Problems come when the mixes incorporate lower rates of each material than would be used for a solo application.
"The hope is that the combination of chemistries is equal in effect to a full dose of one," he says. "The reality is (in some cases) you're putting lower concentrations into the vineyard."
That may leave more survivors overall passing on their resistance to new generations, Gubler says.
Mix it up
He suggests opening a season's powdery mildew program with oils or sulfur, then switching to chemical sprays in a rotation.
Oil, a contact fungicide, "eradicates so beautifully and we get great coverage early in the season," he says.
New boscalid fungicides offer promise as well, but he cautions against overuse.
Every new fungicide works well, but for resistance purposes, he says,
"We don't want to overuse them or they'll get old before their time."
Cultural techniques such as canopy management to reduce shaded areas where the fungus thrives will help prevent powdery mildew infections and hinder reproduction, Wilcox says.
Opening up the fruiting zone is vital to Wagner's powdery mildew program.
All his plantings now feature the Scott Henry trellis system, exposing the fruiting zone to better air flow and more sunlight—conditions that discourage powdery mildew.
Wagner also includes sulfur in every spray application. "If all else fails, we know that will work," he says. Sulfur alone isn't practical, however.
A shorter residual period requires more frequent applications that would be "prohibitively expensive," he says.