Chiang Mai Journal of Science

     Citrus canker, caused by Xanthomonas citri pv. citri (Xcc), is a destructive disease limiting citrus production worldwide. The widespread use of copper-based bactericides has led to copper-resistant (Cu^R) Xcc isolates, reducing control efficacy. This study investigated pathogenic diversity, copper sensitivity, and biological control alternatives for managing citrus canker in Thailand. Forty one citrus samples showing canker symptoms were collected from nine provinces. Pathogenicity tests on key lime (C. aurantifolia) confirmed 41 isolates. Copper sensitivity was evaluated on mannitol glutamate yeast extract (MGY) medium amended with 0–350 mg/l CuSO₄·5H₂O. Xcc isolates were classified as copper-sensitive (Cu^S, <150 mg/l), copper-tolerant (Cu^T, 150–200 mg/l), or copper-resistant (Cu^R, >200 mg/l). All 34 Cu^S isolates lacked the copL-copA-copB genes, whereas of the seven Cu^R isolates, three contained copA and copB, while four carried the copL-copA-copB gene cluster. The biocontrol potential of Bacillus velezensis (BB10, BB19, BB31) and B. siamensis (BB36) was tested against Xcc isolates, Cu^S and Cu^R, using dual culture techniques. All Bacillus sp. isolates showed inhibitory effects, with B. velezensis BB10 showing the strongest activity. Greenhouse trials confirmed the efficacy of B. velezensis BB10 and its formulated biological product in controlling citrus canker in tangerine (Citrus reticulata), caused by both Cu^S and Cu^R Xcc isolates. Disease incidence ranged from 0–10% when unwounded leaves were treated 24 hours before pathogen inoculation (Cu^S and Cu^R Xcc isolates). For wounded leaves, pre-treatment reduced disease incidence to 40–50%. These findings highlight B. velezensis BB10 as a promising, eco-friendly alternative to copper-based bactericides for the sustainable management of citrus canker.