Prevalence of Mutations Associated with QoIs, QiIs, QioSIs, and CAA Fungicide Resistance Within in North America and a Tool to Detect CAA-Resistant Isolates.

Grape downy mildew, caused by , poses a significant 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 outside inhibitor (FRAC 11), quinone inside inhibitor (FRAC 21), carboxylic acid amide (CAA, FRAC 40), and quinone inside and outside inhibitor, stigmatellin binding mode (FRAC 45) fungicides in . populations in the eastern United States and Canada and to evaluate whether these mutations are linked to fungicide resistance correlated with specific . clades. A total of 658 . samples were collected from commercial vineyards across different states and years in the eastern United States and Canada and sequenced for the and genes and the internal transcribed spacer (ITS)1 region. The results showed that . clades , , and were prevalent in the eastern United States and Canada. Quinone outside inhibitor resistance was widespread, and the A-143 resistant genotype was prevalent in .  clades and . The G143A mutation did not show specificity based on clade differentiation. CAA resistance, associated with the G1105S mutation, was mainly identified in . clade from Georgia, New York, and Ontario. G1105-S1105 mixed-genotype samples were observed in . clades and from Wisconsin, Michigan, and New York. However, mutations associated with quinone inside and outside inhibitor and quinone inside inhibitor fungicides were not detected. A TaqMan probe-based assay was developed to detect the G1105S mutation in . conferring CAA fungicide resistance. The TaqMan assay demonstrated sensitivity at low DNA concentrations and specificity in distinguishing between sensitive and resistant genotypes. The assay accurately distinguished the G1105S mutation in leaf and air samples. This study provides insight into the geographic distribution of fungicide resistance in . populations and presents a reliable method for detecting CAA resistance in . . These findings can be utilized to implement effective fungicide resistance management strategies in viticulture.