(Wines&Vines) - As researchers continue to try and determine how the red blotch virus is spreading through vineyards, they are also starting to look at how the associated disease affects grape composition and wine quality.

It appears the effects of red blotch can vary depending on grape cultivar and vineyard, and reducing a crop to try and minimize those effects doesn’t seem to work.

At the Sonoma County Grape Day event held Feb. 18 in Santa Rosa, Rhonda Smith, the University of California viticulture farm advisor for Sonoma County, and Anita Oberholster, University of California, Davis, assistant cooperative extension specialist, discussed their work to better understand how the virus can limit grape development and what it does to wine made with grapes from virus-positive vines in projects funded by the American Vineyard Foundation. 

Smith said she and other researchers spent two years monitoring between 20 and 30 infected vines of Cabernet Sauvignon, Merlot, Chardonnay and Zinfandel in four vineyards located in Sonoma and Napa counties. Grapes from vines infected with the virus produced berries with lower Brix levels and often with higher levels of malic acid. In 2014, Smith said Chardonnay vines with the virus exhibited “significantly decreased” clusters, cluster weights and berries per cluster. Zinfandel yields were also depressed, although the disease did not appear to affect other cultivars infected with red blotch.

Dropping red blotch fruit
In 2013, using the same virus-infected Cabernet Sauvignon and Chardonnay vines, researchers removed flower clusters to ensure all fruit-bearing shoots only had one cluster. The resulting grapes were then analyzed for total soluble solids, pH and titratable acidity and compared to grapes from vines without the virus. Researchers found there was no significant difference. “These results indicate that dropping fruit to improve quality in vineyards with red blotch disease may not produce the desired effect,” Smith said in a report about the research.

Oberholster then discussed some of the analysis being done at UC Davis on grapes from red blotch-infected vines as well as anecdotal insights picked up from the field. In extreme cases of red blotch disease, vines produce pink grapes that never get past 19° Brix, Oberholster said. In other cases, vines can test positive for the virus but show no symptoms; or symptoms will appear one vintage but not the next. Oberholster said she’s hoping to continue evaluating grapes and wine from red blotch-positive vines to help growers and winemakers better understand the disease and how to mitigate some if its effects through viticultural and winemaking practices.

She said the first study (in 2013) on Napa County Cabernet Sauvignon grapes from virus-infected vines found the grapes had significantly lower levels of total anthocyanins, tannin and phenols. Wines made with the grapes did not have much less anthocyanin and phenols—and actually had a bit more tannin—but it still tasted vastly different.

Wide range in Brix levels
In 2014, researchers harvested Chardonnay, Merlot and Cabernet Sauvignon grapes from vines that tested positive for the virus as well as those that did not. Most of the grapes from vines with red blotch had lower Brix levels, although that ranged from 4% less to 20% less, depending on the samples. One Chardonnay sample exhibited no difference in Brix between grapes from a virus-positive vine and one without the virus.

The grapes then underwent analysis with the Adams-Harbertson tannin assay, which found little difference between virus-positive and virus-negative grapes, even in those samples that exhibited a large difference in Brix levels.

Initial testing on wines made with the grapes did find some differences. One of the Cabernet wines made with virus-positive grapes actually had more polyphenol compounds than the wine made of grapes harvested from a virus-negative vine, but Oberholster said it definitely didn’t taste any better.