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

<i>Bacillus atrophaeus</i> 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 <i>B. atrophaeus</i> 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 <i>Cytospora pyri</i> (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.