Fungicide Sensitivity Survey of Plasmopara viticola Populations in Georgia Vineyards

Abstract

Grapevine downy mildew, caused by Plasmopara viticola, is among the most damaging diseases of grapes globally and in the viticultural regions of Georgia (U.S.A.). Although management of this disease typically involves fungicide applications, resistance development in P. viticola can render chemical management ineffective. The objective of this study was to survey fungicide sensitivities of P. viticola populations in vineyards across three regions in Georgia. Samples were collected from eight vineyards in 2017 and four in 2018 and tested phenotypically for sensitivity to quinone outside inhibitor (QoI), carboxylic acid amide (CAA), and phenylamide (PA) fungicides using leaf disc bioassays. In addition, DNA was extracted from P. viticola samples collected from 2015 through 2018 in these 12 vineyards and six others for a total of 18 vineyards sampled. All available DNA samples were tested for the presence of the known resistance-causing mutations G143A (QoI) and G1105S (CAA) using polymerase chain reaction (PCR). This study documented widespread occurrence of QoI resistance in P. viticola in Georgia, with 83.0% of isolates collected in 2017 and 2018 testing positive for QoI resistance based on bioassay and 82.9% of isolates collected from 2015 to 2018 testing positive for QoI resistance through PCR testing. In total, 94.4% (17/18) of surveyed vineyards had confirmed QoI resistance by the conclusion of the survey period. No reduced sensitivity to CAA or PA fungicides was identified. Due to widespread QoI resistance, these fungicides should not be relied upon for downy mildew management in Georgia vineyards with a long history of their use.

Sensitivity of Plasmopara viticola to selected fungicide groups and the occurrence of the G143A mutant in Australian grapevine isolates

“…Similar results were found in Georgia in 2020, where phenotypic and genotypic resistance were not detected in any P. viticola isolates. 40 Mandipropamid is a relatively new fungicide in Australia, and as cross-resistance has been demonstrated between dimethomorph and mandipropamid, 24,25 it is important to continue monitoring for resistance to mandipropamid.…”

“…Leaves were blotted dry on sterile paper towel and 10 mm discs were excised using a sterile cork borer. Isolates collected from 2017 to 2018 were tested by floating 15,28,38,39 or dipping 40 leaves in fungicide suspensions, and for those collected from 2019 to 2022 the protocol was amended to spraying fungicides 26,41 …”

“…Leaves were blotted dry on sterile paper towel and 10 mm discs were excised using a sterile cork borer. Isolates collected from 2017 to 2018 were tested by floating 15,28,38,39 or dipping 40 leaves in fungicide suspensions, and for those collected from 2019 to 2022 the protocol was amended to spraying fungicides. 26,41 For the floating method, ten leaf discs (pseudoreplicates) were placed abaxial side up and floated on the surface of the preprepared fungicide suspensions contained in each of two replicate 150 mm glass Petri dishes per treatment.…”