Guignardia bidwellii (Ellis; anamorph Phyllosticta ampelicida), a hemibiotrophic fungus responsible for black rot (BR) disease in grapevine, could turn into a serious threat in the near future. Causing potential severe crop loss, it currently benefits from the withdrawal of several synthetic fungicides and the lack of commercial specific biocontrol alternatives. Within the framework of the French research project VITAE, combinations of management options, such as biocontrol and vine genetic resistance, are investigated. To evaluate the efficiency and mode(s) of action of a biocontrol experimental candidate, i.e. a Bacillus velezensis strain, culture supernatants were tested via in planta efficacy bioassays in greenhouse (cv. Artaban & Marselan), and in vitro direct confrontation assays. The application of the bacterial supernatant significantly reduced both BR incidence and severity, with protection rates reaching 90% in planta. Two major modes of action accounting for the supernatant efficiency are put forward and deeply investigated: a direct effect against the pathogen and the elicitation of grapevine defense responses. Thus, the B. velezensis supernatant could act as an efficient biocontrol tool in a future strategy of grapevine protection. The mechanisms of action and their relative importance will be further discussed by quantifying the antibiosis capacity under different conditions and at different infection stages, as well as deciphering the defense induction pattern.