Alternative Effects Yeast-Based Biostimulants Against Downy Mildew in CV Cabernet Sauvignon

Grapevine downy mildew, caused by Plasmopara viticola, is one of the most destructive vineyard diseases worldwide, with the potential to devastate up to 90% of harvests under adverse conditions. Traditional chemical-based control strategies are increasingly restricted due to environmental concerns and the development of resistant pathogen strains, prompting the search for sustainable alternatives. This study evaluated the efficacy of two yeast-based biostimulants (YE1 and YE2) in mitigating P. viticola infections in Vitis vinifera cv Cabernet Sauvignon under open field conditions. Field trials were designed to monitor the effects of these bioproducts on grapevine metabolism, with a focus on the biosynthesis of phenolic compounds linked to plant defense responses. Both biostimulants demonstrated significant efficacy against downy mildew, achieving protection levels above 93% during critical growth stages. Metabolic analyses of leaves revealed distinct impacts of YE1 and YE2 on secondary metabolite dynamics. YE1 primarily stimulated the accumulation of flavonols, such as quercetin glycosides and kaempferol glucoside, which contribute to pathogen resistance through antimicrobial activity and ROS detoxification. In contrast, YE2 enhanced the biosynthesis of caftaric and coutaric acids, known for their antioxidant and antimicrobial properties. The stilbene piceid was also induced at early stages by treatments, indicating its role in enhancing plant defenses. The results suggest that these yeast-based bioproducts act as elicitors, effectively enhancing grapevine immunity and reducing reliance on synthetic inputs. This study provides novel insights into the metabolic mechanisms underlying the efficacy of yeast-based biostimulants against downy mildew and offers practical guidance for their strategic application.