Grape growers spend millions battling powdery mildew

California grape growers spend an estimated $200 million a year or more in labor, materials and management time wrestling with powdery mildew.

While fungicide resistance is a challenge in efforts to control this widespread disease, some of the most important management steps come long before the first spray is even applied.

"Canopy management is mildew management; that's the oldest tool we have," said Michelle Moyer, Washington State University Cooperative Extension statewide viticulture specialist. "Canopy management affects your microclimate and it affects your coverage. Anything that opens up the canopy helps you manage mildew, and if you want to manage mildew resistance, you have to manage mildew."

Moyer made her remarks as researchers from around the world came to discuss strategies to contain powdery mildew with the growers and vineyard managers at the 2019 Sustainable Ag Expo & International Sustainable Winegrowing Summit in San Luis Obispo.

While the good news that mildew resistant winegrape varieties may be on the way was announced at the summit, management will always begin with avoiding the moist, warm, but not hot, microclimate the pathogen loves.

"Mildew is a weak pathogen," Moyer said. "Let Mother Nature kill it for you; it will do a pretty good job if you let it. We also know it doesn't grow well in high temperatures and we can use that to know when we don't have to spray."

Nature will reduce powdery mildew if sun and wind are allowed to reach deep into the canopy.

"It's important to prune and train the vines to open up the canopy," agreed Shunping Ding, Cal Poly San Luis Obispo assistant professor in the wine and viticulture program. "This also increases the coverage of fungicides."

Ding cited the 2014 study by researchers at the Robert Mondavi Institute Center for Wine Economics on the University of California, Davis, campus that concluded between 4 and 6% of the revenue received for growing and harvesting grapes goes to efforts to control powdery mildew.

Ding's current research into fungicide resistance in San Luis Obispo and Santa Barbara counties indicates that even in vineyards that have not had resistance problems, it is advisable to apply strobilurin materials early in the season, before resistant pathogens blow in from the outside.

She sampled mildew pathogens from 15 vineyards five times over the course of the season and found that even locations that did not show resistance early in the season had resistant pathogens later on.

"If you have a powdery mildew problem and want to apply a strobilurin you can, but you have to do it early in the season," Ding advised.

"Stylet oil and sulfur are less susceptible to resistance but they need to be applied more frequently because they have shorter efficacy," she said.

Weather models are available that reduce the risk of resistance by pinpointing the times the disease is most likely to be severe.

"Some of you may be lucky because the pathogen does not do well when it gets above 90 degrees," Ding said. "The Gubler-Thomas UC Davis model can guide you; it is a risk assessment index."

That model starts the clock on powdery mildew risk after there have been three days with at least six hours of temperatures between 73 and 86 degrees.

Once that threshold has been crossed, a combination of temperature and moisture data lets farmers assess the severity of powdery mildew risk.

"You have to manage the disease all season long," Ding said. "This is a severe problem in California—all grape varieties are susceptible and we have the right environment because it grows fastest between 73 and 86 degrees."

While all of the current varieties are susceptible to powdery mildew, a researcher from France announced at the conference that they are beginning to release a small number of resistant varieties.

"We have two red varieties and two white with resistance to powdery mildew and partial resistance to downy mildew," said Sabine Wiedemann-Merdinoglu, research scientist at the Institute National de Recherches Agronomique in the Alsace region.

The French have put a full court press on mildew management because, with winegrapes accounting for 3% of cultivated acreage and 30% of fungicide use, the government has called for fungicide use in vineyards to be cut in half by 2025.

The new mildew resistant varieties—the first was registered in 2018—come nearly two decades after the French resumed winegrape breeding.

In the 19th century, breeders began crossing with American grape material to produce hybrids with resistance to Phylloxera, but the government banned the use of American hybrids before World War II on the grounds that the wine was not up to standard.

Since the resumption of crosses in 2000, the French researchers have identified two sources of resistance to powdery mildew and five with resistance to downy mildew they consider worth crossing into future varieties.

In the meantime, powdery mildew management comes back to one of the most basic tools in the vineyard, the sprayer.

"The sprayer may be your most important tool," Moyer said. "You should calibrate your sprayer every time you change your program, but at least once a year. Wettable paper is the cheapest investment you can make in checking your sprayer. Check for clogged nozzles, leaking tanks, rate controllers gone awry, broken fan blades or uneven air distribution."

Resistance management will improve exponentially as growers spray only when necessary, and hit the target with precision.

"We need to refocus not on what we need to spray, but how and when to spray," Moyer said. "You have to reduce the number of escapes. We've ignored spray technology and timing for a long time because we had materials that work very well. No more."

(Bob Johnson is a reporter in Sacramento. He may be contacted at bjohn11135@gmail.com.)