Lodi, Calif.—Late pruning followed by a spray application of fungicide is considered the best strategy to prevent fungal infections of grapevines that cause Eutypa dieback and canker diseases. New biofungicide and paint products to spray onto pruning wounds and prevent fungal infections could be available in the near future. Preventing and managing Eutypa dieback and grapevine canker diseases was the topic of a forum presented by the Lodi Winegrape Commission (LWC) on July 14. A past study estimated that the costs of managing canker diseases—combined with vine losses and yield declines—amount to $260 million per year for California grapegrowers.

Overview
University of California, Davis, plant pathologist Dr. Doug Gubler provided an overview of grapevine canker diseases. Research by Gubler and others has contributed significantly to the understanding of canker diseases and more effective control methods in recent years. For many years, Eutypa dieback, caused by the fungus Eutypa lata, was considered the main dieback disease of grapevines. It results in a gradual decline in yields by causing the death of spurs and cordons; eventually it moves into the trunk to kill the entire vine. Gubler observed, “Most of what we’re finding in grapevine cankers is not just E. lata, but a combination of fungal species.”

Gubler listed other major types of fungal pathogens/diseases found in grapevines: Botryosphaeria species that cause “bot canker,” Esca or black measles, Young esca or young vine decline, Blackfoot disease and Phomopsis canker. He focused on Eutypa and the bot canker pathogens that produce similar symptoms: A dark, wedge-shaped (also called V-shaped or pie-shaped) canker seen in crosscuts of cordons or dead spurs.

Eutypa can cause stunted shoots and foliar symptoms such as leaf chlorosis, but bot canker produces no foliar symptoms. Cankers and symptoms are commonly seen in vines 10 or more years old, but infection can occur in susceptible vines after only four or five years.

E. lata is the main Eutypa species affecting Northern California grapegrowing regions, but another species, E. leptoplaca, was discovered during the past decade in Napa and Sonoma counties. It also causes dieback in grapevines.

Ten Botyrosphaeria species have been identified from grapevine cankers in California, and 14 bot canker species have been found worldwide. Some can colonize and spread faster than E. lata. Gubler’s research identified the two fastest spreading Botryosphaeria species in California—Lasiodiplodia theobromae, and Neofusicoccum parvum. L. theobromae can colonize and move through a grapevine as much as 5.5 inches per year.

It moves faster toward the trunk than toward the end of the cane. L. theobromae is more commonly found in California’s warmer grapegrowing regions—the south San Joaquin Valley and Riverside County. Another bot species, Diplodia seriata, is found throughout the state but is a less aggressive pathogen.

E. lata overwinters in diseased wood and during the damp fall and winter months produces perithecia (fruiting bodies) that release spores soon after rainfall. Other canker species also release spores primarily in fall and winter, after rainfall that must land on a pruning wound or injured vine to gain entrance, colonize and form cankers. Spores can enter vines through wounds or breaks, including injuries from mechanical harvesting, but infection overwhelming occurs through pruning wounds.

Several California native tree and shrub species found in grapegrowing regions are hosts for canker species. These include: Fremont cottonwood, California bay laurel, California buckeye, big leaf maple, ceanothus, and oak and willow species. Other tree and fruit crops can be hosts, including apricot, almond, pear, cherry, kiwi, and blueberry, and landscape plants such as oleander. Gubler advised being aware of the vegetation and ecosystem surrounding the vineyard. If cankers are found in dying tree branches, removal of deadwood may help with prevention. He said the main source of spore infection in most vineyards is from the vineyard itself.

Late pruning helps
Mechanical pruning, also called minimal pruning or box pruning, reduces the number of cuts, particularly if done later in the dormant period. This can significantly reduce infection and dieback compared with spur pruning, while maintaining yield and quality.

Cane pruning also reduces dieback. Double pruning is another preventative measure, utilizing mechanical pruning in fall or early winter, followed later by hand pruning before bud break to remove potentially infected wood below the first pruning cut. Studies in the North Coast showed that neither E. lata nor bot species were found more than 1.5 inches below the pruning cut.

If possible, pruning should be done during dry weather—ideally when rain is not forecast for one week or more. Gubler observed, “Regardless of the species, our research in inoculating pruning wounds with pathogens shows that susceptibility declines as the pruning wounds become older. The highest infection occurs right after the vines are pruned.”
 
Research trials of several registered chemical fungicides sprayed onto new pruning wounds on Chardonnay vines using tractor spray applications showed that some are effective in preventing infection against some fungal canker species but not others. For single product applications, Topsin M provided the best overall control across all species. Rally alone provided the best protection against E. lata. However, a combination of three fungicides—Enable plus Rally plus Topsin M—was the most effective control treatment for all pathogens.

