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Seeing Leafroll More Easily

September 2009
 
by Peter Mitham
 
 
    HIGHLIGHTS
     

     
  • There are at least six strains of grapevine leafroll virus, which presents itself in a variety of ways.
     
  • A shortage of certified stock means many Washington state growers are forced to buy uncertified grapevines.
     
  • Cheaper, more reliable testing methods using leaf reflectance are being developed.

Rick Hamman, director of viticulture at Hogue Ranches in Prosser, Wash., can't forget the Tri-Cities area vineyard that highlighted the impact a bad leafroll infection can have on grapevines.

The leaves of the entire block were a rich purple, and fruit development on the Merlot vines was about four weeks behind those on healthy vines in adjacent blocks. Brix levels wouldn't budge beyond 22.5º. "It needed to be 24.5º, 25º, and just never could ripen," Hamman recalled.

Perhaps the only upside was that the problem was too visible to miss--something that's not always the case. That's why accurate testing is so important to keep new cuttings from introducing virus infections into the vineyard. While much attention has focused on scale insects and nematodes as potential vectors of grapevine viruses, many in Washington state's wine industry now recognize how the shortage of clean plant material during the planting boom of the past decade was the major factor in spreading grapevine leafroll and other viruses through the state.

When clean stock isn't available, many growers will use the next-best stock. Unfortunately, it's usually up to the grower to determine what's "best" in any given circumstance, and even then the best-available stock may have viruses lurking within its tissues. It's a big reason why grapevine leafroll and other diseases are a growing problem in Washington and elsewhere.

"I think it would be very difficult to find a red bottle of wine made in Washington state that didn't have a little bit of leafroll in it," Hamman said. "There's a lot of leafroll out there."

Six strains of leafroll have been identified, as well as rugose wood complex and grapevine fanleaf disease. But testing remains expensive with current diagnostics running at $50 a pop.

Hamman said he knows one grower who spent upwards of $6,000 on testing to identify blocks that could supply 400,000 cuttings with the least chance of introducing disease to his vineyards, but growers don't want to spend that much. It's "a monetary thing--how much can you keep spending on testing, and so forth," he said.

But with the threat of virus-borne diseases rising, and greater awareness of the damage viruses cause to both vineyards and wine quality, growers are looking for faster, more reliable tools to identify infected vines.

leafroll sensor
 
Grape virologist Naidu Rayapati and Eileen Perry, assistant director of the Center for Precision Agricultural Systems, use a spectral radiometer to measure light reflected from grape leaves.

Dr. Naidu Rayapati, a virologist at Washington State University's Irrigated Agriculture Research and Extension Center in Prosser, says visual inspection isn't a conclusive indicator of disease in many cases. While leaves may discolor after veraison--and even roll late in the season--other symptoms often aren't present.

"The grower might think any color or discoloration is because of leafroll, which may or may not be true. And lack of this kind of symptom doesn't mean that there's no virus in the case of white grape cultivars," Rayapati said.

There also are variations in how grapevine leafroll manifests itself in the Washington environment, and in conjunction with other diseases. The complications are a key reason why Rayapati and researchers at WSU's Center for Precision Agricultural Systems are seeking alternative indicators for the presence of infection.

A ray of light
One promising indicator of infection may be the degree to which grape leaves reflect light.

A healthy leaf reflects a different spectrum of light than an infected leaf, a phenomenon that allowed Rayapati's team to develop a protocol that may help growers to determine if a vine is infected simply by the kind of light it reflects. The reflectance of leaves is used to determine plant health in various field crops, but research published by Rayapati this spring in the journal Computers and Electronics in Agriculture shows the method may help grapegrowers, too.

The principle is simple: When light falls on the surface of a leaf, it reflects certain wavelengths depending on the leaf's pigmentation. The difference in pigmentation is what the human eye recognizes as the healthy color (or symptomatic discoloration) of the leaves. By assessing the spectrum of light that leaves reflect, however, Rayapati hopes to help growers detect changes that indicate disease, speeding up virus identification and in turn the removal and replacement of infected vines.

"The reflectant spectrum will be different compared to a healthy leaf," Rayapati said. "So we are trying to see if we can capture that kind of difference between a grapevine leaf infected with the leafroll disease, and if we can use that kind of information as a diagnostic tool which is a non-destructive, non-invasive means of diagnosis."

