Bacillus velezensis LT-22 Volatiles for the Biocontrol of Phytophthora capsici: Antifungal Action and Underlying Mechanisms

<i>Phytophthora capsici</i>, an invasive oomycete pathogen causing blight in crops like cucurbits, tomatoes, and peppers, has led to significant economic losses. Due to the limitations of conventional control methods, research has focused on effective and eco-friendly biocontrol alternatives. The bacterial strain LT-22, identified as <i>Bacillus velezensis</i>, exhibits broad-spectrum antifungal activity and plant growth-promoting potential. This study investigated the biocontrol efficacy of volatile organic compounds (VOCs) from strain LT-22. LT-22 VOCs strongly inhibited <i>P. capsici</i> PC153 growth in vitro and reduced disease symptoms in pepper fruits. Microscopic observations showed that LT-22 VOCs caused significant morphological changes in <i>P. capsici</i> PC153 mycelium by disrupting cell wall and membrane integrity. The treatment increased malondialdehyde (MDA) levels and reduced cellulose content, further indicating that LT-22 VOCs compromise cell integrity to exert antifungal effects. Furthermore, 2-methylbutanoic acid was identified as the primary antifungal VOC, with an EC₅₀ value of 0.095 µL/mL against <i>P. capsici</i> PC153, lower than the reported plant-derived compounds. Together, these results indicate that strain LT-22 and its VOCs, especially 2-methylbutanoic acid, have significant potential as an effective biocontrol agent for managing <i>Phytophthora</i> blight.