Good Fruit Grower

March 1

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Page 18 of 47 Good Fruit Grower MARCH 1, 2016 19 Grape Pest Management R esearchers are making strides in efforts to combat crown gall disease and to develop clean plants free of the pathogen that causes it, but there's still much to be learned about how to prevent, diagnose and eliminate it in grapevines. That said, if there's any place that should be able to grow disease-free vineyards, it's Washington, with its desert, sandy soils and lack of the wild grapevines that serve as reservoirs for the disease elsewhere, said Dr. Thomas Burr, Cornell University professor of enology and viticulture. "You have a real advantage in Wash- ington," he told attendees at the inaugu- ral Ravenholt Lecture Series at the Ste. Michelle Wine Estates WSU Wine Science Center in Richland in January. "If you can plant a vineyard with clean plants, you certainly are not going to eliminate the presence of crown gall forever, but if you can get a healthy vineyard started, you're way ahead." The pathogen that causes crown gall disease, Agrobacterium vitis, can live silently in the vines until an injury initi- ates an infection, most commonly those caused by freezing temperatures or at graft unions. The infection causes galls that crush the vascular tissue of the plant, restricting the movement of water and nutrients, and impede the vine's ability to heal the wound. The infection also can cause necrotic lesions on roots. The disease kills young vines that develop it at graft unions and can stress older vines, depending on the level of infection, significantly reducing vigor and yield. Once established in a vineyard, there is no chemical control. New knowledge about A. vitis Researchers at Cornell took cuttings from 10 different grapevines in the winter of 2014, sampled from nodes and inter- nodes, starting at the base and working their way out through the cane. They hypothesized that the pathogen would be at its highest level at the cane base and at nodes, Burr said. They were wrong. It turns out the bac- terium can be randomly distributed in dormant grape canes. "It's not only at nodes, but it's also at internodes. It's not only at the base, you can find it way out," he said. "It's not what we wanted, obviously. If you're going to be sampling a certain block for a grower, you want it to be simple." That means that if a sample is positive, researchers can positively say that crown gall is present. But if it's negative, they can't say for certain that it's not present somewhere else in the vine. "The more we study this bacterium, the more places we find it," he said. That includes in wild grapevines. In another study, researchers sampled wild grapevines in New York in 2013 and 2014 for tumorigenic strains of the pathogen, finding that roughly one-third were posi- tive for it — 18 of 54 samples in 2013, and 12 of 41 samples in 2014. Similar tests of wild grapevines in riparian areas in Cal- ifornia showed 24 of 87 positive samples in 2014 and 2015, he said. "It's a bit shocking, but good to know Controlling CROWN GALL Keys to combating crown gall disease in wine grapes are clean plants, avoiding freeze damage and pairing appropriate varieties to sites. by Shannon Dininny

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