Rayapati and his team have spent the past two years measuring the reflective character of Cabernet Sauvignon and Merlot vines near Prosser, sampling both healthy tissues and those infected with grapevine leafroll virus 3. The measurements are helping identify the range of light that healthy and infected leaves reflect, so that future testing will diagnose infections accurately.

The key differences in the reflectance of vines occur at the peak of green wavelengths (about 550 nanometers) and red wavelengths (about 680 nanometers). The differences indicate the lower chlorophyll absorption that occurs in leaves on vines infected with leafroll virus.

Rayapati's team has been gathering its da ta using a portable spectrometer from Analytical Spectral Devices in Boulder, Colo. WSU's Center for Precision Agricultural Systems would likely develop a similar device that vineyard workers could hold in their hands or mount on equipment to scan vines. Aerial surveying isn't possible, because only the lower leaves of vines display symptoms of leafroll disease.

Rayapati doesn't expect the device to be available for field trials for another "three or four years," meaning general adoption wouldn't occur much before the middle of the next decade; but the benefits could be significant.

Faster, cheaper diagnosis
Scanning the vines promises to be less intrusive, faster and cheaper than current testing methods. Growers now must send samples of leaves they suspect to be infected to a lab, where separate tests are required to determine the presence of each virus strain. The cost of identifying which virus is infecting the vines easily mounts into the hundreds of dollars.

Kevin Corliss, director of vineyard operations for Chateau Ste. Michelle, said the new method would improve the accuracy of detection, and help make the economic argument for testing.

Right now, it just doesn't make sense for even a large operation like Chateau Ste. Michelle to spend more than a few thousand dollars for tests that reduce the likelihood of infection but never quite eliminate it.

"If you're sampling, you're doing a statistical test. You're trying to catch that disease based on X-number of samples per acre. And as you are able to sample more vines more cheaply and faster, you get much closer to reality versus a guess," he said. "Naidu's work on rapid through-put and multiple virus testing and things like that really help facilitate doing a lot of testing, testing a lot of vines in a short period of time and more economically."

But the challenges that make accurate diagnosis so costly today are also why it will take WSU time to develop an accurate diagnostic device. Different viruses may cause leaves to reflect different strains of light, for example, and Rayapati's team has only been working with grapevine leafroll virus 3 to date, because it's the most prevalent strain in Washington.

Researchers also know that different cultivars reflect different spectra of light, and need to expand the protocol they're developing beyond Merlot and Cabernet Sauvignon vines to include Chardonnay, Pinot Noir and others.

"It's not just one virus, one plant situation," Rayapati said. "Leafroll disease symptoms are more visible in red grapes compared to white grapes, so right now we have done some work with the red grape cultivars, but we would like to see what kind of differences we get in a white grape cultivar like, for example, Chardonnay. And whether there are any unique features or signatures between red grapes and white grapes."

Our Northwest correspondent Peter Mitham is a freelance agriculture writer based in Vancouver, B.C. Look for his weekly dispatches at winesandvines.com Headlines. Contact him through edit@winesandvines.com.

Nurseries could benefit, too
 

 
A shortage of certified vines--and of the varieties in demand--is a key issue as Washington state growers seek to fight the spread of grapevine viruses.

While growers are being told to use certified plant material, rapid growth of the industry has boosted pressure on propagators like Inland Desert Nursery in Benton City, Wash. Inland's general manager Kevin Judkins estimated that a quarter of the 6 million vines he produces annually are Washington State Certified. Certified vines sell out every year, leaving growers with little choice but to take the next-cleanest material.

"Where people are planting hundreds of acres, that's the only way to really fill the need," Judkins said.

Moreover, equal stocks of certified material are not available for every variety or clone. Whether it's an established variety like Chardonnay, which has seen an uptick in demand this year, or a new clone of Cabernet Sauvignon, Judkins said it's hard to keep abreast of planting trends.

"There's always a new variety or clone or something; you just can't really outguess what the demand's going to be," Judkins said. "We just can't get there very quickly."

The shortfall requires him to develop relationships with growers he can trust to produce clean vines--or vines that are as clean as possible. And even then, vines may harbor disease because certification merely indicates a reduced risk of disease rather than virus-free status, "unless you go out there and do testing," he said. "But the testing is so costly."

P.M.
 
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