Pruning during dry periods and pruning late can help control trunk cankers.
In Washington State, aging vines and winter injury can be a recipe for grapevine trunk diseases. While eutypa cankers are the most common of trunk diseases, there are other causes of dead arm in vineyards, says a Washington State University pathologist.
Canker diseases are reported in both vinifera and labrusca cultivars of grapes, with eutypa (Eutypa lata) and Botryosphaeria spp. common causes of dead arm, said Dr. Gary Grove, WSU pathologist and AgWeatherNet director. Ten years ago, eutypa was the common cause of trunk cankers in Washington, he said. "But now, bot cankers (Botryosphaeria) are also involved with trunk diseases in the Pacific Northwest."
Two species of bot canker were recently found in British Columbia, Canada, vineyards, reports the Pacific Agri-Food Research Centre in Summerland. Several different species have been found in California.
Both cankers, caused by fungi, are generally seen in vines older than five or six years, Grove said.
Both diseases require predisposition, he said. "The vine needs to be predisposed to infection by some environmental factor or man-made factor, such as being hit by a tractor. In our area, the predisposition usually comes in the form of winter damage."
Symptoms of both diseases include delayed shoot emergence in the spring, tattering of leaves, and chlorosis early in the season, he said, adding that a wedge-shaped canker can be seen when the trunk is cut in a cross section. Yields are eventually reduced from the dead wood of the vine.
"You cannot distinguish these two diseases in the field by looking at the symptoms," Grove said. "The only way to distinguish between the two—and by and large it's academic because they're both managed in the same way—is through direct isolation on the growth medium or by PCR [polymerase chain reaction] techniques."
Although WSU's diagnostic laboratory at the Prosser station is not equipped yet to run the tests, Grove said that growers can take samples to a lab at Oregon State University.
Both groups of cankers have alternate hosts that provide sites for the fungus to overwinter. The hosts most commonly reported are apricots and cherries, crops that are widely grown in eastern Washington and near vineyards.
"The life cycles of the cankers are closely tied to their hosts," Grove explained, adding that the spores of the diseases overwinter on the infected plant material of apricots, cherries, or grapes, and are released in the spring during rain. The released spores are splashed or windblown into pruning wounds and then form a canker on the grapevine.
"If that canker is not removed, it will complete the disease cycle," he stated.
Grove described the pathogens as being opportunist because they attack weakened or damaged vines. "One way to ward off infections is to maintain general vine health and eliminate all infected wood of stone fruit and grapevine in the area, " he said. Eliminating infected wood helps reduce the inoculum load of the area.
For grape growers in Washington State, winter injury can complicate vine health maintenance, Grove said, adding that he learned about winter injury and vine health from personal experience.
In 2004, cold winter temperatures damaged Grove's vineyard. "My Merlot was just smoked and totally wiped out by the cold temperatures. But in some of the warmer parts of the vineyard, the Cabernet Franc didn't appear to be damaged as bad, so I decided to grow them out the next year without retraining."
The following year, his Cabernet Franc vines looked fine, but he began noticing dead arms in 2007 on the Cabernet Franc vines that were lightly damaged but not retrained. "Lo and behold, I do have cankers. Winter injury is something to factor in when you're making the decision whether to retrain vines," he said. "If there's any question that there's been damage to the vines, I would recommend that you retrain."
To help control the diseases, growers are advised to prune during dry periods to avoid washing spores onto pruning cuts.
He also recommends that growers prune late in the dormant period when pruning wounds heal more rapidly. For example, pruning cuts made in January take four to five weeks to heal, compared with cuts made in April, which may only take 10 to 14 days.
"This disease complex is manageable, but it takes an integrated approach," he concluded.
Grove spoke at the Washington State Grape Society meeting last November.