Sensitivity of Erysiphe necator and Plasmopara viticola in Virginia to QoI Fungicides, Boscalid, Quinoxyfen, Thiophanate Methyl, and Mefenoxam

The sensitivity of downy mildew (DM, Plasmopara viticola) and powdery mildew (PM, Erysiphe necator) of grape (Vitis sp.) to commonly used non-demethylation 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 mu g/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 mu g/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.