Bacillus subtilis YN2014090 Water-Dispersible Granular Formulation for Controlling Powdery Mildew On Cucumber

Cucumber powdery mildew which is a common and severe fungal disease caused by Podosphaera xanthii (syn. Podosphaera fuliginea, Sphaerotheca fuliginea, and Sphaerotheca fusca), throughout the reproductive period of cucumbers and can cause 10–40% yield reduction. Bacillus subtilis YN2014090 (YN90) is a biocontrol bacterial strain with a good control effect against powdery mildew. The present study developed a water-dispersible granule (WG) of YN90 and evaluated its application effect as an active ingredient. The optimal WG formulation was confirmed by screening the composition and dosage of the carrier, binder, dispersant, disintegrant, and UV-protecting agent. In addition, the main performance indexes, storage stability, and the control effect of the formulation on cucumber powdery mildew were determined. The findings showed that the optimal WG formulation of YN90 was 30% bacterial powder (6.4 × 10¹¹ CFU g⁻¹), 10.8% urea, 1.2% ammonium sulfate, 8% D425, 2% magnesium aluminum silicate, 0.05% carboxymethyl cellulose, and the carrier bentonite added to final 100%. After testing, the WG consist of bacterial load of 1.8 × 10¹¹ CFU g⁻¹ with a pH value of 7.8 ~ 8.0 and less than 5% of contaminated bacteria. The 85% particle size was between 1.0 ~ 4.75 mm and the moisture content was less than 2%. After 12-month storage at room temperature, the WG contains 2.43 × 10¹⁰ CFU g⁻¹ of YN90. Our greenhouse study confirmed that 1000 times dilution of YN90 WG effectively prevented cucumber powdery mildew by 76.49% after 11 days, outperforming the 10 times dilution of YN90 (1.8 × 10⁸ CFU mL⁻¹) and 500 times dilution of 75% Chlorothalonil wettable powder. The chlorophyll a and b of cucumber leaves sprayed with YN90 WG were increased by 72.03%, and 76.11%, respectively. The powdery mildew spores of cucumber were cleaved in the presence of YN90 WG. The findings of this study contribute to the establishment of a stable dosage form for B. subtilis and the development and application of biocontrol agents.