Gubler summarized, “Our recommendation is to prune as late as possible (late February or March): The wound is less susceptible to infection in late winter and spring. If you can add a fungicide, or coat the wound to prevent infection on top of that, then protection from infection can be increased to 95% or more.” 

Applying fungicides to pruning wounds even in early-season pruning can help in dry weather—if the application is able to dry on the wound. An application of Rally or Topsin M can provide protection for about six weeks, unless it’s washed off by rainfall. Gubler said wounds can be susceptible to E. lata for as long as six weeks, and for some Bot species up to eight weeks. Vine bleeds from the wounds after pruning actually protects from spore entry. In this case, no follow-up chemical application should be done. Applying fungicide to a wet or bleeding wound is not effective.

Potential new products Gubler is optimistic about the future potential for new biofungicides that can be sprayed to protect pruning wounds. Although in recent years several biofungicides have been tested with mixed results, Gubler said a newly discovered bacterium has provided 60-day protection from E. lata infection in lab experiments.

In addition, a species of Trichoderma, a soil fungus, shows good promise in protecting against canker disease pathogens for periods of two months or longer. Gubler also discussed a new paint product that may soon become available—a diluted version of the paint used on the Space Shuttle that will last one year without cracking. It could be applied by spray equipment to coat pruning wounds and prevent fungal infection.

Susceptibility by varietal
Although no grapevine is considered immune from canker disease infection, grape varieties differ in their susceptibility to canker diseases and can pathogen species. Gubler said physiology of the vine plays a role. Vines less susceptible to Eutypa have higher amounts of organic acids. UC Cooperative Extension viticulture advisor for San Joaquin County Paul Verdegaal grouped varieties into three categories based on generally accepted knowledge and observations.

• High susceptibility: Chardonnay, Chenin Blanc, Sauvignon Blanc, French Colombard, Cabernet Sauvignon, Petite Sirah.
• Moderate susceptibility: Gewürztraminer, Muscat, Alicante Bouschet, Cinsault, Pinot Noir, Syrah, Tannat, Zinfandel.
• Low susceptibility: Pinot Grigio, Riesling, Sémillon, Verdelho, Barbera, Cabernet Franc, Malbec, Merlot, Mourvèdre, Petit Verdot.

Growers weigh the costs
A panel of Lodi growers and vineyard consultants, who are also members of the LWC research committee, discussed experiences in managing vineyards with dieback. Grower and PCA Steve Quashnick has implemented dieback removal in a Petite Sirah vineyard where 40% of the vines showed canker disease. Over several years, infected cordon and trunk wood was removed from the vine below the diseased tissue, while new sucker shoots were allowed to grow from the vine trunks then trained into the canopy along trellis wires. The cut vine wounds were painted with B-LOCK, a paste with boric acid that forms a water resistant coating to prevent re-infection. Quashnick has also practiced late pruning followed immediately by a treatment with Rally. He summarized, “This disease will cost you one way or another. You have to consider the factors at each site and decide how much you can spend to maintain productivity.”

Lodi grower Brad Goehring reported success in bringing back a Cabernet Sauvignon vineyard by removing infected wood below the cankers and retraining suckers into the trellis. He noted that this results in loss in grape production from a vine for one or two years and requires bringing it back into balance. In some cases, the new suckers and shoots can be over-vigorous when the vine’s mature root system is putting all its energy into the new growth. Goehring and others advised using a “Sawzall” (reciprocating saw) as the best tool for cutting off diseased wood from vines with clean and precise cuts.

Kautz Farms vineyard manager Joe Valente said Kautz began using mechanical box pruning in vineyards with dieback and was able to maintain yields and grape quality for several more years before the disease took a severe toll. “Our management plan was ‘let’s get what we can as long as it’s economically viable until we have to pull out the vines and replant the vineyard,’” Valente said.

Large farm operations face special challenges, he observed, “Late pruning sounds great, but when you have hundreds of acres like we do, we can’t really wait and try to prune all our acreage in March.” Valente advised weighing all the factors when dieback is found (age of vineyard, potential yield, type of variety and price per ton, lifespan of the trellis/infrastructure, and whether the vines have other problems). “Look at all the factors and see what option pencils out best, because whatever you do, it will cost money,” he advised.

The growers discussed disposal of diseased wood prunings. One strategy is to remove diseased prunings from the vineyard, stack them in piles and burn them, if allowed by the local jurisdiction. If the vineyard is tilled, prunings will be broken up and disked under the soil where the wood will be unable to produce spores. Larger cut cordons and trunk wood can be run through a shredder/chipper and disked into the soil or disposed of elsewhere.

Consultant Stan Grant of Progressive Viticulture is a proponent of late pruning, double pruning and applications of pruning wound protection. Noting successes in vineyards of susceptible varieties such as Cabernet Sauvignon and Syrah, Grant said, “These practices do work.”