Pre‐infection efficacy of resistance inducers against grapevine powdery mildew

Plant resistance inducers (PRIs) are of increasing interest for disease control in sustainable viticulture, but their efficacy in vineyards is still under investigation. We studied the effects of four commercial PRIs, of natural and microbial origin, on the development of powdery mildew (PM) on grapevine leaves over a 3‐year period. Disease and sporulation severity were evaluated following the artificial inoculation of PRI‐treated leaves with Erysiphe necator conidia at 1, 3, 7 and 12 days after treatment (DAT). Laminarin (LAM) and cos‐oga (COS) were the most effective in reducing both disease (57.1% and 49.5%, respectively) and sporulation (85.3% and 90.2%, respectively) severity; the reduction in PM severity provided by cerevisane (CER) was lower (27%), and that of Pythium oligandrum (PYT) was intermediate (44.9%). LAM was effective from 1 DAT, while COS and PYT were more effective when E. necator was inoculated at 7 to 12 DAT. These results open new perspectives on how PRIs are used as single, preventative interventions performed 1 to 3 days before E. necator infection, depending on the product used. The effect of PRIs on the sporulation of E. necator colonies also suggests a broader and longer effect on PM epidemics.