Vineyard Location Key Factor of Pinot Leaf Curl

Disease one of several issues discussed at Sonoma County Grape Day Event

by Andrew Adams
leaf curl
Pinot Leaf Curl was less prevalent during 2012, according to Rhonda Smith, UC extension advisor for Sonoma County.
Santa Rosa, Calif.—Pinot Leaf Curl, the vine disorder that struck Sonoma County’s Pinot Noir vineyards with increasing severity during the cooler vintages of 2010 and 2011, appears to be a nitrogen disorder mainly triggered by the site conditions of a vineyard.

University of California extension advisor for Sonoma County Rhonda Smith said the disorder was far less prevalent during the past vintage. Smith announced her research at the Grape Day session held Feb. 20.

Other topics covered during the morning session attended by about 150 people included some of the latest findings on mealybugs and leaf curl viruses, phylloxera and rootstocks as well as the looming threat of red blotch disease.

During last year’s event, Smith and University of California, Davis, professor Doug Adams described the leaf curl disorder and theorized about possible causes. Based on the symptoms, they suspected a nitrogen disorder that led to the plants producing the toxic compound putrescine.

The disease can cause vine leaves to curl downward above necrotic sections of vine shoots. Severe cases of the sickness can lead to thick bands of necrotic tissue that stunts shoots and reduce yields.

Smith said 2012 produced far fewer cases of the disease, and those vineyards that did exhibit the condition suffered only mild symptoms. She sampled 14 vineyard blocks pulling symptomatic and normal-looking vine leaves. Those leaves were analyzed for the presence of putrescine and 12 of the 14 samples showed higher levels of the compound in leaves exhibiting symptoms. She stressed there is no “normal” level of putrescine in blade tissue.

Because of the nitrogen connection, Smith said growers that have had to deal with the disorder are advised to not make any nitrogen applications to their vines in early spring. Smith said it ultimately appears that the largest contributing factor to the severity of the disorder is the vineyard location and especially minimum daily spring temperatures.

Red blotch and clean plant material
Dr. Keith Striegler, the outreach coordinator for the National Clean Plant Network, discussed the network’s process for ensuring disease-free materials for growers in several industries before giving the audience an update on the grapevine red blotch-associated virus, GRBaV.

He said the disease symptoms are quite similar to those of leaf roll, but plants infected with GRBaV exhibit red or pink veins on the undersides of leaves.

Striegler said researchers first observed the disease in 2008 in Napa, Calif., but then positively identified it in 2012 through studies in New York and California. Infected vines have been identified in California, New York, Virginia, Maryland, Pennsylvania, Texas and Washington state, and the disease has struck nearly every major winegrape variety. Because of the range of infection, Striegler said it’s thought GRBaV is spread through propagation material. The virus can inhibit ripening and reduce a vine’s sugar accumulation by up to 5° Brix. Once the virus infects a vineyard there is no cure.

As Striegler discussed red blotch and planting materials, the same subject was the focus of the annual meeting of the UC Davis Foundation Plant Services held the same day in Davis, Calif.

Striegler said all of the vines at the new Russell Ranch Foundation Vineyard Block tested clean, while only three of 1,600 vines tested in the Classic Foundation Vineyard were positive for the virus. He said the testing cost $165,000, and test records can be found on the FPS website.

The FPS later released a statement about its findings later the same day. The infected vines were Chardonnay 68 (a private clone no longer listed as available), Thomcord 02 (a table grape) and Ruby Cabernet 02. “The most urgent research need now is to determine how the virus spreads,” Deborah Golino, a cooperative extension plant disease specialist and FPS director, said in the statement. “Due to the distribution of the virus in many parts of the United States and evidence that it can be transmitted by grafting, we suspect that red blotch disease is widespread wherever grapes are grown.”

She encouraged growers to evaluate vineyards for the disease and be prepared for news of other viruses and diseases because of new DNA-sequencing technology. “Some of those will be disease agents, some beneficial, and some neutral,” she said.

In Santa Rosa, growers questioned Striegler about how they could be sure they’re receiving clean nursery stock—especially if they plan to plant this spring. He said he understands their concerns but just didn’t have any answers for them and cautioned that there may not be enough answers for the next two years. “What we really need to know is how it’s spreading,” he said.

More information about the virus can also be found here.

Mealybugs and spreading leafroll
Red blotch may have stayed undetected for years because growers assumed the symptoms were for the similar grape leafroll virus, or GLRaV. The primary vector for that disease is the mealybug, and Dr. Rodrigo Almeida, an entomologist with UC Davis, offered an update about current leafroll research.

He said researchers have identified several different types of GLRaV, but GLRaV-3 is the most common—especially in Napa Valley. In an experiment to analyze virus transmission, he found that mealybugs with the virus only needed one hour to infect more than 25% of the plants in the study. “This process of transmission happens very quickly,” he said.

In a July 2011 field trial conducted with Napa County UC Extension advisor Monica Cooper, a Napa Valley Cabernet Franc vineyard planted in 1994 was inoculated for 48 hours with 10 grape mealybugs carrying the virus.  After the inoculation period, the vineyard received the same standard cultivation practices including spraying. Almeida said vines that were not inoculated for negative control did not become infected, and there was no natural spread. By August 2012, the plants with the virus were exhibiting significant symptoms. “This was pretty surprising to us that from one leaf to infecting the entire plant in just a year,” he said. The symptomatic grapes also were about 2° Brix behind the uninfected plants by late September.

Researchers in Napa also found variants of the virus in a vineyard block that did not appear to come from adjacent vineyards. Almeida said those findings indicate the disease and its vector can be brought in on the wind. “Even if your adjacent neighbors are doing a good job, these infections can come from far away,” he said.

Phylloxera and rootstock
Dr. Andrew Walker, who develops new rootstocks at UC Davis offered an overview of phylloxera and addressed some concerns about the pest and the popular 101-14 rootstock. Walker said he has heard from growers concerned if the rootstock is becoming more vulnerable to phylloxera.

Walker said vines with the rootstock at the Oakville Field Station in Napa Valley and elsewhere has exhibited nodosities, but there has been no decline or tuberosities. Walker said he can understand the concern, as about 40% of Northern California and the Central Coast are planted with the rootstock, but said the issue more likely can be attributed to the root structure.

101-14’s roots usually grow out in a thin horizontal pattern. This structure can be compromised in heavy clay soils that can be too wet in the spring and crack in the summer. The roots also can exhibit poor regeneration and are not well adapted to deficit irrigation. However, phylloxera strains found on the rootstock have exhibited better reproductive capacity. 101-14 strains can reproduce in less than two weeks, and adults can produce more than a dozen eggs per day, which can lead to “extremely large” populations that may weaken the vine.

The popularity of the rootstock also may explain why growers may be noticing more issues with it. “If we’re using huge amounts of 101-14, we’re going to see more problems with it,” Walker said.

Walker, who also has devoted a great deal of research into cultivating vines resistant to Pierce’s disease, also gave the audience a brief update on his work in that area when asked about any recent developments.

He said he’s proud to note he’ll be hosting a series of tastings featuring wines made with vinifera vines that are 97% resistant to PD. He said he’ working to identify five to 10 cultivars that would be good for winegrape production. “They taste like winegrapes to me, act like winegrapes and they are Pierce’s disease resistant,” he said.

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