By Erika Holmes, Viticulture & Enology
PULLMAN, Wash. – Researchers at Washington State University have documented seven fungal species that cause cankers in grapevines. These new findings could reduce the incidence of grapevine trunk disease in Washington vineyards by preventing the problem before it becomes widespread.
Fungi infect the wood of grapevine trunks (or cordons) through pruning wounds, resulting in cankers that enlarge over time and ultimately kill the plant.
“We found a diversity of canker-causing fungi, and knowing there are seven species allows us to address each one specifically,” explained Leslie Holland, who completed her master’s degree in plant pathology at WSU in June. “Because the fungi differ in their biology and dispersal, we can work on customizing management methods now that we better understand the causes.”
Building upon research by Dean Glawe, a WSU plant pathology professor who retired in June, Holland conducted a statewide survey to gauge how common grapevine trunk diseases are in vineyards in Washington. She expanded Glawe’s research to wine grapes, which were not widely grown in Washington when his initial study linked grapevine trunk disease to the fungus Eutypa armeniacae.
In the 1970s, Glawe worked with Concord grape growers in Washington to understand why they were seeing stunted shoots and distorted leaves on some vines. As agricultural technology advanced during the previous years, grapevines had been pruned from four-arm to two-arm trunks to accommodate mechanical harvesting.
Glawe found that the cuts had allowed fungal pathogens to enter the vines. Because cankers can take a decade or so to develop and cause problems, the infected Concord vines had been slowly dying unbeknownst to vineyard owners from what would come to be known as trunk canker disease.
Last summer, under the guidance of Glawe and Gary Grove, director of the WSU Irrigated Agriculture Research and Extension Center in Prosser, Wash., Holland collected wood samples from symptomatic vines growing in seven vineyards in the Yakima Valley and the Horse Heaven Hills areas. Foliar symptoms are best seen from early May to June, and the disease is hard to detect during the rest of the growing season.
The surprise came when Holland analyzed the diseased wood samples to identify canker-causing fungi based on morphological features and gene sequencing. Not only did she discover more fungal species could cause cankers in Washington than was previously known, but she also saw a correlation between vineyard age and symptom incidence.
She found the highest incidence of trunk disease in a vineyard with 33 percent of its vines showing symptoms. This was also the oldest vineyard sampled, with plants from 40-42 years old. The trend continued as she crunched the numbers.
Though Benjamin Franklin was referring to fires when he said, “An ounce of prevention is worth a pound of cure,” the quotation illustrates the power of sustained agricultural research, a service WSU has provided since 1892.
“Washington is at an advantage because the wine industry is young, with most vineyards planted from 10 to 30 years ago, and we have had scientists involved since the beginning,” Glawe said. “The predictive value of Leslie’s research is substantial, as we are just beginning to see trunk disease become a bigger problem.
“It helps that the Washington wine industry is very engaged and forward-looking,” he said. “They want to be involved in research, and this study would not have been possible without the participating vineyard owners.”
Holland plans to continue studying canker fungi on other perennial crops when she begins a Ph.D. program at the University of California, Davis in September. Through sequence analyses, she found that fungal isolates in Washington are similar to fungal isolates in other grape-growing regions of the world, including California, Portugal and Australia.
Understanding management approaches used elsewhere could help shape Washington’s mitigation plans.
Holland sees potential in further studies on how different grape varieties are affected by various fungal species, whether the incidence of symptoms is similar in vineyards in western Washington and potential sources of inoculum.
For example, dead grapevines are often stacked in piles surrounding healthy vineyards following removal. Could this practice spread fungal spores to healthy vines? Some of the canker-causing fungi also live on poplar trees, which are commonly planted as a windbreak in vineyards, suggesting that they could also be a source.
Holland’s complete thesis is “Characterization of Fungal Pathogens Associated with Grapevine Trunk Diseases in Washington State.” The research won second place in the graduate student poster session at the Washington Association of Wine Grape Growers annual conference in February.
A technical summary of the research can be found in the Spring 2015 issue of Viticulture and Enology Extension News (http://wine.wsu.edu/research-extension/files/2010/07/2015-VEEN-Spring-FINAL.pdf).
Current management practices can be found in the WSU Extension publications “Pest Management Guide for Grapes in Washington” (https://pubs.wsu.edu/ItemDetail.aspx?ProductID=13362) and the “Field Guide for Integrated Pest Management in Pacific Northwest Vineyards” (https://pubs.wsu.edu/ItemDetail.aspx?ProductID=15589).