Antifungal Lipopeptide Production and Tomato Early Blight Control by Bacillus velezensis B157

Bacillus strains used as biocontrol agents (BCAs) for plant diseases produce antimicrobial metabolites, especially cyclic lipopeptides (LPs), that protect aerial plant parts against pathogens. The Bacillus strain B157 was previously isolated from coffee and shown to reduce coffee leaf rust severity in the field. The biocontrol mechanism of B157 was attributed to antibiosis, but the nature of the antifungal metabolites and the correct identity of this BCA remained unclarified. We investigated the diversity of LPs of B157 and evaluated their antifungal activity against the tomato pathogen Alternaria linariae. The efficacy of the crude organic extract of B157 in controlling tomato early blight in the greenhouse was also tested. Multilocus phylogenetic analyses (16S rDNA, gyrA, rpoB) identified B157 as Bacillus velezensis. Twenty-seven LPs were detected in the extract of B157 and identified as iturins (n=5), fengycins (n=13) and surfactins (n=9). A well-diffusion assay with chromatographic fractions of the bioactive extract showed that all fractions having antifungal activity contained mixtures of fengycins or the combination of fengycins and iturins. In two independent assays, preventive treatments of tomato plants with the crude extract of B157 (1 g/L) significantly reduced the severity of early blight. The disease control efficacy (>90%) was similar to the fungicide treatment (chlorothalonil). Our results suggest that biochemical fungicides enriched in LPs may be used for protecting aerial plant parts against phytopathogens. This approach may complement the common use of Bacillus cell suspensions containing diluted and variable amounts of LPs that can give inconsistent control results in the field.