Think you’ve been seeing more trunk diseases in vineyards than you’re used to? You’re not alone.
These diseases, which produce cankers on vine trunks, are caused by wood-infecting fungi. Doug Gubler, a University of California plant pathologist, has been tracking these diseases – botryosphaeria dieback (bot canker), eutypa dieback and esca (black measles) – for more than two decades.
“We’ve seen a pretty good increase in canker disease infections over the past few years,” he says. “Last season, the amount of black measles was the worst I’ve ever seen.”
He thinks this surge reflects the impact of the past four years of drought.
When short of water, plant tissues undergo stress and become more susceptible to fungal infection, he said. Preliminary research conducted in his lab reveals that water stress can just about deplete the levels of three proteins in plant sap. Two of these proteins are known to suppress fungi. "We think, in their absence, the fungi can grow and spread in the vascular tissue.
“When we apply esca fungi on sap from water-stressed vines and unstressed vines, they grow faster on the sap from the stressed vines,” Gubler says. “Although not yet proven, this would explain why these pathogens move faster through stressed wood than unstressed wood and why we’ve been seeing an increase in canker diseases in vineyards during the drought.”
A total of 21 different fungi cause the various canker diseases, not only in grape vines but a wide range of other hosts. Depending on the type of disease, they include almonds, blueberry, walnuts, apple, pear and cherry trees, as well as some riparian shrubs and trees.
The fungi which cause these diseases produce structures that allow them to over-winter in and on diseased wood parts of the vine. These spores are released by rains in the fall, winter and spring and are carried by the wind or splashed by rain drops onto vines. There, they spread the disease by entering the plant primarily through pruning wounds. Mechanical injury such as that caused by harvesting equipment ripping off cordons or spurs can also provide an entry point. The larger the wound, the greater the chance of infection, Gubler notes.
Trunk diseases can infect vines of all ages and, if not controlled, can spread over the entire vineyard. A vineyard and even individual vines may be infected with more than one of these diseases at the same time.
It can take a number of years before the cankers grow large enough to restrict the flow of water and nutrients. As a result, symptoms may not be noticeable until about five years after infection, in the case of Eutypa. Bot canker may be evident in 2 years.
In the meantime, fungi grow in the plant tissue producing enzymes and substances that break down cell walls, allowing the fungi to spread internally in the vine. Most of the fungal species advance no more than 1 or 2 inches a year through the vine. However, some botyrosphaeria species can move as much 5 inches annually.
As they grow, these fungi produce toxins, which are transported by the vine’s vascular system into the foliage and fruit.
These trunk diseases share some common symptoms, including dead spurs and buds and stunted shoots.
The toxins released by Eutypa lata stunt shoot growth and cause zig-zag internodes and tattering and yellowing of leaves. Infected shoots are likely either to die back later that growing season or the spur from which they originate will die the following year
The esca toxins cause interveinal striping of leaves, often limited to individual shoots or those growing from the same spur or cane and irregularly-shaped dark spots on berries.
Botysophaeria, the most common and widespread trunk disease, does not produce any foliar or fruit symptoms. However, infected spurs or canes will die back completely during the growing season.
A University California study in 2001 estimated the annual cost to growers, statewide, from trunk disease, including the value of lost production, treating the disease, retraining new cordons and trunks and replacing vines, at $260 million.
“That study involved just one of the pathogens,” Gubler says. “But we now know more than one pathogen was causing damage.
"The only way to eliminate trunk disease from a vineyard is to prune out infected wood and ensure that no cankers have developed below that area," he notes. Otherwise, the pathogen will continue to grow.
“As soon as you see diseased wood, prune it out immediately and retrain new cordons,” Gubler says “The longer you leave the disease in your vineyard, the harder it becomes to prune it out, and you run the potential of increasing inoculum.
“In a severe case with cankers moving down the trunk, train up suckers from scion material and let it grow during the summer. Then, prune the diseased wood out in the winter and retrain the suckers for a new vine.”
The best way to prevent trunk diseases is to prune as late in dormancy as possible and treat each wound with a protectant, he says. Late pruning takes advantage of more heat units in early spring which speeds up the healing process.
University of California research shows that wounds created in late-dormancy pruning are about 90 percent less susceptible to trunk disease than those pruned in December or January, Gubler reports.
He’s also studied an option for pruning earlier in the dormant period by using a rotary saw to leave five to seven bud position spurs and then coming back in late February or early March to prune the vine back to two buds. Vines inoculated with Eutypa and Botryosphaeria in late February or early March had about 95 percent fewer cankers than those inoculated in December or January.
In addition to treating infected vines with Topsin M or Rally fungicides, several wound protectants are available that offer longer protection. The fungicides are effective for about two weeks. So they must be re-applied depending on rainfall events because wounds can remain susceptible to infection for eight to 12 weeks. “This short term protection is why we asked for Rally and Topsin M to be registered for tractor application,” Gubler says.
One of these wound protectants, VitiSeal, is an acrylic paint. It was introduced in the past few years. Containing three essential oils, it can be hand-painted or sprayed on vines to provide about three months of protection. “So far, it has worked well in our trials,” Gubler says.
Gubler is also testing Spurshield, another wound protectant, introduced this past winter.
“These products are more expensive but I think growers should look at this as the cost of either spending the money for protection or for vine surgery and retraining the vineyard.”