The invention relates to a strain of Saccharomyces cerivisiae characterized in that it is selected from strain No. I-3936 deposited on Mar. 4, 2008 at the CNCM, strains No. I-3937 deposited on Mar. 4, 2008 at the CNCM, strain No. I-3938 deposited on Mar. 4, 2008 at the CNCM, and strain No. I-3939 deposited on Mar. 4, 2008 at the CNCM. The invention also relates to a phytosanitary composition and to a method for treating or protecting plants against diseases caused by pathogens, using said strain.