Abstract: To improve the control efficiency of Bacillus velezensis YJ02 against peanut southern blight caused by Sclerotium rolfsii, this study explored the potential of integrating B. velezensis YJ02 with chemical fungicides for disease management. The compatibility of nine chemical fungicides commonly utilized for peanut southern blight control with B. velezensis YJ02 was evaluated using the dilution plating method and colony counting technique. The in vitro toxicity of compatible fungicides and B. velezensis YJ02 against S. rolfsii was determined via the toxic medium method, and the optimal mixing ratio of compatible fungicides to B. velezensis YJ02 was selected based on the toxicity ratio. Pot and field experiments were further conducted to verify the synergistic control effect of the screened composite formulation against peanut southern blight. The results demonstrated that three fungicides, namely thifluzamide, azoxystrobin, and trifloxystrobin, maintained excellent compatibility with B. velezensis YJ02, and thifluzamide exhibited the highest toxicity against S. rolfsii among them. The composite formulation consisting of B. velezensis YJ02 (1× 10² CFU/mL) and thifluzamide (0.1 mg/L) at a volume ratio of 5:5 showed a significant synergistic effect with a toxicity ratio of 1.95. In pot experiments, the high-concentration 5:5 composite formulation (the final concentration was 0.5× 10⁸ CFU/mL B. velezensis YJ02 and 5 mg/L thifluzamide) achieved the optimal preventive control efficacy, which was 55.24% and 46.04% higher than that of B. velezensis YJ02 and thifluzamide applied alone, respectively. Notably, the preventive efficacy of all treatments was significantly better than their corresponding curative efficacy. In field experiments, the control efficacy of the aforementioned high-concentration 5:5 composite formulation was comparable to that of thifluzamide applied at the recommended field dosage, while reducing the application amount of thifluzamide by more than 90%. This study provides a specific technical strategy and theoretical foundation for the green and efficient management of peanut southern blight, contributing to the sustainable control of this soil-borne disease in peanut production.

Key words: Bacillus velezensis YJ02, peanut southern blight, Sclerotium rolfsii, combanition, synergistic effect