Practical resistance to QoI fungicides in New York populations of Uncinula necator associated with quantitative shifts in pathogen sensitivities

• Author(s): Burr, J.; Riegel, D.; Wilcox, W.; Wong, Frank;
• Abstract: In 2002, poor control of powdery mildew occurred in multiple NY Vitis vinifera vineyards where QoI fungicides were applied solo during the critical early postbloom period. In experimental Chardonnay plots, a standard rotational program using azoxystrobin (AZ) provided only 37% control of cluster infections, whereas substituting BAS 516F (pyraclostrobin (PY)+boscalid (BAS 510F)) for AZ increased control to 86%. Seasonal programs of PY vs. boscalid applied solo provided 31 vs. 97% control, respectively. Leaf-disc assays of U. necator sensitivities utilizing discriminatory doses of 0.031 mg/l AZ and 0.008 mg/l PY showed pronounced quantitative shifts relative to baseline, but little to no evidence of qualitative resistance. Similar results were obtained in assays of AZ sensitivities from affected commercial vineyards. Apparently, practical resistance to QoI fungicides can result from quantitative as well as qualitative shifts in pathogen sensitivity distributions.
• Publication Date: Jun 2003
• Journal: Phytopathology