Wood rot pathogens pose a serious threat to vineyards worldwide, but grapevine trunk diseases are a relatively new issue requiring attention from growers in the Pacific Northwest.
Growers would be wise to keep an eye out for the diseases and implement management at the first sign of symptoms, instead of allowing inoculum sources to build up in vineyards, said Kendra Baumgartner, a plant pathologist for the U.S. Department of Agriculture in Davis, California, who specializes in trunk diseases.
She spoke about her research into prevention and management of these fungal pathogens at the Washington State Grape Society annual meeting in November.
Many growers believe trunk diseases are inevitable, but good management practices can reduce their toll. “I’m on a campaign to educate growers that yes, you can do something about it,” she said.
And the sooner the better, according to economic models she shared, comparing the cost of proactive management with vineyard productivity.
Expensive preventative practices such as double pruning or delayed pruning, protecting pruning wounds with fungicides, and trunk surgery for retraining healthy new vines after diseased cordons are removed work well under California conditions.
But many other regions lack registrations for protective fungicides and biological control products. Moreover, regional differences in climate also play a significant role in the behavior of these fungal pathogens and, therefore, the best steps for managing them.
More regional research is underway.
“Knowing these diseases, I would be surprised if it’s not a problem everywhere grapes are grown,” said Jóse Ramón Úrbez Torres, a research scientist at the Summerland Research and Development Centre in British Columbia.
But the slow-moving pathogens tend to slip off the radar in some places, as growers face more immediate and visible concerns such as winter injury, botrytis and powdery mildew, he said.
When researchers talk about trunk diseases, they are actually referring to a large group of fungal pathogens that cause wood rot. There are over 100 now identified, but common trunk diseases include eutypa dieback, esca, botryosphaeria dieback and phomopsis dieback.
Understanding the primary pathogens driving trunk decline is important for researchers studying disease epidemiology or testing fungicide efficacy, but for growers, trunk disease management is all approached the same, Baumgartner said.
The basics of disease management include orchard sanitation, removing diseased wood where spores are produced, retraining vines when necessary, and protecting vulnerable infection points, such as pruning wounds. Most of the pathogens are spread by rain splashes, so in California the winter rains are the key period of management, which overlaps with pruning, Baumgartner said.
“We have a lot of growers here in California that have protected pruning wounds by pruning later in the dormant season. It seems to lower the risk for infection, with less rain then to bring the spores out, but also the pruning wounds tend to heal faster as we get close to March and the vines are coming out of dormancy,” she said in a follow-up interview with Good Fruit Grower.
Mechanical prepruning can also prevent infection, though the practice is less common in California vineyards.
“If it does happen to rain and spores from trunk pathogens land on the top of those cuts, the pathogens don’t grow so fast that they would make it into the cordon before you come back in February to cut them away,” Baumgartner said. “They are very slow growing, localized fungi. You just get many different infections at different pruning wounds over time.”
Given labor concerns, delaying pruning isn’t feasible for large vineyards, so she recommends a protective fungicide spray after pruning. However, no fungicides are yet registered for dormant season use in Washington or Canada; trials are underway.
Lastly, Baumgartner recommends vine surgery — basically removing an infected vine at the trunk with a chainsaw and tapping the healthy root system to push up a productive new vine. It’s important to note that this also creates a wound that needs to be protected from fresh infection.
It’s a labor expense, but economic modeling shows that both vine surgery and fungicide sprays can be well worth the investment for vineyard productivity in the long term, provided the management starts early enough. Symptoms of trunk disease tend to appear when vineyards are about 10 years old and then increase rapidly over the next five or six years if not managed.
In Washington, trunk diseases have not been a management priority, although a survey by Baumgartner launched last summer found significant levels of esca infection and a smattering of other pathogens.
“We haven’t had the widespread outbreak of trunk diseases and the economic losses associated with them,” said Washington State University extension viticulturist Michelle Moyer. Washington growers occasionally retrain vines due to winter injury, and many mechanically preprune. These practices may lower the threat from trunk diseases, she said.
Baumgartner’s findings show the importance of prevention for Pacific Northwest growers, as more vineyards mature, Moyer said. “Intentional cordon renewal will go a long way to manage things, so we don’t have to spray,” she said.
The top priority in British Columbia: identifying products to spray. Úrbez Torres called the pathogens a primary threat to vineyard lifespan in the region, along with winter injury.
“Here in British Columbia, trunk diseases have been long overlooked and in many cases the symptoms were confused with the result of winter injury and crown gall, but as the industry matured, growers have started to pay more attention to trunk disease,” he said. He began his research program at the Summerland Centre in 2010 to help the industry learn about how to tackle these issues.
His research program includes epidemiological research to understand how the pathogens behave in the region’s climate, with the goal of developing an effective integrated pest management program. Climate dictates spore release and infection risk, and Úrbez Torres found that delayed pruning — as practiced in California — doesn’t work in British Columbia.
“Our first inoculum pressure is at the end of winter. The fungi is not stupid; they know they don’t have any chance to survive if they release spores in under freezing temperatures,” he said. His trials show that pruning in December or January makes wounds less susceptible to infection in March and April when the fungal spores are present, and that winter pruning can reduce infection risk by 50 percent.
That’s a complicated recommendation, given the risk of cold damage and snow making it difficult to access vineyards, Úrbez Torres acknowledged.
“The ultimate goal is to have products, both fungicides and biocontrols, to protect pruning wounds,” he said. “That’s going to make the difference, but in the meantime, we have to keep the vineyards as clean as possible.”
While growers should make sanitation a priority, it can often be difficult, as these pathogens have many hosts, including apple and cherry trees and other native woody plants.
More research needed
Many questions remain about how to handle trunk diseases in the long run, such as the potential for cultivar resistance, the role of environmental stress on susceptibility, the efficacy of biological protectant products, and the role of clean planting material in minimizing the threat.
While trunk diseases are best understood in mature vineyards, another group of fungal pathogens strikes young vines, causing a more rapid decline.
“Here in British Columbia, where we are still quite a young grape-growing region, we have a lot of new vineyards and a lot of new material has come in,” he said. “One of the issues we face in B.C. is these pathogens coming in in the planting material.”
British Columbia, of course, is not alone in dealing with infected planting material, and Úrbez Torres said his research has shown that the pathogens can be present in asymptomatic material. That’s why he plans to study how stresses such as drought or overcropping may contribute to pathogen activity in young vines.
To explore these questions and more, this summer British Columbia will host an international workshop on grapevine trunk diseases — the first time the event will be held in Canada. The scientific workshop runs July 7 to 12, and although the agenda was not yet final at press time, one day will be devoted to industry outreach presentations, he said. •
—by Kate Prengaman