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POSTERS: Chemical control

Fungicide sensitivity and efficacy profiling of Georgia Plasmopara viticola isolates causing grapevine downy mildew
Phillip Brannen - University of Georgia. Sarah Campbell- University of Georgia, Harald Scherm- University of Georgia

Georgia vineyards provide an ideal environment for downy mildew epidemics. The causal oomycete, Plasmopara viticola (Pv), can cause near 100% crop loss and vine death if left unmanaged. Management relies on multiple fungicide applications, often leading to resistance development. Although fungicide-resistant populations of Pv were identified in other viticultural regions, no study has evaluated fungicide sensitivities in Georgia. Pathogen samples were collected throughout Georgia and subjected to leaf disk bioassay with seven concentrations of three fungicide classes: (1) a quinone outside inhibitor (QoI), azoxystrobin, (2) a carboxylic acid amide (CAA), mandipropamid, and (3) a phenylamide (PA), mefenoxam. In addition, previously identified point mutations conferring QoI (G143A) and CAA (G1105S) resistance were evaluated utilizing polymerase chain reaction. Furthermore, fungicide efficacy trials with eleven treatments were conducted at three vineyards in North Georgia. Results from bioassays and mutation testing confirmed QoI resistance in 94.4% of Georgia vineyards. All samples were sensitive to both CAA and PA fungicides. Field efficacy trials showed reduced efficacy of QoI fungicides, with disease levels statistically equivalent to those of untreated vines. Based on the frequency of QoI resistance and field trial failures observed where QoIs were applied, QoIs are no longer effective for downy mildew disease management in most Georgia vineyards.