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Diseases & Disorders for grape powdery mildew BE PREPARED Spray smart, using cultural and chemical strategies. by Melissa Hansen I n eastern Washington State's relatively dry climate, grape powdery mildew is easily controlled most years. But every once in a while, the disease catches some growers off guard. Last year was a prime example, says a Washington State University viticulture extension specialist. The year before, in 2010, it was botrytis bunch rot, rarely a problem in eastern Washington's grape-growing regions, that thrived in the unusually wet fall weather and took growers by surprise. But in 2011, powdery mildew took center stage, as conditions were conducive for much of the growing season. It caused significant damage and losses for many growers. By midsummer, reports were coming in to WSU from throughout the state concern- ing increasing levels of powdery mildew on fruit clusters. Instead of the hot summer tem- peratures that usually visit eastern Washington by mid-July, last summer's mild temperatures, moderate humidity, and often cloudy skies created perfect conditions for the disease, said WSU's Dr. Michelle Moyer, who joined WSU about a year ago. Moyer developed a powdery mildew risk assessment model while working on her doctorate in plant pathology from Cornell University. What went wrong? Growers who got by in past years with longer spray intervals needed tight ones last year, she said. Spray coverage was also a problem in many vineyards. "If you're having a high-pressure year, then you need to use the higher end label rates, tight spray intervals and, most importantly, slow down the tractor speed," Moyer told grape growers last fall during statewide juice and wine-grape talks. She recommends that when infection risk is high, tractor speeds should be slowed to 2.5 miles per hour instead of 4 to 4.5 mph. Checking sprayer calibration before the spray season starts is also important. Clogged and worn nozzles are common problems. "If you're not getting the chemical into the canopy, that's one way for the fungicide not to work. And, watch your wind speeds to keep sprays on target." She noted that for a full canopy, higher spray volumes are needed (100 gallons per acre). "Last summer, a lot of people lost control because of inadequate coverage—they drove too fast or had broken nozzles." Assess 2012 conditions Growers approaching the 2012 growing season can't predict the weather during bloom or harvest. However, vineyards that had powdery mildew problems last year will have more disease carryover than those with minimal problems. Additionally, some vinifera varieties are more susceptible to mildew than others, she said. "Essentially, what happened last year impacts this year and affects next year," Moyer said, adding that the fungus overwinters in dormant buds and can survive in temper- atures as low as minus 40°F. "The real driving force in how quickly things will develop the coming season is how quickly things developed in the past season." In late winter, the overwintering cleistothecia release A Grape powdery mildew was a problem in many Washington State vineyards last year, infecting entire bunches like this example. special seminar on grape powdery mildew management is scheduled for February 23 at 9 a.m. at Washington State University's Tri-Cities campus at Richland. Dr. Kathy Evans of the University of Tasmania in Australia will discuss how to manage disease according to risk. Dr. Walt Mahaffee, with the U.S. Department of Agriculture, Corvallis, Oregon, will explain how inoculum spreads in the vineyard and throughout the growing season. The session will be broadcast to WSU's Prosser AgTech Conference Room and WSU's Mount Vernon research center. To register, call (509) 372-7224. ascospores when temperatures are above freezing and there's been 0.1 inch of rain. Primary infection is then poised to begin when temperatures warm up to at least 50°F. Under perfect weather conditions, powdery mildew can reproduce every five to seven days. But it can only develop and grow on green tissue, she explained, which is why the disease shows up on young shoots and leaves but not old leaves or growth. Powdery mildew likes a broad temperature range from 50 to 95°F, which is why it can reproduce so rapidly. 34 FEBRUARY 15, 2012 GOOD FRUIT GROWER Models Washington grape growers, through WSU's AgWeatherNet, have access to both grape powdery mildew and botrytis bunch rot risk assessment models on the Internet. The models track temperature, moisture, and humidity to indicate when vineyards are at low, moderate, or high risk of infection. A table of fungicides on the AgWeatherNet site can help growers choose appropriate chemistries to avoid disease resistance. A smart- phone application allows growers to access the information in the field. WSU is developing an alert system that will send disease warnings directly to a phone or e-mail. Smart spraying While some growers might want to protect the canopy from mildew incidence to keep inoculum levels low for the following year, protecting the grape cluster from disease is really the goal. Diseased fruit can impart unwanted flavors, causing winemakers to reject or discount fruit loads with high disease levels. Timing of sprays is everything with powdery mildew, Moyer said. To protect fruit clusters, mildew sprays must target the prebloom to fruit set stage. As fruit begins to accumulate sugar later in the season, fruit are no longer susceptible, so there is no use in spraying the fruiting zone after veraison. At the end of the season, after verai- son or later, sprays will only manage powdery mildew on the canopy—not on the cluster. But the situation is different for Concord growers. The rachis in the Concord cluster is susceptible to disease throughout the growing season, she said, adding that Concord clusters can have late-season powdery mildew infection coming from the rachis. www.goodfruit.com GARY GROVE