Inhibitory Effect of Bacillus velezensis dhm2 on Fusarium oxysporum f. sp. cucumerinum and Synergistic Activity of Crude Lipopeptide Extract with Chemical Fungicides

Fusarium oxysporum f. sp. cucumerium, a resilient saprophytic fungus, poses a significant risk to cucumber crops. The research investigated the suppressive impact of Bacillus velezensis dhm2 on this pathogen and the synergistic performance of its crude lipopeptide extract with synthetic fungicides. Strain dhm2 inhibited the pathogen by 52.27% in confrontation culture. Its fermentation supernatant showed peak activity at 4 h bacterial age and 60 h fermentation duration, while the crude lipopeptide extract had an EC50 of 9.99 g L−1. Among the six chemical fungicides, prochloraz exhibited the highest toxicity, with an EC50 value of 0.03 μg mL−1. In all mixed combinations of the crude lipopeptide extract and chemical fungicides, there existed synergistic mixing ratios, particularly with difenoconazole (volume ratio 7:3, synergistic ratio 5.88) and propiconazole (7:3, 3.41), as confirmed by Wadley tests. Pot experiments revealed that the combined use of the crude lipopeptide extract and difenoconazole controlled cucumber Fusarium wilt by 80.95%. The mixture showed the highest SOD (315.76 U g−1 FW min−1), POD (281.63 U g−1 FW min−1), and CAT (23.39 U g−1 FW min−1), with increases over single treatments. This study provides an eco-friendly strategy for managing cucumber wilt, advocating reduced fungicide use via synergistic formulations.