Do plant resistance inducers reduce infection on grapevine berry clusters at different growth stages?

Plant resistance inducers (PRIs) are promising alternatives to chemical fungicides. Their effectiveness against grapevine downy mildew (DM) has been demonstrated for leaves, yet research on berry clusters is limited. We investigated the efficacy of six PRIs on clusters of cv. Barbera and Merlot from the end of flowering (growth stage GS 69) to fruit setting (GS 71) and inoculated with a sporangial suspension of Plasmopara viticola (Pv) at 1, 3, 6, 12, and 19 days after treatment (DAT). Cerevisane (CER) and Pythium oligandrum (PYT) did not reduce DM severity compared to the nontreated control (NT). Fosetyl-Al (FOS) reduced DM severity at 1-6 DAT, with >50% reduction compared to NT. Laminarin (LAM) was effective at 6, 12, and 19 DAT, with 42.7%-to 50.0% efficacy compared to NT, while cos-oga (COS) and K-phosphonate (PHO) were effective at 12 and 19 DAT (60% efficacy). PRIs were also applied to clusters at initial flowering (GS 60), GS 69-71, and pea-sized berries (GS 75), which were then inoculated with Pv at 7 DAT. At GS 60, treatments with LAM and PHO reduced disease severity compared to NT by 77.5% and 83.6%, respectively. At GS 69-71, LAM, PHO, and FOS caused 54.7%-75.7% disease reduction. At GS 75, all PRIs exhibited a disease reduction > 75%. The efficacy of PRIs increased as the cluster developmental stage advanced, indicating an interaction with the ontogenic resistance of berries. Our results are relevant for the practical use of PRIs in protecting grapevine clusters from DM.