Powdery mildew has long been an almost routine headache for grape growers, but labor shortages, climate change, and resistance to chemical treatments, are forcing changes in vineyard management, growers and pest control advisors say.

“Powdery mildew is something we take for granted,” says Bart Haycraft, vineyard manager at Santa Rosa, Calif.-based Jackson Family Wines. “We’ve been lucky the last 20 years to have effective treatments. Powdery mildew is one of those rare items that, as a PCA ,that you don’t scout for at threshold — you always treat it prophylactically, so it’s a line item in your budget.”

Growers used to be able to use one treatment in a season, then move to another the following year. But that no longer works, he says.

“While it looks like we have good chemistries, a lot of them are overlapping, and a lot are being used several times in a year,” Haycraft said during a workshop at the Unified Wine and Grape Symposium at Sacramento. “It looks like there are going to be different elements we’re going to need to manage if we want to keep these chemistries around and keep them useful.”

Powdery mildew is a problem for all types of California-grown grapes — raisin, fresh, and wine. Initial symptoms appear on leaves as chlorotic spots on the upper leaf surface, and signs of the pathogen appear a short time later as white, webby mycellum on the lower leaf surface, according to the University of California Statewide Integrated Pest Management Program. As spores are produced, the infected areas take on a white, powdery, or dusty appearance.

DESTROYS QUALITY, YIELD

Powdery mildew — caused by the pathogen Erysiphe necator — can also appear on fruit and rachises (stems) as white, powdery masses that may colonize the entire surface. Uncontrolled powdery mildew can destroy quality and yield.

Workshop participants noted that management of powdery mildew has changed in the past 10 years as more has been learned about the life cycle of the fungus, and fungicide chemistry has advanced. But it still has an economic impact on vineyards, costing growers statewide an estimated $189 million annually for fungicides and their applications, according to a 2014 UC-Davis study.

Last year, with its rainy spring and intermittent heat waves during the summer, was a particularly rough season for powdery mildew, the growers said. During the Labor Day heat wave, Jackson Family Wines had grapes drop because of powdery mildew and sunburn, Haycraft said.

“2017 was the hardest year I’ve ever had in 15 years of farming,” said Chris Storm, director of viticulture at Vino Farms. In a given year, the operation grows wine grapes on between 11,000 acres and 12,000 acres in the Napa, Sonoma, Lodi, and Clarksburg areas. Spray rigs were moving constantly through the various vineyards last year, and more fungicide was applied than Storm would have liked, he says.

MORE DELIBERATE, PRECISE

With instances of powdery mildew seemingly becoming more severe, and with consumers, buyers, and government regulators growing more wary of the use of chemicals, farms are having to be more deliberate and precise about the way they approach the pathogen.

Beckstoffer Vineyards in the Napa Valley sells grapes to many different customers, and the bar for cleanliness and the pressure to reduce chemical use are both “very high,” says Blake Wood, who manages vineyards in the Oakville and St. Helena areas.

When Wood develops a plan for managing powdery mildew, he creates a timeline that includes the critical periods for disease development — including bud break, pre-bloom, bloom, and fruit set — as well as limitations he may encounter for treatments, such as heat and deficit irrigation. He then considers how the plan could fail and makes improvements, and he stands ready to change the plan at mid-season if the need arises. This approach enables him to limit use of some of the “preferred chemistries.”

“Placement is key,” Wood says. “Picking the window of when you can apply a product most effectively is more important than just the general statement that coverage is key.”

Researchers several years ago mapped the genome of the grape powdery mildew pathogen — a development that could lead to new resistant varieties. In the meantime, UC-Davis offers a risk index to determine disease pressure, based on weather, and to aid in the timing of sprays. The UC IPM program advises vineyardists to rotate applications, using fungicides that have a different mode of action. Research has shown that sequential use of fungicides that have the same mode of action can reduce the pathogen’s sensitivity to the active ingredients.

In general, treatment may be discontinued for wine and traditionally trellised raisin grapes when fruit reaches 12º Brix, but should be continued up to harvest for table grapes, and three weeks to four weeks before cane severance for dry-on-the-vine raisin grapes, the UC advises.

For detailed advice on managing powdery mildew, visit the UC IPM website at ipm.ucanr.edu.