Alternative Effects Yeast-Based Biostimulants Against Downy Mildew in <i>Vitis vinifera</i> cv Cabernet Sauvignon

Grapevine downy mildew, caused by <i>Plasmopara viticola</i>, 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, an experimental formulation, and YE2, Romeo, a commercial preparation containing Cerevisane^®) in mitigating <i>P. viticola</i> infections in <i>Vitis vinifera</i> 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 promoted the accumulation of flavonols, such as quercetin glycosides and kaempferol glucosides, which are known to contribute to pathogen resistance through antimicrobial activity and ROS detoxification. Compared to the untreated sample, quercetin glycosides levels in YE1 leaves were approximately 700 µg/g higher (+9.6%), in comparison to control conditions, on a dry weight basis after the first treatment in 2019 and about 900 µg/g higher (+13.7%) after the final treatment in 2020. A similar trend was observed for kaempferol concentrations, which were 70 µg/g (+98.7%) and 100 µg/g higher (+24.6%) on the same dates. In contrast, YE2 enhanced the biosynthesis of caftaric and coutaric acids, known for their antioxidant and antimicrobial properties. These compounds were induced by YE2 over time and, after the last treatment, in 2019 caftaric acid levels were approximately 345 µg/g higher (+25.9%), than control condition, while coutaric acid levels were 30 µg/g higher (+33.8%). The stilbene piceid was also induced at early stages by treatments, indicating its role in enhancing plant defenses. In 2019, after the first treatment, both YE1 and YE2 applications induced an increase in piceid concentration, compared to the control, of approximately 2.5 µg/g (+104.8%) for YE1 and 1.3 µg/g for Y2 (+55.0%). In contrast, in 2020, the effect was more pronounced for YE2, with concentrations exceeding the control by 2 µg/g (+46.2%). 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.