Latent infections inside a cluster can take over the bunch by harvest time. PHOTO COURTESY OF CORNELL UNIVERSITY
Last year’s cool season not only challenged growers and winemakers with slow fruit ripening, it also brought bunch rot to some vineyards, causing significant damage from a lack of disease awareness.
The 2010 growing season was wet throughout, with 40 days of measurable rain recorded in Prosser by Washington State University’s AgWeatherNet system compared with 18 days the previous year. Moreover, it was wet at critical times during the growing season, said Dr. Gary Grove, WSU plant pathologist. “It was wet during bud burst and harvest. That’s why we saw so many problems with powdery mildew and botrytis (Botrytis cinerea) or bunch rot.”
In normal years, eastern Washington growers control powdery mildew through vigor and canopy management (shoot thinning, shoot positioning, and leaf removal) and well-timed fungicide applications during bloom, he said. Bunch rot is rarely a problem and is easily controlled by vigor and canopy management when the weather is dry.
During a recent seminar held at WSU’s Tri-Cities campus, Grove said his biggest concern was that Washington growers would overreact in the coming season.
“We obviously got smacked by bunch rot last year,” he said. “But the worst possible thing that growers can do—if weather returns to its normal dry pattern—is to react in 2011 and treat bunches like we’re still in a wet weather pattern. We obviously need to do things differently, but I don’t want our reactions to be overreactions.”
Rick Hamman, Hogue Ranches viticulturist, saw bunch rot last year, particularly in tight-clustered, white varieties in low-lying vineyards of the Yakima Valley.
In a 140-acre vineyard with Pinot Gris, Sauvignon Blanc, Riesling, Semillon, Chardonnay, and Gewürztraminer, yield losses from bunch rot totaled almost 40 percent, he said. The crop was estimated to be 5.3 tons per acre in July; actual yield was 3.2 tons per acre. In another Pinot Gris vineyard, he watched the crop shrink from around five tons per acre to less than one ton per acre at harvest.
As rot symptoms increased near harvest in one vineyard, Hamman’s vineyard management team was unsure what to do. Vines had been leafed, as they often are, but that didn’t make the disease go away. Because bunch rot is so rarely seen in Washington, he didn’t have past experience or knowledge of botrytis fungicides to rely on.
“We asked ourselves: ‘Should we spray, and if so, with what?’ That’s part of the problem—if you don’t have an ongoing program, what do you spray with?”
Though he tried rescuing the fruit with a hydrogen peroxide product, in the end, the crop was abandoned.
Winemaker Coman Dinn of Hogue Cellars points out that Washington is blessed with a benign climate for grape growing compared to other regions, like Germany, New York, and northern California, that deal with cool, wet weather on a regular basis. In hindsight, he believes the industry should have been thinking “late, cool year” and thought more about what to do when ripening was behind schedule and what should be done with rot-prone varieties.
“We shouldn’t get lulled into a false sense of security because of our beautiful dry region,” Dinn advised. “We need to recognize these unique seasons when they come upon us and work together—grower and winemaker—to make adjustments.”
Though the state had some rot problems in some varieties in 2010, most red varieties had great color and balance and the whites had good acids overall. A record 160,000 tons were harvested, according to recent figures released by the National Agricultural Statistics Service.
In California, powdery mildew and bunch rot are problems that growers deal with regularly. Bunch rot seems to be cyclical, University of California’s Dr. Doug Gubler said, noting that every three to four years the industry has a widespread problem, and 2010 was such a year.
Control strategies in California include leaf removal, decreasing fertilization to reduce the amount of succulent tissue, applying fungicides, and manipulating cluster architecture. Fungicides are only part of the solution, he said. “With fungicides, the best we can ever do is about 50 percent disease control, and it doesn’t seem to matter what fungicides are used.”
Gubler said that in California, leaf removal is the grower’s first line of defense. When timed properly, it also improves fruit quality. Research has shown that leaf removal, when done at cluster set, provided significantly better disease control than three or four fungicide sprays. Up to eight nodes (leaves) have been removed in the UC trials without causing sunburn. Removing leaves at veraison is too late, but when done at cluster set, the epicuticular wax on the berry thickens to prevent sunburn. He reports that fully exposed Semillon clusters, leafed at cluster set on both sides of the canopy row, did not show sunburn even after exposure to 108°F.
Reporting results of trials comparing leaf removal and fungicides, Gubler said the control (no leaf removal, no fungicides) had 28 percent disease. Applying one, two, and three fungicide applications, without leaf removal, resulted in only slightly better control. Leaf removal by itself resulted in 6 percent of the grapes having disease; leaf removal with one or two fungicides resulted in 6 and 3 percent disease, respectively.
Leaf removal increases the vine’s evaporative potential and improves airflow, he explained. Researchers have found that the evaporative potential increased by 1.5 times in leafed vines compared to nonleafed.
Gubler added that the canopy management technique is also helpful in controlling powdery mildew because it lets ultraviolet rays into the fruiting zone, and the mildew fungus can’t handle direct UV rays.
Melissa Hansen is the research program director for the Washington Wine Commission. Hansen previously was an associate editor at Good Fruit Grower from 1996 through 2015.
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