Fermentation Conditions and Wettable Powder Formulation of Biocontrol Agent Bacillus atrophaeus YL84 in Control of Pear Valsa Canker

Bacillus atrophaeus has considerable potential for development as a microbial pesticide. Optimization of fermentation conditions and the wettable powder (WP) formulation is critical for its industrialization and application in sustainable agriculture. In this study, the fermentation of B. atrophaeus YL84 was optimized using single-factor experiments and response surface methodology. Based on these results, a WP formulation was developed and further optimized. The optimal carbon, nitrogen, and inorganic salt sources were sucrose (13.9 g·L−1), tryptone (11.8 g·L−1), and MgSO4 (5.9 g·L−1), respectively; optimal fermentation conditions were pH 7.0, 32 °C, and 210 r·min−1. After optimization, the inhibition rate and OD600 reached 83.71% and 1.758, respectively. The optimized formulation comprised attapulgite-based powder (79%, as carrier), sodium alkyl naphthalene sulfonate (5.4%) as a wetting agent, PEG-6000 (12.6%), CaCO3 (2%), and vitamin C (1%). The resulting WP exhibited a spore viability of 2.63 × 109 CFU·g−1, and its 50-fold dilution demonstrated antagonistic activity in vitro against Cytospora pyri (Korla pear valsa canker agent) and biocontrol efficacy in vivo on detached-branch assays. These findings demonstrate that the YL84 WP is a promising candidate for the biological control of Korla pear valsa canker.