Do Plant Resistance Inducers Reduce <i>Plasmopara viticola</i> 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 the cultivars Barbera and Merlot from the end of flowering (growth stage [GS] 69) to fruit setting (GS 71) and inoculated with a sporangial suspension of <i>Plasmopara viticola</i> (<i>Pv</i>) at 1, 3, 6, 12, and 19 days after treatment (DAT). Cerevisane (CER) and <i>Pythium oligandrum</i> (PYT) did not reduce DM severity compared with the nontreated control (NT). Fosetyl-Al (FOS) reduced DM severity at 1 to 6 DAT, with >50% reduction compared with NT. Laminarin (LAM) was effective at 6, 12, and 19 DAT, with 42.7 to 50.0% efficacy compared with 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 to 71, and pea-sized berries (GS 75), which were then inoculated with <i>Pv</i> at 7 DAT. At GS 60, treatments with LAM and PHO reduced disease severity compared with NT by 77.5 and 83.6%, respectively. At GS 69 to 71, LAM, PHO, and FOS caused 54.7 to 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.