Of 20 Plasmopara viticola isolates collected in four locations in Virginia and northwest North Carolina in 2005, 16 were resistant to QoI fungicides. The resistance factor was over 100, and label rates of formulated azoxystrobin and pyraclostrobin provided little or no control of these isolates. Additional sampling in 2006 revealed at least 15 additional vineyards with QoI-resistant P. viticola in Virginia, Maryland, and Pennsylvania. Of 22 isolates of Erysiphe (Uncinula) necator collected in 2005 from five Virginia locations, 20 isolates from 4 locations showed resistance to QoI fungicides. The G143A mutation for resistance was detected in several isolates of both pathogens. This is the first detection of this type of resistance in P. viticola in North America, and the second North American report of QoI resistance in E. necator. Accepted for publication 26 November 2007. Published 11 February 2008.

Colcol, J. F., Rallos, L. E., and Baudoin, A. B. 2012. Sensitivity of Erysiphe neeator to demethylation inhibitor fungicides in Virginia. Plant Dis. 96:111-116.Grape powdery mildew {Erysiphe necator) isolates were collected from 2(K)5 to 2007 from vineyards mostly in Virginia but also some in Maryland, North Carolina, and Pennsylvania. Using a leaf disc assay, the isolates were tested against five demethylation inhibitor (DMI) fungicides. Most isolates exhibited reduced sensitivity to the live DMIs when compared with a sensitive group (H = 12) and compared with unexposed populations reported from other areas. The median resistance factor (RF) was highest for tebucona/ole (RF = 399) and myclobutanil (RF = 378), followed by tritlumizole (RF = 70), triadimefon (RF = 62), and fenarimol (RF = 44). Tbe sensitive group used as the basis for comparison appears to have been more sensitive than unexposed isolates in New York and California. Our finding that the greatest resistance shift occurred with tebucona/ole and myclobutanil contrasts with earlier reports from New York and California, whore the greatest resistance shift was observed with triadinielbn or triadimenol. Sensitivities to all five DMI fungicides were strongly correlated (pairwise r values of 0.70 to 0.87) but our data suggest that some may retain greater utility than others.

The sensitivity of downy mildew (DM, Plasmopara viticola) and powdery mildew (PM, Erysiphe necator) of grape (Vitis sp.) to commonly used nondemethylation inhibitor, single-site fungicides in and near Virginia was determined from 2005 to 2007, with more limited additional sampling in subsequent years. In grape leaf disc bioassays, 92% of the P. viticola isolates were quinone outside inhibitor (QoI, azoxystrobin) resistant but none were resistant to mefenoxam. In all, 82% of the E. necator isolates were QoI resistant. Most of the QoI-resistant P. viticola and E. necator isolates contained >95% of the G143A point mutation, which confers high levels of QoI resistance. In contrast, QoI-sensitive P. viticola isolates contained less than 1% of G143A. In total, 1 of 145 and 14 of 154 QoI-resistant P. viticola and E. necator isolates (able to grow on azoxystrobin concentration ≥1 μg/ml), respectively, contained <1% G143A. In total, 61 E. necator isolates from 23 locations were tested against thiophanate methyl, and the majority grew well on leaf tissue treated with 50 and 250 μg/ml. Through 2012, none of the E. necator isolates were resistant to boscalid and quinoxyfen. However, in 2013, quinoxyfen-resistant E. necator was detected in one vineyard experiencing difficulties with powdery mildew control. No 50% effective concentration value could be calculated but these isolates tolerated labeled rates with only limited inhibition. QoI (E. necator and P. viticola) and benzimidazole (E. necator) resistance were widespread in Virginia, rendering these materials inadvisable for control of these diseases. The practical importance and current distribution of quinoxyfen resistance needs further investigation.