Biocontrol of citrus bacterial canker caused by Xanthomonas citri subsp. citri by Bacillus velezensis

Microorganisms with biocontrol capabilities against plant pathogens are considered as one of the most promising approaches for healthy crop management. In this study, ethyl acetate extracts of 25 Bacillus strains were investigated for their antagonistic effect on Xanthomonas citri subsp. citri (Xcc), which causes the citrus bacterial canker (CBC) disease. Among them, 21 strains exerted antibacterial activity against wild-type Xcc strains. Based on the strength of the antibacterial activity, nine Bacillus strains were selected for 16S rRNA analysis. 16S rRNA sequence homology revealed that several strains were closely related to B. velezensis, where strains with no antibacterial activity grouped as the soil-associated community of B. amyloliquefaciens. B. velezensis Bv-21 exhibited the highest antibacterial activity against wild type and streptomycin resistant Xcc with inhibition zones of 22.91 ± 0.45 and 20.28 ± 0.53, respectively. Furthermore, B. velezensis Bv-21 strain was tested for biocontrol activity against a streptomycin-resistant XccM4 in detached susceptible citrus leaves. The strain reduced the incidence of CBC by 26.30% and pathogen density of XccM4 by 81.68% over control. The results of the study strongly suggest that B. velezensis can be used as an effective and eco-friendly biocontrol agent either by itself or as an active compound, against both, the wild-type and streptomycin-resistant Xcc.