Antagonistic Activity and Potential Mechanisms of Endophytic Bacillus subtilis YL13 in Biocontrol of Camellia oleifera Anthracnose

Anthracnose, caused by the fungus Collectotrichum fructicola (C. fructicola), is a major disease affecting the quality and yield of Camellia oleifera (C. oleifera); it reduces C. oleifera yield by 40%-80%. Bacillus subtilis (B. subtilis) YL13 is an antagonistic endophytic bacteria strain isolated from healthy C. oleifera leaves. This study was aimed at investigating the potential of YL13 for the biocontrol of C. oleifera anthracnose and the possible mechanisms involved. In in vitro assays, YL13 demonstrated remarkable antagonistic activity of C. fructicola. Its cell-free filtrates displayed antagonistic activity, which suggested that the metabolites of YL13 might play important roles. In vivo tests showed that the disease index of YL13-treated plants was obviously reduced under greenhouse conditions. YL13 secretes a variety of bioactive metabolites, including protease, cellulase, and siderophore, which might participate in the resistance to C. fructicola. In addition, C. oleifera treated with the fermentation broth of YL13 demonstrated different defense responses, e.g., accumulation of hydrogen peroxide (H2O2) and activation of the defense-related enzyme peroxidase (POD), which might contribute directly or indirectly to overcome external stresses. The significant biocontrol effect and host defense-induction activity of YL13 suggested that this B. subtilis strain as well as its metabolites have the potential to be exploited as microbial control agents for the efficient management of C. oleifera anthracnose.