerotiorum (Lib.) de Bary is a major fungal disease of canola and other oilseed rape worldwide, including Australia. Prevalence of the disease was investigated in eight major northern mustard growing districts of Bangladesh where sclerotinia stem rot is present in all districts occurring as petal and stem infections. The existence of Sclerotinia minor Jagger was also observed, but only from symptomatic petals. The management of stem rot relies primarily on strategic application of synthetic fungicides throughout the world. An alternative biocontrol approach was attempted for management of the disease with 514 naturally occurring bacterial isolates screened for antagonism to S. sclerotiorum. Three bacterial isolates demonstrated marked antifungal activity and were identified as Bacillus cereus (SC-1 and P-1) and Bacillus subtilis (W-67) using 16S rRNA sequencing. Dual culture assessment of antagonism of these isolates toward S. sclerotiorum resulted in significant inhibition of mycelial growth and restriction of sclerotial germination due to the production of both non-volatile and volatile metabolites. The viability of sclerotia was also significantly reduced in a glasshouse pot trial with soil collected from the field. Spray treatments of bacterial strains reduced disease incidence and yielded higher control efficacy both on inoculated cotyledons and stems under controlled environment conditions. Three field trials with application of SC-1 and W-67 at 10% flowering stage of canola showed high control efficacy of SC-1 against S. sclerotiorum in all trials when sprayed twice at 7-day intervals. The application of SC-1 twice and a single application of the recommended fungicide Prosaro 420SC showed greatest control of the disease. When taking into consideration both cost and environmental concern surrounding the application of synthetic fungicides, B. cereus SC-1 may have potential as a biological control agent of sclerotinia stem rot of canola in Australia.As a biocontrol agent against sclerotinia stem rot disease of canola both in vitro and in vivo, B. cereus SC-1 was further evaluated against lettuce drop caused by S. sclerotiorum. Soil drenching with B. cereus SC-1 applied at 108 CFU mL-1 in a glasshouse trial completely restricted infection by the pathogen and no disease was observed. Sclerotial colonisation was tested and results showed that 6-8 log CFU per sclerotia of B. cereus resulted in significant reduction of sclerotial viability compared to the control. Volatile organic compounds produced by the bacteria increased root length, shoot length and seedling fresh weight of the lettuce seedlings compared with the control. B. cereus SC-1 was able to enhance lettuce growth resulting in increased root length, shoot length, head weight and biomass weight compared with untreated control plants. The bacteria were able to survive in the rhizosphere of lettuce plants for up to 30 days. These results indicate that B. cereus SC-1 could also be used as an effective biological control agent of lettuce drop.