Abstract: To improve the control efficacy of pear fire blight and reduce the use of chemical bactericides, this study aimed to screen for synergistic combinations and optimal mixing ratios between the biocontrol strain Bacillus velezensis FX1 and compatible chemical bactericides. The biocompatibility of FX1 fermentation broth with seven tested bactericides was evaluated using plate colony counting and turbidimetry assays, while the synergistic effects of the combinations were determined by the Horsfall method. The control efficacy of the mixtures was further assessed through experiments on detached flowers of Pyrus sinkiangensis and potted P. betulifolia seedlings. Results showed that FX1 strain had poor compatibility with 5% zhongshengmycin WP, 20% thiazide copper SC, 20% thiazole zinc SC, 3% benziothiazolinone WDG, and 46% copper hydroxide WDG, moderate compatibility with 40% Kasugamycin thiazole zinc SC,and the best compatibility with 2% Kasugamycin AS. At low concentrations and recommended field application concentrations, the inhibition rate of 2%Kasugamycin AS on B. velezensis FX1 was less than 14%, and the endospore germination rate remained above 83% after 30 days of mixing culture. The in vitro toxicity of a mixture of FX1 fermentation broth and 2% Kasugamycin AS against Erwinia amylovora was determined by co-culturing, and the optimal compounding ratio was determined using the Horsfall method. The combination of FX1 fermentation broth and 2%Kasugamycin AS at a volume ratio of 6∶ 4, 5∶ 5 and 4∶ 6 inhibited the growth of E. amylovora by more than 70%, with a synergic ratio value (IR) greater than 1.7, exhibiting a significant synergistic effect. The control efficacy of these mixtures was evaluated using detached flowers of Pyrus sinkiangensis and potted P. betulifolia seedlings. Through spraying the combination of FX1 broth (1×10⁶ cfu/mL) and 2% kasugamycin AS (2000 μg/mL) with volume ratios of 6∶ 4 and 5∶ 5, it was determined that the protective efficacy of over 85% against blossom blight on detached P. sinkiangensis flowers after 3 days treatments, while both protective and therapeutic efficacy reached over 80% on P. betulifolia seedlings after 7 days, and maintaining above 71% after 10 and 14 days. These results were significantly higher than that of single agent treatments or other combination treatments. Based on all the above results, and the principle of reducing chemical bactericides usage, the optimal mixing ratio of FX1 fermentation broth and 2% Kasugamycin AS was determined to be 6∶ 4. Notably, the combination of B. velezensis FX1 and 2% Kasugamycin AS not only decreases chemical usage， but also significantly enhances control efficacy for pear fire blight.

Key words: fire blight, Bacillus velezensis, kasugamycin, compounding, synergistic effect