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Fighting grape disease

By Sharon Omahen
University of Georgia

Bobby Scott Jr. had two choices. He could yank up his grape vines or watch as Pierce's disease ate through his earnings.

He chose option three.

When the Aiken, S.C., wine grape grower called University of Georgia professor C.J. Chang, he needed hope, help and answers. "Pierce's disease is the major limiting factor for the success of the wine industry in the southern United States," Chang said.

Replace or import

Within two to four years of contracting the disease, most grape vines originating from Europe die, he said. To fight the disease, wine grape growers must either replant vines periodically to replace the diseased vines or import grapes from other regions to keep their businesses going.

Since the early 1980s, Chang has been searching for a way to control Xylella fastidiosa. The bacterium is associated with three major crop diseases in Georgia: Pierce's disease of grapes, phony peach disease and plum leaf scald.

When Scott contacted Chang, he was running out of options. No plant pathologists in South Carolina were studying Pierce's disease. He turned to Chang in desperation.

Since that first phone call, the two have developed a partnership in the fight against Xylella fastidiosa. Scott has been crossbreeding European grape varieties with bunch grape-muscadine hybrids resistant to Xylella fastidiosa. To date, he's bred thousands of young seedlings. Oddly enough, he needed Chang to inoculate the fledgling vines with the bacterium.

Breeding for resistance

"I start in the greenhouse with the new crosses," Scott said. "Then they have to be inoculated so we can see how many survive."

Scott then plants the new potentially-tolerant vines in the field. Many of his crosses don't survive to live outside the greenhouse and many die in the field.

"We lose a lot of crosses along the way, but I know eventually we're gonna be successful," he said. "I feel good about the progress we're making. But it's taking a little longer than I had hoped."

Scott now has 3,000 Pierce's disease-tolerant vines in his family's Montmorenci Vineyard. Vineifier grapes are used to introduce good quality wine genes.

"We both acknowledge that as long as we work hard, the fruit of our labor will be tolerant wine grapes for the Southern region," Chang said.

Besides breeding disease-tolerant grape varieties, UGA researchers have been searching for other ways to control the bacterium. In 1979, scientists found it can be slowed down by tetracycline treatments. The find turned out to be a breakthrough in controlling diseases caused by Xylella fastidiosa.

Tetracycline successfully suppressed the symptoms of plum leaf scald disease and oak leaf scorch. But researchers hit a snag when it came to using it on food-bearing crops.

"Unfortunately, EPA frowned on the use of tetracycline as a control method," Chang said. "They were concerned over the environmental hazards, and they think it could produce a bug that's resistant to tetracycline. So we can use it on trees like oaks and sycamores, but not on food crops."

Plant-derived control

Undeterred, Chang set out to find a plant-derived compound for control. His answer came in a product called terpene, developed by the Eden Research of Oxfordshire, U.K.

Scott is allowing Chang to test these potential control methods in his vineyard. More than 500 test-plot wine grapes are now part of Scott's 20 acres of wine grapes.

The terpene solution is being fed to the grapevines through the vineyard's drip irrigation system. Comparing the treatment to untreated control vines, the grower is seeing dramatic results. The treated vines are thriving while the untreated are suffering the effects of the disease.

Chang is now working to develop a strategy to put in place in case wine-grape growers in north Georgia begin to see disease symptoms. Time is on the side of Georgia growers as the disease spreads slowly at higher elevations.

"The disease spreads slowly in vineyards that are 1,600 to 1,800 feet or more above sea level," Chang said. "There are some resistant muscadine and American wild grape hybrids available, but relying on these flavors alone limits marketability for Southern growers."

As Chang and Scott work to develop ways to fight Pierce's disease in grapes, growers wait.

"Until we can control it, our county agents are recommending that growers yank out the diseased vines," Chang said. "It sounds harsh, but right now it's the most effective control around."

(Sharon Omahen is a news editor with the University of Georgia College of Agricultural and Environmental Sciences.)

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