Good Fruit Grower

February 15th

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GRAPE DISEASES drag down wine quality Merlot wines from diseased fruit were lower in anthocyanins, phenolics, tannins, and alcohol. by Melissa Hansen T WSU students harvest Merlot grapes in a side- by-side study of normal and diseased vines from the same block. he effects of grapevine leafroll disease and other viruses on wine quality are real, not some vague generalities. Growers can easily see physical symptoms and notice production losses from viruses through declining yields, but declining wine quality often is not as easily noticed because vines may be symptomless, and good and bad grapes are mixed together during crush. Washington State University plant pathologist Dr. Naidu Rayapati has collected extensive yield, fruit, and wine quality data in the last four years in side-by-side studies of diseased and normal vines in the same block, from different appellations, and of various cultivars. The results of his studies, the first of their kind, showed just how devastating grape leafroll and other viruses can be to Washington State vineyards. Leafroll virus, one of the most complex viral diseases, is a major concern for Washington's wine grape industry, said Rayapati, who devotes his time to studying grape dis- ease aspects and educating the industry about the impor- tance of using clean plant material. Once a vineyard becomes infected, there is no cure, and eventually, unproductive vines must be replaced. The principal means of spreading the disease is through propagation of infected cuttings, as well as by vectors of mealybug and scale insects. In one of the first statewide surveys looking for grape leafroll, conducted in 2000, about 6 percent tested posi- tive for grapevine leafroll-associated virus type 3. But since then, more thorough testing and improved detec- tion technology have found vines with mixed viruses and more than one type of grape leafroll virus in vineyards across the state and in registered blocks of nurseries. In addition, some virus isolates show no symptoms, Rayapati said, adding more complexity to past surveys that were conducted based on visual symptoms. "Given all this information, I would imagine that up to 30 percent of acreage in the state might be infected with leafroll and other viruses," he stated. The severity of infec- tion within a vineyard can have a dramatic effect on fruit and wine quality—a small amount might not be noticed, but a larger percentage of infected vines will negatively impact fruit uniformity. Comparative studies In Rayapati's side-by-side studies in a Yakima Valley ATTENTION AMBROSIA™ GROWERS McDougall & Sons, Inc., in partnership with Columbia Marketing International, is the exclusive licensee of the PLANT IMPROVEMENT CO. LTD (PICO) to grow and market the Ambrosia™ apple variety in the United States. Under the terms of this exclusive agreement, all Ambrosia™ tree plantings and treestock propagation or topworking to Ambrosia™ in the U.S. for 2005 and forward is prohibited. For further information, please contact Scott McDougall McDougall & Sons, Inc. 305 Olds Station Road Wenatchee, WA 98801 509-662-2136 32 FEBRUARY 15, 2012 GOOD FRUIT GROWER Merlot vineyard, he found reductions in yield, in the number of clusters per vine, cluster weight, and pruning weight. Of the four years of the studies, the highest reduc- tion in cluster weight per vine was 28 percent and the lowest 15 percent. The range in reduction of cluster num- ber was from 10 to 18 percent, and the range of pruning weight reduction was from 11 to 35 percent. Rayapati found similar differences in the weight of clusters from other cultivars, including Cabernet Franc, Cabernet Sauvignon, and Pinot Noir. Samples collected from Horse Heaven Hills, another appellation, did not show as dramatic differences as the Yakima Valley vine- yard, but Rayapati believes that fruit and wine quality dif- ferences between leafroll-infected and normal vines are similar in Washington appellations. He plans to extend the yield, fruit and wine quality studies to other varieties in the future. Fruit quality impacts Rayapati's side-by-side comparison of fruit quality aspects in the same Yakima Valley Merlot block for four years found the following: • Infected vines were up to 9 percent lower in Brix. • Infected vines were more acidic than normal vines, with the pH level of the juice almost 3 percent lower. • Titratable acidity was up to 8 percent higher in infected vines. • Skin tannins were 13 percent lower in leafroll vines but seed tannins were slightly higher (4 percent). The WSU pathologist also identified that the fruit qual- ity differences between normal and leafroll-infected vines www.goodfruit.com COURTESY WASHINGTON STATE UNIVERSITY

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