Synergistic Effects of [Ile⁷]Surfactin Homologues with Bacillomycin D in Suppression of Gray Mold Disease by<i>Bacillus amyloliquefaciens</i>Biocontrol Strain SD-32

We previously reported that Bacillus amyloliquefaciens biocontrol strain SD-32 produces powerful antifungal lipopeptides, C17 bacillomycin D homologues. In the course of the investigation we found that the antifungal activity of the culture supernatant of this bacterium was not ascribed exclusively to bacillomycin D. We attempted to identify metabolites other than bacillomycin D to gain insight into the mechanism for the biocontrol by this bacterium. After purifying the fractions of the culture supernatant exhibiting synergistic activity with bacillomycin D, we isolated two new cyclic lipodepsipeptides, anteiso-C13 and iso-C13 [Ile(7)]surfactins, together with three known [Ile(7)]surfactins. Interestingly, [Ile(7)]surfactins showed synergistic activities with bacillomycin D to gray mold disease on cucumber leaves but not to Botrytis cinerea itself in vitro, suggesting that the synergistic effects might be on infection processes of the fungus. Actually, we observed that they did not show synergistic actions on conidial germination or mycelial growth of B. cinerea on the leaves.