Spray-Induced Silencing of Grape Powdery Mildew Genes to Reduce Powdery Mildew Growth

Powdery mildews are widespread pathogens of grapevine that are difficult to control. Resistance has emerged against all current fungicides and health consequences associated with extensive use of sulfur are beginning to surface. Therefore, the grape industry is in great need of new methods of limiting powdery mildew disease. The potential of spray induced gene silencing (SIGS) in agricultural pest control has been recently realized. The method is also useful in characterizing gene function. The efficiency of SIGS has been demonstrated to control the growth of Fusarium in barley and Botrytis in several plant host species. With the first year of AVF funding support, we showed that SIGS can also be effective in silencing powdery mildew genes, resulting in reduced powdery mildew growth and reproduction. We optimized dsRNA design, application method, dosage, timing of application and powdery mildew growth assessment for testing of SIGS against powdery mildew target genes in both powdery mildew G. orontii -Arabidopsis and powdery mildew E. necator-grapevine systems. With the continued support of AVF in Year 2, we screened powdery mildew genes prioritized to impact metabolic and regulatory pathways critical to powdery mildew colonization, growth, and reproduction. All selected G. orontii target genes had homologs in E. necator and are conserved among powdery mildews. dsRNA against individual target genes were designed, applied exogenously and the growth of powdery mildew was quantified. We had a 60% success rate in identifying efficacious novel targets, with reductions in powdery mildew proliferation ranging from 2- to 5-fold compared to the controls. The initial screening of target genes was done using the Arabidopsis-powdery mildew system as it was faster than grapevine. We then selected 6 genes that showed significant reduction in G. orontii growth on Arabidopsis via SIGS and tested them using the grapevine system. SIGS against each of these six E. necator genes showed significant reduction in powdery mildew growth and reproduction on grapevine. This growing season, we are testing SIGS against two genes that showed maximum reduction in powdery mildew growth on grapevine in research field trials, performed with the assistance of cooperator UCCE viticulture advisor George Zhuang. dsRNAs are biodegradable, flexible, and specific, with reduced negative environmental and health impacts compared with existing fungicides giving them excellent potential as future powdery mildew disease mitigation agents. Introduction of this research to vineyard managers, growers, owners and other stakeholders at the UC Davis Continuing and Professional Education course ‘Current Wine and Winegrape Research’, held in February 2019, and the ‘8th Annual Vineyards & Wineries Continuing Education Class Series’ organized by Napa County Farm Bureau Foundation and the Ag Commissioner’s Office in November 2019 was well received, and productive discussions ensued. We have successfully developed the methodology to identify novel powdery mildew targets that when silenced reduce powdery mildew disease of grapevine and are translating and testing this approach in the vineyard in a highly timely manner.