Plasmopara viticola (Pv), the causal agent of downy mildew, is one of the most damaging pathogens affecting grapevine. Current control strategies largely depend on copper-based fungicides and synthetic chemicals, raising increasing concerns related to environmental sustainability and pathogen resistance. This study evaluated the efficacy of a novel Saccharomyces cerevisiae extract (U) as an inducer of resistance in the grapevine–Pv interaction. Microscopic observations revealed the ability of U