Grape Powdery Mildew Reduction

SUMMARY Spray-Induced Silencing of Grape Powdery Mildew Genes to Reduce Powdery Mildew Growth PI: Dr. Mary Wildermuth, University of California, Berkeley Senior researcher: Dr. Jyoti Taneja, University of California, Berkeley Powdery mildew is the dominant disease of grapevine. It infects all grape varietals and vines are typically treated 9-11 times over the growing season. Despite well-planned treatments, infection with powdery mildew can still occur and tolerance levels on grapes is very low. Many commonly used fungicides for powdery mildew control are becoming less effective as powdery mildews develop resistance and there is a demand for safer treatments. In this multi-year project, we are developing a novel biological powdery mildew control. The process or technology is called Spray Induced Gene Silencing (SIGS), where the RNAi molecules are designed to target powdery mildew genes that are essential for infection development and growth. Using endogenous RNAi machinery, the designed long double-stranded (ds) RNA is processed into multiple small interfering RNAs (siRNAs) that result in cleavage of the powdery mildew target gene transcript and its degradation resulting in reduced powdery mildew growth and reproduction. In previous funding years, we developed and optimized the method for SIGS assay using CYP51 as target gene, which is a target for FRAC group-3 fungicides. Using our optimized multi-pronged method for selecting conserved powdery mildew genes likely to be critical to powdery mildew growth and reproduction, we screened numerous target genes for their ability to control powdery mildew when silenced using SIGS, and have filed a PCT patent application with >200 of predicted and tested targets. We developed two pathosystems – the Arabidopsis-G. orontii powdery mildew system for ease of use and the grapevine-E. necator powdery mildew system for commercial relevance. Effective SIGS targets identified in the Arabidopsis powdery mildew system were also effective in the grapevine powdery mildew system. Furthermore, effective SIGS targets identified in growth chamber/greenhouse studies with Arabidopsis and grapevine powdery mildew systems were also effective in limiting powdery mildew in vineyard trials (performed at 2 sites in 2021 growing season). Our results suggest SIGS treatment could be used as a replacement for systemic fungicides in mid-growing season. Furthermore, the SIGS treatment had no impact on canopy or berry development, or berry chemistry. In 2022, we performed i) pilot studies to assess the systemic action of our applied dsRNA, ii) developed protocols to isolate high quality RNA from grape leaves and berries for quantification of the applied dsRNA and derived siRNAs, iii) tested two formulations/delivery methods in lab and one in a pilot field test with a new commercial collaborator to ascertain whether the formulation/delivery methods would be suitable for use and able to increase efficacy of the dsRNA treatment in reducing powdery mildew proliferation, and iv) tested two new dsRNAs against novel powdery mildew targets.