Forum Delves Into Grapevine Disease

Research implies that vine microbe populations could help control fungus and viral disorders

by Jon Tourney
Andy Walker powdery mildew
Professor Andy Walker of UC Davis provided an update on breeding grapevines for powdery mildew resistance.
Davis, Calif.—A forum about current wine and grape research presented progress toward understanding and managing grapevine trunk diseases, vine microbiota, powdery mildew and grapevine leafroll-associated viruses.

The third annual day-long educational meeting Feb. 24 at the University of California, Davis, (UCD) was organized and supported by the American Vineyard Foundation (AVF), the National Grape and Wine Initiative, the Viticulture Consortium West (VCW) and UCD Extension. Dr. Deborah Golino, UCD’s director of VCW and Foundation Plant Services, helped plan the event and served as moderator.

Vine microbiota

Johan Leveau from the UCD Department of Plant Pathology described a potential new frontier in managing grapevine pathogens. He envisioned this emerging from new research to analyze and understand plant microbial diversity on grape leaves and berries, which include populations and interactive communities of bacteria, yeasts and fungi. Human microbiota already are being studied and applied to human health issues such as stomach microbiota and their relation to digestion and obesity, he explained. Leveau posed the question, “How is the establishment of vine pathogens such as powdery mildew affected by the vine’s microbiota? Could it be influenced either positively or negatively?”

Leveau outlined his research objectives:
• Build a database about microbiota diversity over time and different locations as well as under different vineyard management conditions.
• Determine how the composition of microbiota on berries and leaves correlates with incidence, severity and formation of pathogens and symptoms.
• Formulate recommendations.

Some preliminary data was gathered from a small sampling of eight leaf samples and two berry samples from Chardonnay in a Clarksburg vineyard in 2009. The leaf and berry samples were carefully washed to remove the microbiota, and the samples went through high-throughput gene sequencing that identified more than 37,000 different microbe gene sequences. Two bacteria genera represented the most abundant microbes in the samples—Pseudomonas and Sphingomonas. Some patterns showed that specific populations favored leaves and others preferred berries. 

Data is currently being analyzed from a 2010 sampling of 176 leaf samples and 176 berry samples from six vineyards in California and Oregon, taken at different times during the growing season. Leveau noted that the number of bacteria and fungi found on leaves increases in diversity and overall population throughout the season.

Leveau summarized, “If we look at a microbiota-based approach to grapevine health and productivity, we may be able to come up with ways to manipulate the structure and populations of microbiota on leaves and berries to manage pathogens and overall vine health.”

This information could be used for management decisions involving choice of cultivars, and the use of vine nutrients and bio-control agents. Ultimately, this information potentially could be incorporated into disease forecasting models and help make the use of vineyard fungicides more efficient.

Grapevine trunk diseases

Trunk diseases are caused by fungi of various species that primarily infect pruning wounds. The most common diseases (and their common names) are Eutypa dieback, Bot canker, Esca (or black measles), Petri disease and young vine decline. Philippe Rolshausen, University of California, Riverside Department of Plant Pathology and Microbiology, explained, “The infection usually starts with the spur, then goes to the cordon, the trunk and affects the entire vine with a decline in productivity over about 10 years.”

Trunk diseases diminish vineyard longevity and productivity in nearly every grape production region worldwide. He noted that in the most recent AVF survey of industry research priorities, trunk diseases ranked ninth of 42 viticultural issues listed. Rolshausen summarized, “You’re never going to get rid of it, but you can delay the disease onset and save your vineyard for several more years before replanting vines.”

Control strategies include vineyard sanitation practices, late pruning, double pruning and pruning-wound protection. Painting fungicidal material on individual pruning wounds provides the best coverage and better disease control, but it is more labor intensive, so current trials are testing applications with a backpack sprayer. Rolshausen is coordinating research with vineyard trials started in 2009, which will continue through 2011 on Syrah in Sonoma County, on Merlot in Santa Clara County and on Flame Seedless in Riverside County.

Pathogen genera found in each location include: Eutypa, Botryosphaeria, Diplodia, Phaeomoniella and Togninia. Part of the research objective is to identify potential organic products to control trunk diseases. Topsin M, Rally, Solubar (a soluble boron spray), and the biofungicides PlantShield, Actinovate AG and Serenade Max are being tested in the trials.

