The effect of mode of application of Bacillus subtilis CA32r on control of Sclerotium rolfsii on Capsicum annuum

Abstract

Sclerotium rolfsii Sacc. the causal agent of collar and root rot of chili (Capsicum annuum L.), is one of the most important soil-borne pathogens in Sri Lanka. Bacillus subtilis CA32r, a stable spontaneous kanamycin resistant isolate, showing antagonism in a Petri plate assay, was selected for greenhouse pot experiments to control S. rolfsii. An ethyl acetate extract of the culture filtrate of B. subtilis CA32r inhibited radial colony growth as well as germination of sclerotia of the pathogen in vitro, indicating the presence of antifungal compound(s) in the culture extract. B. subtilis CA32r was investigated for its effectiveness as a biological control agent against S. rolfsii infecting transplanted chilli seedlings in greenhouse pot experiments. CA32r significantly decreased the disease incidence in terms of lesion development on stem base and roots depending on the mode of the bacterial application. Seed bacterization and soil application alone did not protect chilli plants, but root bacterization prior to the transplanting of seedlings significantly decreased the disease incidence caused by S. rolfsii. However, even in the combined treatment, seed bacterization and soil application, did not protect chilli plants. The best protection was achieved by combination of root bacterization prior to transplant and soil application of CA32r. Root bacterization resulted in maintaining higher numbers of bacteria at the collar region of chilli plants and may have shielded the most vulnerable area from the pathogen, resulting enhanced protection. Since the application of CA32r resulted in a significant reduction of the number of viable S. rolfsii propagules in the soil indicates that B. subtilis CA32r possesses not only protective but also eradicative potential.