An Endophytic Bacillus amyloliquefaciens ZN-S18 Suppresses Fusarium oxysporum f. sp. lycopersici Through Synergistic Activity of Surfactin and Iturin A

Fusarium wilt, caused by the soil-borne fungus Fusarium oxysporum f. sp. lycopersici (Fol), is a devastating disease that severely constrains tomato production worldwide. In this study, an endophytic bacterium, designated ZN-S18, was isolated from healthy tomato stems and identified as Bacillus amyloliquefaciens based on phylogenetic analyses of the 16S rRNA and rpoB genes. Dual culture assays demonstrated that ZN-S18 exhibited strong antagonistic activity against Fol and significantly inhibited fungal mycelial growth. The cell-free culture filtrate of ZN-S18 markedly suppressed Fol spore germination and mycelial growth, and the inhibitory effect on spore germination was irreversible. LC-MS analyses confirmed that ZN-S18 produces the cyclic lipopeptides surfactin and iturin A. In planta assays further demonstrated that pretreatment with ZN-S18, its culture filtrate, or the combination of surfactin and iturin A significantly reduced Fusarium wilt in tomato seedlings. Moreover, the culture filtrate disrupted fungal cell wall integrity, as indicated by the increased sensitivity of Fol to Congo red stress. Notably, the combined application of surfactin and iturin A exhibited significantly stronger antifungal activity than either compound alone under Congo red-containing conditions, suggesting a synergistic effect on fungal cell wall integrity. Collectively, these findings indicate that the endophytic strain B. amyloliquefaciens ZN-S18 is a promising biocontrol agent against tomato Fusarium wilt, primarily through the synergistic action of surfactin and iturin A. This study provides an environmentally friendly alternative to chemical fungicides and establishes a foundation for future investigations into the molecular mechanisms underlying lipopeptide-mediated antagonism.