Mechanisms of the biofungicide Serenade (Bacillus subtilis QST713) in suppressing clubroot

Clubroot is a serious threat to canola production in western Canada. The biofungicide Serenade (R) (Bacillus subtilis QST713) reduced the disease substantially in controlled environment, but showed variable efficacy in field trials. To better understand how this biofungicide works, two of the product components, i.e., B. subtilis and its metabolites (product filtrate), were assessed under controlled conditions for their relative contribution to clubroot control. The information may be used to optimize the product formulation. The bacterium or product filtrate alone was only partially effective against clubroot, reducing disease severity by about 60% relative to untreated controls. In contrast, Serenade controlled the disease by over 90%. This pattern of response was mirrored in quantitative PCR assessment on P. brassicae DNA within canola roots; the lowest and highest amounts of pathogen DNA were found in roots of Serenade treatment (0.02 and 0.01 ng/g) and controls (0.52 and 13.35 ng/g), respectively, at 2 and 3 weeks after treatment. During this period, the amount of DNA changed little in Serenade-treated roots but increased by almost 30-fold in the control. The product filtrate or B. subtilis also reduced the pathogen DNA substantially (0.03-1.16 ng/g). Serenade decreased the germination and viability of P. brassicae resting spores only marginally. It is suggested that biofungicide Serenade controls clubroot largely via suppressing root-hair and cortical infection by P. brassicae zoospores. The bacterial metabolites in the product formulation possibly assist B. subtilis in rhizosphere colonization and clubroot control by minimizing the competition from other soil microbes.