Disease Detectives Tackle the Mystery of the Fall Colors

WSU plant pathologist Naidu Rayapati and his colleagues are carefully unraveling the intricate biochemistry and molecular biology of grapevine leafroll disease.

Grapevine leafroll is a complex viral disease that can cause a marked decline in grapevine vigor, grape quality, and fruit productivity, according to Rayapati. The disease can reduce yields as much as 50 percent or even more, depending on the severity of infection. A few years ago, it was estimated that nearly 10 percent of Washington’s vineyards have grapevine leafroll disease. Anecdotal evidence suggests that the disease is more wide spread than previously thought, raising alarm among industry stakeholders. Grapevine leafroll disease accounts for about 60 percent of the production losses of grapes worldwide, Rayapati said.

One of the most perplexing questions Rayapati is addressing is one of symptomology: why do some grape varieties show the “fall colors” symptoms in their leaves while others do not?

The reds and oranges in leaves we see in fall are due to a fascinating group of natural chemical compounds called anthocyanins. Anthocyanins are valuable to plants, giving fruit red, blue and purple colors, and providing protection against UV radiation while attracting pollinating insects and animals that can aid in seed dispersal. And, of course, it’s anthocyanins we see in action each fall as the leaves on deciduous trees change color before dropping.

The thing with grapevines, though, is that it’s the fruit that is supposed to turn color, not the leaves. Grapes change color during véraison, with red varieties deepening to their characteristic purple while undergoing other complex changes not visible to the eye but that result in an accumulation of sugars and other beneficial compounds prized by winemakers and table-grape eaters.

When vines are infected with leafroll disease, however, fruit ripens unevenly while, at least in the red varieties, the leaves turn color after véraison. This is puzzling because, in fact, the viruses associated with grapevine leafroll can be detected in diseased vines throughout the season.

“There must be something going on in the bioregulation of anthocyanin pathways,” Rayapati said. In living organisms, things rarely occur in straightforward ways. Instead, growth, ripening, and other functions occur through complex networks of interactions called bioregulatory pathways. If there is a blockage or perturbation anywhere along that chain, as with a disease, there are often unexpected and undesirable results.

“What we suspect,” Rayapati hypothesized, “is that the virus is present in the phloem so that the translocation of nutrients may be affected.” The phloem is akin to the circulatory system in an animal in as much as it transports or translocates nutrients. The leaves of a grape plant should, under normal, disease-free conditions, metabolize sugar and carbohydrates and transfer these nutrients through the phloem to the berries.

But that’s not happening. Instead, infected plants produce berries with less color and fewer sugars. It appears that anthocyanins are not being synthesized in berry skin. And that’s not only a mystery to scientists, it’s a problem for growers.

Rayapati, along with his colleague James Harbertson, a wine chemist in the WSU School of Food Sciences, are working to shed a little light on the mystery of anthocyanin bioregulation in various grape cultivars.

“We have the tools enabling us to very sensitively measure which compounds are being accumulated where in the plant,” Harbertson said. “And what we see is that, indeed, specific anthocyanin compounds are accumulating in the symptomatic leaves of Merlot grapevines.”

Rayapati and his team’s molecular studies on anthocyanin pathway genes complement these results. Their collaborative research has recently been published in the journal BMC Plant Biology, where it is marked as “highly accessed,” indicating broad interest due to the number of times the article has been downloaded.

“The value of this research,” Rayapati said, “is that we are now able to use this information to establish a baseline in order to measure symptom expression in other cultivars. Using these tools, we should be able to better understand the differences in impact the leafroll viruses have on various cultivars.

“We are the first group to address this kind of research,” Rayapati said. “We are approaching the disease at a molecular level to dissect the cascade of events occurring at different levels in the grapevine.” Rayapati and Harbertson’s collaborators include Linga Gutha, a post-doctoral research associate in Rayapati’s lab, who conducted studies on gene expression and real-time PCR and Luis Casassa, a graduate student in Harbertson’s lab, who conducted HPLC analysis of anthocyanins and other compounds identified in this study.

Understanding the symptomology of grapevine leafroll disease at the molecular level is critical if growers want to correctly identify the cause of changes in leaf color.

“From a practical point of view, this kind of research helps us to differentiate disease symptoms from nutrient disorders, which can also cause color change and reddening, and make appropriate recommendations to growers,” Rayapati said.

Exactly how grapevine leafroll disease affects the anthocyanin pathway remains a mystery and is the subject of ongoing research. As Rayapati pointed out, the disease is caused by a complex of viruses. If identifying the pathology of a single virus is a bit like hunting down a needle in a haystack, then Rayapati and his colleagues are searching for multiple needles hidden not only in haystacks but in piles of fall leaves, as well.

The research has great value not only in terms of understanding the disease but also because, in hunting for those needles, the molecular regulation of the beneficial compounds prized by winemakers is exposed. It’s a bit like taking a watch apart to remove a speck of dust and learning how a complex set of gears interact to tell time.

What can growers do to protect their vineyards against grapevine leafroll disease?

First of all, Rayapati advises, keep a sharp eye out. Look for the symptoms in red-grape cultivars. If there are symptoms, or if a grower suspects an infection in white cultivars where symptoms won’t be visible, test for the presence of virus.

“We take random samples to test for infection,” Rayapati said. Keeping in mind that grapevine leafroll disease is caused by a complex of viruses, not just one, testing needs to be done for several viruses. The ELISA test is about $4 per sample per virus while PCR from a commercial lab is more like $40/sample/virus.

“Once we know the level of infection, we can develop a strategy with the grower,” Rayapati said.

For Washington growers, best management practice should consist of roguing and replanting with virus-tested cuttings as a strategy for eradicating the disease from virus-infested vineyard blocks. And planting new vineyards with virus-tested planting materials is one of the most critical decisions that a grower will live with for the life of the vineyard. Procuring planting material from reliable sources like certified nurseries deserves careful consideration.

For more information

This article is based on the paper “”Modulation of flavonoid biosynthetic pathway genes and anthocyanins due to virus infection in grapevine” by Linga R. Gutha, Luis F. Casassa, James F. Harbertson and Rayapati A. Naidu in BioMedical Central Plant Biology. The paper can be accessed online at http://bit.ly/9NTaFd.

Download Naidu Rayapati’s WSU Extension publication “Grapevine Leafroll Disease” from http://bit.ly/cxBHUy.

Rayapati and undergraduate research conducted a survey of grapevine leafroll disease in a well-known Red Mountain vineyard; read “Iraq War Vet Comes Home to Wine Country” at http://bit.ly/cpuNdW.

In “The Virus Fighters,” science writer Brian Clark visits Rayapati’s lab in Prosser, Wash., and learns about some new tools that may help with disease diagnosis: http://bit.ly/apzpxa.

Deficiencies in plant micronutrition can also lead to color change; for more on that topic, see Clark’s article “A Pinch of This, A Dash of That” at http://bit.ly/9qPPEV.