Scientists Closer to Understanding Grapevine Red Blotch Disease

4/20/2013 7:00 AM
By Helen Margaret Griffiths New York Correspondent

GENEVA, N.Y. — Viruses are a major economic issue in grape production, with 56 viruses currently known to be associated with grape diseases.

For a number of years, grape producers and Extension educators have been frustrated with their inability to identify the cause of leaf reddening, reduced sugar level and delayed grape ripening, specifically in red wine grape cultivars.

Thanks to research performed by a team of scientists from California and New York state, that may soon change.

The disease has been named grapevine red blotch disease and is thought to be associated with a new virus, grapevine red blotch-associated virus (GRBaV).

The disease was the topic of a recent webinar funded by the National Clean Plant Network.

Tim Martinson, a senior Extension associate with Cornell University in Geneva, N.Y., moderated the sessions. Marc Fuchs, associate professor of plant pathology at Cornell, Rhonda Smith, a viticulture farm adviser at University of California, Sonoma County, and Deborah Golino, director of Foundation Plant Services at University California, Davis, discussed identifying the virus, current understanding of the disease, ongoing research and implications of the disease for grape producers.

Fuchs explained how the team of scientists from Ithaca, Geneva and California identified the virus associated with the disease.

"There is a very high correlation between presence of GRBaV and red blotch leaf symptoms," he said.

The availability of molecular biology tools made it possible to identify this as a circular, single-stranded DNA virus which, along with other characteristics, suggests a geminiviridae-like virus. To date there is only one other DNA grape virus, with the majority being linear RNA viruses.

The diagnostic tools are now available at some commercial laboratories, and growers concerned about symptoms on their grapes should contact their Extension educator for the name of their closest laboratory.

"Over 20 different grape cultivars in many different states in the U.S. have currently tested positive for GRBaV," Fuchs said.

Grape red blotch disease has been reported in Canada (British Columbia), Australia and New Zealand. Scientists are just starting to understand the biology and epidemiology of the disease.

GRBaV is graft transmissible, but whether it can be dispersed in other ways, such as insects, is yet to be determined.

"There has been an increase in incidence over time in vineyards in some locations," Fuchs said.

How the virus spreads within the plant or vineyard over time is not yet clear, although the virus has been detected in one-year-old and also 20-year-old vines.

Red wine-producing cultivars, such as Cabernet Franc, Cabernet Sauvignon, Merlot, Petite Sirah, Petit Verdot and Zinfandel, seem to be particularly susceptible, although the virus has been detected in white wine grape cultivars and table grape cultivars as well.

Symptomatic vines show greatly reduced sugar levels, with brix from red-leaf vines being 4 to 5 units lower than green-leaf vines. In addition, there may be a lack or delay in grape maturity. Both factors are major issues for the wine producer.

"If additional viruses are present in the plant will there be an interaction? Currently we don't know," said Fuchs, stressing the importance of using "clean" foundation stock.

Prior to about 2007, site issues, such as soil, drainage and geographic exposure, were often blamed when red blotch symptoms were observed on grapes and no known virus could be detected, said Smith, of the University of California.

The effect of GRBaV on vines of white grape cultivars is not clear.

"In red cultivars we see different symptoms within and between cultivars and also at different times in the season," Smith said.

Researchers are currently studying effects on yield, fruit and wine quality.

"Some growers are postponing replanting until more information is available about GRBaV and red blotch disease," Smith said.

Considering commercial diagnosis has only been available since October 2012, good progress has been made in studying the disease.

Foundation Plant Services at University of California, Davis, is one of only three locations in the country where grapes can be imported.

"The clean-up’ of imported materials can take years," said Golino, adding that many grape growers are not aware of the damage that grape viruses can cause.

Plant virus detection technology has advanced over the years at FBS, Golino said.

In 1990, ELISA (an antibody-based technique) was available for many grape viruses. In 1993, the molecular technique known as conventional RT-PCR became the method of choice. That was replaced in 2001 by the very sensitive qPCR (real-time quantitative PCR), the routine detection method currently used.

Golino briefly discussed next generation sequencing technology, which she likened to a full body scan. FPS scientist Maher Al Rwahnih, who was an early user of the technique, teamed up with USDA scientist Mysore Sudarshana and generated important information about the virus status of vines showing the symptoms associated with red blotch disease.

"Many new viruses and other microbes are likely to be detected in grapevines in the next few years due to this new technology," Golino said.

Not all will cause disease but some will, and determining their role will take time, but the ultimate outcome will be to have "healthier" vineyards.

Growers wishing to plant new vines are advised to purchase vines which are certified free of GRBaV and, if possible, plant them in separate blocks if they are going in an established vineyard.

Growers with plants of red grape vine cultivars showing symptoms of red blotch — late-season leaf blotching, reddening of primary, secondary and tertiary veins, with no leaf roll or cupping — are encouraged to contact their Extension educator for assistance with testing for GRBaV.

As scientists obtain more information about this emerging disease, Extension bulletins will be updated.

Currently, there are some helpful publications for growers and managers which show disease symptoms and provide additional information.

View them online at :

http://iv.ucdavis.edu/files/162288.pdf

http://iv.ucdavis.edu/files/157508.pdf

http://iv.ucdavis.edu/files/164812.pdf


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