Abstract: 【Objective】Fire blight is a devasting bacterial disease caused by Erwinia amylovora, which seriously affects the production safety of rosaceous plants, such as pears and apples. To explore potential bacterial strains as biocontrol agents for managing fire blight,antagonistic bacteria were isolated from fruits and rhizosphere soil of Malus spectabilis.【Method】Biocontrol strains were isolated using plate dilution method, using E. amylovora as the indicator bacterium. Strains with antagonistic effects were screened and rescreened using plate confrontation method and Oxford cup diffusion method. The antagonistic bacteria were identified by morphological observation, physiological and biochemical tests and 16S rDNA sequence homology analysis. The fermentation of antagonistic bacteria were optimized using the single-factor procedure and orthogonal experiments. The fermentation broth of antagonistic bacteria was used to determine the in vitro antagonistic activity against E. amylovora on the fruit of wild apple（Malus sieversii）.【Result】Four strains with strong antagonistic effects were isolated （JK1, JK2, JK3 and JK4）, and the strain JK2 showed the strongest antagonistic effect on E. amylovora with a bacteriostatic potency of 359.7 mm/mL. Based on morphological,physiological and biochemical characteristics and 16S rDNA sequence analysis, the strain JK2 was identified as Bacillus velezensis. The optimized culture conditions were determined as follows: initial pH of 6.66, culture temperature of 30.6℃ , inoculation size of 3.9%, and rotation speed of 208 r/min. Under these conditions, the number of viable counts was 1.29×109 CFU/mL, a significant increase（53.1%）as compared to the control. The control effects in the fruit of wild apple in greenhouse showed that the disease index of the protective test inoculated with strain JK2 significantly reduced, reaching 65.88%, and the protective effect was better than the therapeutic effect.【Conclusion】Strain JK2 demonstrates considerable potential for controlling fire blight, which is a promising candidate for use as a biological control agent.

Key words: fire blight disease, biological control, Bacillus velezensis, fermentation conditions