Prevalence of Mutations Associated with QoI, QiI, QioSI and CAA Fungicide Resistance Within Plasmopara viticola in North America and a Tool to Detect CAA Resistant Isolates.

Abstract

Grape downy mildew, caused by Plasmopara viticola poses a threat to grape cultivation globally. Early detection of fungicide resistance is critical for effective management. This study aimed to assess the prevalence and distribution of mutations associated with resistance to Quinone oxide inhibitors (QoI, FRAC 11), Quinone inside inhibitors (QiIs, FRAC 21, cyazofamid), Carboxylic acid amides (CAA, FRAC 41), and Quinone inside and outside inhibitor, stigmatellin binding mode (QioSI, FRAC 45, ametoctradin) in P. viticola populations in the eastern United States and Canada; and evaluate whether these mutations are linked to fungicide resistance correlate with specific P. viticola clades. A total of 658 P. viticola samples were collected from commercial vineyards across different states and years in the eastern United States and Canada and sequenced for the PvCesA3 and cytb genes and the ITS1 region. Results showed P. viticola clades aestivalis, vinifera, and riparia were prevalent in the eastern United States and Canada. QoI resistance was widespread, with the A-143 resistant genotype prevalent in P. viticola clades aestivalis and vinifera. CAA resistance, associated with the G1105S mutation, was mainly identified in P. viticola clade aestivalis from Georgia, New York, and Ontario. A TaqMan-probe based assay was developed to detect G1105S mutation in P. viticola conferring CAA fungicide resistance. The TaqMan assay demonstrated sensitivity at even low DNA concentrations and specificity in distinguishing between sensitive and resistant genotypes. This study provides insights into geographic distribution of fungicide resistance in P. viticola populations and presents a reliable method for detecting CAA resistance in P. viticola.