Results to date show that Topsin M applied with a backpack sprayer provides good control and is the most effective treatment when there is low to no rainfall after application. The treatment was washed off by rain in the Sonoma County trial in 2009.

Rolshausen cautioned, “Bio-control agents work in the lab, but not always in the field.” Future research will continue to evaluate the above materials, confirm the efficacy of Topsin M sprays and look at boron and zinc applications.

Asked if some grape varieties are more resistant to trunk diseases, Rolshausen said anecdotal observations indicate that Merlot and Zinfandel are more resistant, whereas Cabernet Sauvignon and Syrah have higher susceptibility.

Leafroll virus and wine quality

As part of ongoing research to evaluate grapevine leafroll viruses (GLRV) and their spread in recent years among Napa Valley vineyards, Golino and winemaker Tod Mostero of Napanook Vineyard (owned by 12,000-case Dominus Estate), addressed the effects of leafroll virus on wine quality.

Golino first discussed the symptoms of GLRV, which include red and rolled leaves; reduced vigor, sugar, color pigments and yields; increased acidity and delayed ripening. She cited studies from throughout the world that showed negative effects on grape and wine quality from GLRV-infected vines. “I’m convinced that we don’t want virus in our vines, but I still get people who argue that a little virus in vines can enhance wine quality,” Golino said.

Napanook is a 103-acre vineyard planted to 85% Cabernet Sauvignon near Yountville, Calif. GLRV, in addition to Eutypa, fanleaf virus and Pierce’s disease, have been identified in the vineyard. Virus-infected vines have been mapped and marked for the entire vineyard.

Mostero performed comparison experiments producing wines using fruit from healthy vines and separately producing wines from GLRV- infected vines. Based on chemical and sensory analyses, Mostero concluded that wines made from GLRV vines have lower alcohol, higher acidity, less color and less color stability over time; lower tannin quantities and qualities and less fruity aromas.

The winery has made a management decision to begin rogueing all infected vines with leafroll symptoms affecting 20% of the vine and to remove infested vineyard blocks that are more than 20 years old. Mostero said, “Our objective is 100% healthy vines.”

Given that leafroll can be spread from other infected vines and neighboring vineyards by mealybug vectors, Mostero said, “This cannot be a problem that any single producer attacks. We need community cooperation in controlling leafroll and its spread in Napa Valley.”

Powdery mildew resistance studies

Dr. Andy Walker, grapevine breeder, researcher and professor with the UCD Department of Viticulture and Enology, described ongoing research to breed powdery mildew-resistant grapevines. Identifying mildew-resistant genes in grapevines, along with breeding and lab and field-testing, is taking place in grapegrowing regions including Australia, France, Italy, Germany and the U.S.

One focus of Walker’s work has been on genetic testing and breeding with hybrid cultivars of Vitis vinifera x Muscadina rotundifolia for resistance genes and traits. In addition, genetic testing of 210 Vitis vinifera cultivars in the UCD collection brought by Dr. Harold Olmo from Turkey, Afghanistan, Pakistan and Iran as well as additional Vitis cultivars from Central Asia and China, showed that 44 of the cultivars had key DNA markers associated with powdery mildew resistance.

Walker summarized, “We need multiple resistance sources to powdery mildew for best results, and we can now track resistance markers from multiple and unique sources of vine material.”

Walker said that mildew problems are rarely seen on young vines up to two years old, perhaps because the small vines receive better fungicide coverage than older vines with large canopies. He noted that there are differences among powdery mildew strains and their resistance to fungicides. In addition, some grape varieties and clonal selections show variations in susceptibility, vineyard site, environmental factors and seasonal differences all affect powdery mildew conditions and severity.

Current research was also presented about vineyard management issues. Enological topic presentations included ethanol levels in wine, the effects of filtration on wine sensory properties, and managing Brettanomyces-related aroma compounds. Video presentations from the event will be posted in the future on the UC Integrated Viticulture website at ucanr.org/sites/intvit.

Currently no comments posted for this article.