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

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, i...

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Bibliographic Details
Published in:	Crop protection Vol. 139; p. 105371
Main Authors:	Campbell, Sarah E., Brannen, Phillip M., Scherm, Harald, Eason, Nathan, MacAllister, Clark
Format:	Journal Article
Language:	English
Published:	Elsevier Ltd 01-01-2021
Subjects:	Downy mildew Fungicide resistance Grape Plasmopara viticola Vitis vinifera Plasmopara viticola Downy mildew Vitis vinifera Grape Fungicide resistance
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Summary:	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. •QoI resistance in Plasmopara viticola populations was confirmed in Georgia (USA).•CAA fungicides currently provide excellent protection against downy mildew.•Potassium phosphite combined with captan or cyazofamid is also efficacious.
ISSN:	0261-2194 1873-6904
DOI:	10.1016/j.cropro.2020.105371