Plasmopara viticola, the causal agent of grapevine downy mildew, is one of the most devastating grape pathogen. Phytochemicals are generally used to control infections, but the appearance of resistant strains and the concern for possible adverse effects on environment and human health are increasing the search for alternative strategies. Biological control has received a great deal of attention as an alternative and promising measure to control different plant diseases. Many antagonistic microorganisms, including Bacillus spp., have been exploited against different pathogens. Bacillus spp. activity results from multiple modes of action including antibiosis, competition, and induction of the host systemic resistance. Bacillus production of the antibiotics lipopeptides, especially surfactin, iturin, and fengycin, is known as an efficient way to suppress diseases. We have isolated the Bacillus subtilis strain GLB191 and demonstrated it has a strong activity against grapevine downy mildew. Assays performed in greenhouses showed a reduction of more than 95 % of the leaf sporulating area. The bacterial activity results from both direct effect against the pathogen, and indirect effect via stimulation of the plant defenses. This strain indeed induces defense gene expression and defense reactions such as callose production. Mutants affected in lipopeptide production were obtained to further investigate the mode of action of this strain. The absence of production of the targeted lipopeptides was confirmed using high-performance thin-layer chromatography (HPTLC). Activity against downy mildew was lost for double mutants affected in both fengycin and surfactin, suggesting a key role of these two lipopeptides in the direct mode of action of Bacillus subtilis strain GLB191. This study shows that Bacillus subtilis strain GLB191 could be a potential biological agent to control grapevine downy mildew. It would be interesting to check activity against other grapevine diseases, especially Grapevine Trunk Diseases, and also against other crop pathogens.