For grape growers, location can determine the scale of resistant powdery mildew.

In California, Doug Gubler, a University of California Extension plant pathologist in Davis, sees the most severe DMI resistance in areas with longer periods of moderate temperatures, including the Delta and coastal counties.

High temperatures can mask the presence of resistance by suppressing pathogen reproduction and lowering inoculum pressure, he says.

Meanwhile, "powdery mildew is endemic" for New York growers, says Mark Wagner, owner and managing member of Lamoreaux Landing Wine Cellars in Lodi, N.Y.

"We can't escape it in our climate. If we don't spray for powdery mildew, we don't have a great crop."

The disease also paves the way for other problems such as botrytis and rots, making control even more critical, Wagner says.

In addition to DMI resistance, New York grape growers struggle with strobilurin resistance, suffering significant crop losses in 2002, says Wayne Wilcox, professor of plant pathology at Cornell University, in Geneva, N.Y.

Strobilurin resistance is black and white—either the fungicide works or it doesn't. Resistance to DMI fungicides comes in shades of gray, Wilcox says.

Higher rates or more-active compounds often work where lower rates or less-active materials fail.

"It's a creeping kind of thing," he says.

But losses to fungicide resistance aren't inevitable.

"The conventional wisdom is there for a reason," Wilcox says.

Resistance management is twofold 

He cites two basics: rotating fungicide sprays among different classes and limiting the fungus' reproduction.

"Resistance is simple," Wilcox says. "It's survival of the fittest."

Mixtures may be a suitable option in some cases, he says.

Pathogens that survive a brush with one of the mix's chemical components likely will succumb to the second.

"It can be a good way to save the materials" and regain some activity, Gubler says.

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.