Abstract: Pear black spot caused by Alternaria alternata is an important disease of pear during the cultivation and post-harvest storage, and its control relies on chemical fungicides, therefore it is urgent to develop safer and more efficient biofungicides. An actinomycetes isolate, Streptomyces odonnellii SZF-179 obtained from our previous study showed high antagonistic activity against A. alternata. To enhance the fermentation level of this strain and determine its application potential to control pear black spot, the fermentation media were optimized through response surface methodology. The optimal shake-flask fermentation conditions and the proper combination of carbon sources, nitrogen sources and mineral salts were achieved through single-factor experiment. Three factors, such as maltose, yeast extract and calcium carbonate significantly affecting the growth inhibition of A. alternata by supernatants from SZF-179 fermentation broth, were determined by Plackett-Burman design. By combining the steepest ascending experiment and Box-Behnken design and response-surface methodology to fit the nonlinear equation between the significant factors and the inhibiting rate of the strain's fermentation broth supernatant, an optimized fermentation medium formula was obtained with the composition of 15.86 g/L maltose, 10.00 g/L potato starch, 8.15 g/L yeast extract, 5.00 g/L sesame meal, 0.50 g/L ammonium sulfate, and 0.20 g/L calcium carbonate. The constructed model predicted that the inhibiting rate of the 50 fold dilution of the fermentation broth supernatant from the optimized medium was 70.39%, with a validation value of 70.51%. The predicted value was in good agreement with the validation value, and the inhibiting rate was about 1.3 times higher than that of the 50 fold dilution from the fermentation broth supernatant (30.70%) before the optimization of the culture medium. The optimized culture of S. odonnellii SZF-179 achieved a control efficacy of 61.21% on pear black spot in the field trial, and reduced the defoliation rate to 14.56% which was significantly lower than the defoliation rate of blank control, Which was 44.25%. This study will support technically the development of new microbial fungicides to control pear black spot.

Key words: Streptomyces odonnellii, pear black spot, fermentation medium, optimization, response surface methodology, inhibiting activity