Bacillus subtilis 0618A Volatiles Inhibit Sclerotium rolfsii and Enhance Soil Health in the Biocontrol of Peanut Southern Blight

Peanut southern blight, caused by Sclerotium rolfsii, is a severe soil-borne disease. Given the adverse effects associated with chemical fungicides, this study investigated the biocontrol potential of volatile organic compounds (VOCs) from Bacillus subtilis 0618A. VOCs produced by B. subtilis 0618A at 1 × 108 CFU/mL almost completely inhibited mycelial growth and sclerotia germination. Compared to common fungicides, the VOCs (1 × 108 CFU/mL) effectively penetrated soil (up to 2 cm depth) to inhibit sclerotia germination and enhance key peanut defense enzyme activities (SOD, POD, and PPO). The indoor efficacy of VOCs was 75.28%, which was significantly higher than those of 100 mg/L tebuconazole (62.92%) and 300 mg/L polyoxin (46.07%). VOCs also significantly increased soil bacteria and actinomycetes populations and improved sucrase activity. Furthermore, 2,4-di-tert-butylphenol (2,4-DTBP) was identified as the primary active VOC against S. rolfsii (EC50 = 0.23 μL/L). Both VOCs and 2,4-DTBP could destroy the integrity of the cell wall and cell membrane, reduce mitochondrial membrane potential, block ATP synthesis, and ultimately lead to fungal death.