Abstract:Objective To explore the antifungal activity and inhibitory mechanism and identify the main antagonistic substances of the crude extract of Pseudomonas protegens FD6 in fermentation.Methods The plate confrontation test was employed to screen the optimal medium, extractant, and solvent and determine the antifungal activity stability of the crude extract. The main antifungal substances in the crude extract were identified by MALDI-TOF/TOF mass spectrometry, and the antifungal spectrum of the crude extract was explored. The inhibition mechanism of the crude extract against Botrytis cinerea was investigated by mycelial morphology observation, spore germination test, PI staining, and conductivity determination.Results The crude extract of FD6 in the potato juice medium without any carbon source displayed the strongest antifungal activity. The methanol-solved crude extract obtained with ethyl acetate as the extractant had the strongest inhibitory activity. The main antifungal substance of the crude extract was identified as a cyclic lipopeptide, orfamide A, based on mass spectrometry data. In addition, the crude extract displayed broad-spectrum inhibitory activities against Phomopsis amygdali, Fusarium graminearum, Colletotrichum gloeosporioides, and Monilinia fructicola. The antifungal activity of the crude extract was stable after treatment at 121 ℃ for 20 min, within the range of pH 4.0-10.0, and with protease K and trypsin. The mycelial morphology of B. cinerea became abnormal after treatment with the crude extract. The treatment with 2.0 mg/mL crude extract completely inhibited the conidial germination of B. cinerea, caused a 50% damage of the cell membrane, increased the conductivity, and led to leakage of intracellular nucleic acids and proteins.Conclusion The metabolites produced by P. protegens FD6 have broad-spectrum and stable antifungal activities, serving as lead compounds for exploration of new green fungicides.