Postharvest biological control of Colletotrichum acutatum on apple by Bacillus subtilis HM1 and the structural identification of antagonists

Bacillus subtilis HM1 was isolated from the rhizosphere region of halophytes for its antifungal activity against Colletotrichum acutatum, the causative agent of anthracnose. Treatment of postharvest apples with the cell culture or with a cell-free culture supernatant reduced disease severity 80.7% and 69.4%, respectively. Both treatments also exhibited antifungal activity against various phytopathogenic fungi in vitro. The antifungal substances were purified and analyzed by acid precipitation, gel filtration, high-performance liquid chromatography, and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). Three compounds were identified as fengycin, iturin, and surfactin. The MALDI-TOF/TOF mass spectrum revealed the presence of cyclized fengycin homologs A and B, which were distinguishable on the basis of the presence of either alanine or valine, respectively, at position 6 of the peptide sequence. In addition, the cyclized structure of fengycin was shown to play a critical role in antifungal activity.