Abstract:[Background] As an important biocontrol actinomyces, Streptomyces caeruleatusWMF106 exhibits good antagonistic effect on the pathogen of walnut blight, Xanthomonas campestris pv. campestris. To promote the preparation and application of antimicrobial agent in the control of the disease, it is necessary to improve the biomass, antimicrobial activity and stability of antimicrobial active substance for strain WMF106. [Objective] The present study aimed to explore an optimal fermentation condition and evaluate the stability and field control effect of antimicrobial active substance for Streptomyces strain WMF106. This study will be benefit for formulating large-scale fermentation procedure and development of bioagent.[Methods] Single factor experiment and orthogonal tests were used to optimize the fermentation medium and conditions. The effects of pH, temperature, light, ultraviolet, storage temperature on the stability of active substances and the polarity nature was determined by the single factor method respectively. To evaluate the results of optimization, the methods of field trials on leaves and fruits were used. [Results] The optimum composition of the fermentation medium was yeast extract 2.0 g/L, octmeal 20.0 g/L, NaCl 0.5 g/L, K2HPO4 0.5 g/L MgSO4· 7H2O 0.5 g/L. The optimized fermentation condition was initial pH 6.0, inoculation amount 8%, medium volume 150 mL/500 mL, fermentation time 9 d, fermentation temperature 28 against the disease that infected leaves and fruits were more than 65.33% and 61.21%, respectively.[Conclusion] The optimal fermentation medium and conditions were obtained. Moreover, it is also concluded that the fermentation broth of strain WMF106 not only possess the great stability, but also have better field control efficacy. Therefore, the strain WMF106 may be considered as potential biocontrol agent against walnut blight.
