Bacillus velezensis: a natural shield against black rot disease on Serbian autochthonous cabbage cv. Futoški

Serbian autochthonous cabbage cv. Futoški is highly susceptible to the plant pathogenic bacterium Xanthomonas campestris pv. campestris, the causal agent of black rot, leading to significant economic losses. This study aimed to isolate indigenous Bacillus spp. and Pseudomonas spp. strains with beneficial traits for biocontrol of black rot. From a total of 115 potential antagonistic strains, four (coded as P-FC 55, RD-FC 88, R-FC 102, And R-FC 114) identified as species Bacillus velezensis were selected as candidates based on strong in vitro antagonistic activity, both as whole cultures and cell-free supernatants, against the most virulent X. campestris pv. campestris strain (XcFC 231) identified in this study. No statistically significant differences were observed between the efficacy of whole cultures and supernatants of biocontrol candidates. Preventive inoculation approaches (seed and foliar) of biocontrol candidates consistently outperformed curative treatments across all strains, highlighting their potential as preferred application strategies. The most effective B. velezensis strain was RD-FC 88, applied in the form of a whole culture during preventive foliar application, achieving 93.86% disease suppression, surpassing copper oxychloride (89.26%) under the same conditions. Chemical profiling (GC/MS and UHPLC–QToF MS) of ethyl acetate extracts revealed the presence of a wide range of antimicrobial organic compounds (e.g., 2,3-butanediol, urea, succinic acid, thymine, phenylalanine, 9H-purin-6-ol) and lipopeptides (surfactins C₁₂–C₁₇), which may be contributing factors to the biocontrol activity of the strains. Next to the surfactins, molecular screening for the presence of genes encoding the production of lipopeptides with antimicrobial activity indicated the potential of strains to produce bacillomycin D and iturins (except R-FC 114) under certain growing conditions. Preventive application (seed and foliar) of biocontrol candidate strains led to an increase in epidermal flavonoid and chlorophyll content, while the opposite trend was observed in curative treatments. The results of this study highlight the strong potential of the B. velezensis strains P-FC 55, RD-FC 88, R-FC 102, And R-FC 114 for controlling black rot disease on the cabbage cv. Futoški, with possible broader applicability to other cabbage cultivars and Brassicaceae crops.

Graphical Abstract