Efficacy of fungicide treatments for Plasmopara viticola control and occurrence of strobilurin field resistance in vineyards in Georgia, USA

Abstract

Grapevine downy mildew, caused by Plasmopara viticola, is a major disease of cultivated wine grapes in Georgia (USA), where the climate is favorable for rapid epidemic development. Although loss of fungicide sensitivity has been documented for P. viticola populations in other viticultural regions, it had not been confirmed previously in Georgia. Downy mildew samples from three vineyards, two managed commercially and one located at a research station, were subjected to polymerase chain reaction (PCR) assays to identify the presence of known mutations conferring quinone outside inhibitor (QoI) and carboxylic acid amide (CAA) fungicide resistance. PCR assays detected the presence of the G143A mutation which confers resistance to the QoI fungicides in 100% of samples (n = 39) and the absence of the G1105S mutation that confers resistance to CAA fungicides at all three locations. To determine fungicide efficacy, field trials were conducted in these same vineyards with 10 oomycete-active fungicides or combinations. A significant reduction in efficacy of QoI fungicides (azoxystrobin and pyraclostrobin), with disease levels statistically equivalent to those of untreated vines, was observed. Based on the presence of the G143A mutation that confers resistance to QoIs and the associated field disease control failures observed where QoIs were applied, this fungicide class may no longer be effective for downy mildew management in many Georgia vineyards.