Preventing Trunk Diseases with Fungicide Applications to Pruning Wounds in Washington Winegrapes

Abstract

<h3>Abstract</h3> <h3>Background and Goals</h3> Grapevine trunk diseases in the Columbia River Basin of eastern Washington include Cytospora dieback, Eutypa dieback, and Esca. Although some of the causal fungi are known (as <i>Cytospora viticola, Eutypa lata,</i> and <i>Phaeomoniella chlamydospora,</i> respectively), basic epidemiology is not. This makes it difficult to time management practices. The common assumption is that these pathogens infect through pruning wounds during the dormant season, as has been shown for causal fungi of some grapevine trunk diseases in California. As such, we evaluated fungicides for protecting wounds after pruning under eastern Washington conditions. <h3>Methods and Key Findings</h3> In March of 2019, 2020, and 2021, we evaluated the protection efficacy of pyraclostrobin + fluxapyroxad and thiophanate-methyl sprayed within 3 days of pruning, at an established <i>Vitis vinifera</i> ‘Chardonnay’ vineyard in Prosser, WA. Within 2 days of fungicide treatment, <i>C. viticola, E. lata,</i> or <i>P. chlamydospora</i> (2,000 spores per wound) were inoculated separately onto spurs, and molecular-detection attempts were made 5 to 8 weeks later (after budbreak). Compared to water-treated spurs, detection rates of <i>C. viticola</i> and <i>P. chlamydospora</i> from thiophanate-methyl-treated spurs were lower in all three study years. Detection rates of <i>E. lata</i> from thiophanate-methyl-treated spurs were lower in one year. <h3>Conclusions and Significance</h3> This suggests that dormant-season spray applications of thiophanate-methyl as a pruning-wound protectant can reduce grapevine spur infection by these pathogens. Little-to-no rain during the dormant season in eastern Washington may limit opportunities for disease spread, but winter injury to the permanent, woody structure of the vine may create additional infection courts.