Sustainable Control of Grapevine Downy Mildew: 24-Epibrassonolide Triggers Distinct Defense Strategies in Grapevine Cultivars.

Grapevine downy mildew, caused by Plasmopara viticola, poses significant threats to viticulture, necessitating sustainable alternatives to chemical fungicides. This study investigates the efficacy of exogenous 24-epibrassinolide (EBR), a brassinosteroid, in enhancing disease resistance across two grape cultivars: susceptible Vitis vinifera Cabernet Sauvignon and resistant Ecolly. EBR application reduced P. viticola colonization in both cultivars, suppressing hyphal growth and sporulation. In Cabernet Sauvignon, EBR delayed pathogen establishment by modulating stomatal closure and boosting antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase, peroxidase) and pathogenesis-related proteins (chitinase, β-1,3-glucanase). Ecolly exhibited inherent resistance, characterized by localized hypersensitive responses and elevated salicylic acid (SA) and jasmonic acid (JA) levels post-EBR treatment, which amplified systemic acquired resistance (SAR) via upregulation of PAD4, EDS1, and PRs genes. Stomatal regulation in Cabernet Sauvignon correlated with EBR-induced abscisic acid (ABA) dynamics, while Ecolly maintained constitutive stomatal defense. Transcriptional analysis revealed cultivar-specific phytohormonal interplay, with SA pathways dominating in Ecolly and delayed SA accumulation in Cabernet Sauvignon. These findings demonstrate that EBR enhances resistance through multifaceted mechanisms-pathogen inhibition, redox homeostasis, hormonal signaling, and genetic priming-with efficacy modulated by genetic background. This study underscores brassinosteroid as a sustainable tool for integrated disease management in grapevines.