Photoprotection of a Stilbenoid Extract for Downy Mildew Control

Stilbenoids are metabolites with inhibitory activity against plant pathogens, such as Plasmopara viticola, the causal agent of grapevine downy mildew. However, their activity is strongly reduced under UV-A exposure because of photodegradation. This study explores strategies to improve stilbenoid photostability in a grapevine cane extract (CE) while preserving its bioactivity. Among these, encapsulating CE stilbenoids in a cellulose nanocrystal-stabilized oil-in-water emulsion achieved 53% encapsulation efficiency and proved most effective, limiting stilbenoid photodegradation to 37.5% after 1 h of UV-A exposure, compared to 73.9% in the control. However, in this case, the biological activity against downy mildew was loosened. Combinations of CE with a flavonoid (hesperidin methyl chalcone or rutin) also improved stilbenoid photostability, although less markedly, while still significantly preserving the activity of CE against P. viticola. These findings highlight that such photoprotection strategies support the realistic use of stilbenoid extracts as eco-friendly alternatives for sustainable disease management in grapes.