Abstract In this study, a Cu2O/TiO2 visible light photocatalytic composite (CuTi) was employed for the treatment of Xanthomonas campestris and Xanthomonas campestris-infected Brassica napus seedlings. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values against X. campestris were determined to be 8 µg ml⁻¹ and 32 µg ml⁻¹, respectively. Transmission electron microscopy analysis demonstrated a direct correlation between the extent of bacterial cell damage and the concentration of CuTi. Noteworthily, a bactericidal rate of 100% was achieved at a concentration of 150 µg·ml⁻¹ over a treatment duration of 120 min. Moreover, alterations in active oxidants and antioxidants, including ROS, GSH, GR, SOD, POD, and CAT within the bacterial cells, were examined to elucidate the underlying mechanism of inhibition by the CuTi. The Brassica napus infected by X. campestris was treated with CuTi, and the efficacy was validated through determination of plant resistance indexes. The combined data confirmed that the CuTi is characterized by a low dose, fast onset, good effect and higher safety for killing X. campestris, and it is expected to be developed as an antimicrobial agent for vegetables.