Aim:To investigate the antifungal efficacy of grape cane extracts enriched in stilbenes against Plasmopara viticola by in vivo experiments on grape plants. Methods and results:Experiments on grape plants were carried out from 2007 to 2008 at commercial vineyards in « Le Tourne » (southwest region of France) on Vitis vinifera L. cv Merlot plants and in a greenhouse, where 45-day-old plants (Merlot) were used. Grapevine plants were treated with stilbene extract (STE) or « Bordeaux mixture » (BM) as positive control. STE, prepared from Vitis vinifera canes, contained 33% of total stilbenes ranging from monomers to tetramers and was used at 5g L -1 . The degree of protection for downy mildew was determined as the percentage of infected leaf surface area and pathogen attack frequency. On young plants in greenhouse, P. viticola inoculation and three treatments with STE were carried out. After the treatments, the disease reduction was from 59 to 69% for pathogen attack frequency and from 83 to 88% for infected leaf surface for the two modes of infestation (artificial and secondary). STE triggered a similar intense decrease in downy mildew infection as recorded for BM. In the vineyard assay, downy mildew infection was allowed to occur naturally. Field grown plants were treated every 7 th day for eight weeks or every 14 th day for eight weeks during June and July. We observed that STE was also able to prevent the infection of P. viticola in vineyard but to a lesser extent. Treatment with STE every 7 th day provided better results in terms of infection frequency and leaf area than that every 14 th day. STE treatment of plants every 7 th day in vineyard reduced the frequency of attack by 16 to 39% and leaf surface infection by 57 to 61%, which was lower than that recorded for BM. STE also displayed in vitro inhibitory effects on the release of zoospores from sporangia and the germination of zoospores of P. viticola. Zoospore germination was completely inhibited at a concentration of 100 mg L -1 and zoospore release was inhibited at 330 mg L -1 . Conclusions:Treatment of young grapevine plants in greenhouse with stilbene extract from grapevine canes offered considerable protection against P. viticola that was identical to that provided by Bordeaux mixture. STE was also able to prevent the infection of P. viticola in vineyard but to a lesser extent. Treatment with STE every 7 th day in vineyard provided better results than that every 14 th day.Significance and impact of the study: Stilbenes from grapevine canes are good candidates as natural fungicides against downy mildew. Given the very large quantity of grape canes available each year, this strategy could be scaled up to control this grapevine disease in a sustainable manner.Abstract manuscript

<p style="text-align: justify;"><strong>Aim:</strong> To investigate the antifungal efficacy of grape cane extracts enriched in stilbenes against <em>Plasmopara viticola</em> by <em>in vivo</em> experiments on grape plants.</p><p style="text-align: justify;"><strong>Methods and results</strong>: Experiments on grape plants were carried out from 2007 to 2008 at commercial vineyards in « Le Tourne » (southwest region of France) on <em>Vitis vinifera</em> L. cv Merlot plants and in a greenhouse, where 45-day-old plants (Merlot) were used. Grapevine plants were treated with stilbene extract (STE) or « Bordeaux mixture » (BM) as positive control. STE, prepared from <em>Vitis vinifera</em> canes, contained 33% of total stilbenes ranging from monomers to tetramers and was used at 5 g L^-1. The degree of protection against downy mildew was determined as the percentage of infected leaf surface area and pathogen attack frequency. On young plants in greenhouse, <em>P. viticola</em> inoculation and three treatments with STE were carried out. After the treatments, the disease reduction was from 59 to 69% for pathogen attack frequency and from 83 to 88% for infected leaf surface for the two modes of infestation (artificial and secondary). STE triggered a similar intense decrease in downy mildew infection as recorded for BM. In the vineyard assay, downy mildew infection was allowed to occur naturally. Field grown plants were treated every 7^th day for eight weeks or every 14^th day for eight weeks during June and July. We observed that STE was also able to prevent <em>P. viticola</em> infection in vineyard but to a lesser extent. Treatment with STE every 7^th day provided better results in terms of infection frequency and infected leaf area than that every 14^th day. STE treatment of plants every 7^th day in vineyard reduced the frequency of attack by 16 to 39% and leaf surface infection by 57 to 61%, which was lower than that recorded for BM. STE also displayed <em>in vitro</em> inhibitory effects on the release of zoospores from sporangia and the germination of zoospores of <em>P. viticola</em>. Zoospore germination was completely inhibited at a concentration of 100 mg L^-1 and zoospore release was inhibited at 330 mg L^-1.</p><p style="text-align: justify;"><strong>Conclusions</strong>: Treatment of young grapevine plants in greenhouse with stilbene extract from grapevine canes offered considerable protection against <em>P. viticola</em> that was identical to that provided by Bordeaux mixture. STE was also able to prevent <em>P. viticola</em> infection in vineyard but to a lesser extent. Treatment with STE every 7^th day in vineyard provided better results than that every 14^th day.</p><p style="text-align: justify;"><strong>Significance and impact of the study</strong>: Stilbenes from grapevine canes are good candidates as natural fungicides against downy mildew. Given the very large quantity of grape canes available each year, this strategy could be scaled up to control this grapevine disease in a sustainable manner.</p>