Close to 60 viruses are known to affect grapes worldwide.
Dr. Naidu Rayapati says that the discovery of mixed virus infections in grape vineyards makes thorough testing more important than before.
The world of grapevine viruses is complicated, Washington State University researchers are learning. Many of the samples taken from Washington vineyards not only have grapevine leafroll virus, but also other viruses, making testing more difficult and time consuming.
Grapes have more documented diseases than any other single perennial crop, according to WSU grape virologist Dr. Naidu Rayapati. Nearly 60 viruses are known to affect grapes worldwide, with grapevine leafroll disease the most common, accounting for about 60 percent of the global grape production losses due to virus diseases. "Inability to accurately diagnose a viral disease has several implications for the wine industry, and our main goal is to take accurate aim at viruses in wine grapes through reliable and sensitive diagnostic methods," he said.
Since grape leafroll disease was detected in Washington vineyards during a state survey in 2000, the grape virology program at WSU has worked to educate growers about grape diseases and improve diagnostic tools for virus testing. Vineyard samples continue to be tested for leafroll as well as other viruses.
"The message is that leafroll is a complex virus disease," said Rayapati. Scientists have now identified nine viruses called grapevine leafroll-associated viruses (GLRaVs), with six of them (GLRaV-1, -2, -3, -4, -5, and -9) found to occur either alone or in combination, in grapevine-leafroll-infected vines in Washington. Initially, three viruses (GLRaV-1, -2, -3) were predominantly found in Washington vineyards.
Mixed infections often result in synergistic effects that lead to more severe damage than from single infections.
"One of the surprising things we've found when we went looking for viral diseases is the mixed virus infections found in combination with leafroll," Rayapati said. "We're finding Rugose wood complex viruses mixed in with leafroll infection. And we have some indications that fanleaf disease could also be present as mixed infection with leafroll viruses at some vineyards."
Grapevine fanleaf disease, caused by grapevine fanleaf virus, is the most widespread of the nepoviruses (transmitted by nematodes) around the world.
The virology team's new discovery of mixed infections with leafroll has implications for the industry that has been working hard in recent years to keep the vineyards free of diseases through the "clean" plant and certification program.
"If you don't test for several viral diseases, only testing for leafroll, you might be missing some," Rayapati said, adding that growers need to know what viruses have been tested for when buying certified planting material.
Certified doesn't mean clean of all viruses, he said. Growers need to keep in mind what certified means and ask specifically what viruses the stock was tested for.
Rugose wood disease
While much attention has been given to grapevine leafroll diseases because of their impact on grape quality, another silent killer could be lurking in Washington vineyards.
Rugose wood complex viruses are known as "trunk" diseases because they cause modifications in the trunk of the affected vine, said Olumfemi Alabi, a graduate student working under the direction of Rayapati in the Department of Plant Pathology at Washington State University. The symptoms are latent in own-rooted vines and only show up on certain rootstock-scion combinations. But this means that grapevines in eastern Washington can be a carrier of the virus without showing symptoms.
"When you have own-rooted vines, you could have the disease but don't know that it's there," Alabi said. "The use of cuttings from sources that are compromised with Rugose wood complex may lead to spread of the disease unknowingly and affect vine performance. That's why we call it the silent killer."
Rugose wood disease is differentiated into four types of disorders based on their effects on the trunk when grafted on specific Vitis indicator hosts, Alabi said. The four include Rupestris stem pitting, Kober stem grooving, LN33 stem grooving, and corky bark.
"Symptoms are not as dramatic as those exhibited by grapevine leafroll disease," he said. Symptoms are usually related to grafted vines and include swelling above the graft union. Only by removing the bark can one see pits or grooves along the trunk's woody cylinder. The viruses interfere with the translocation of phloem, and over time, cause vine decline. Trunk alterations can occur on the scion, rootstock, or both. Vines become less vigorous, may look dwarfed, and eventually die.
The symptoms of all four viruses look similar in the field, making them difficult to distinguish, he noted.
Any type of swelling above the graft union, marked difference between the diameter of the scion and rootstock, poor bud take, graft incompatibility or decline, slow growth, or delayed bud opening are signs of Rugose wood complex, according to WSU's grape virology Web site.
Scientists are still learning about the multifaceted Rugose wood diseases, but have found four viruses associated with different types of Rugose wood diseases. WSU researchers have found three of the four in different wine grape cultivars in Washington vineyards—grapevine Rupestris stem pitting-associated virus, grapevine virus A, and grapevine virus B. Grapevine virus D has not yet been documented.
Alabi also found it common for vines to have one or more virus combinations. Many samples that tested positive for grapevine leafroll disease also had Rugose wood virus.
"The take-home message is sanitation," Rayapati said. "If you start with virus-free material, the likelihood is that the vineyard will stay virus free."
With so many viruses that could be lurking in the plant, testing is not as simple anymore. More than one type of test will be needed because leafroll disease is not the only one present in Washington vineyards. Growers should test both the scion and rootstock before grafting any vines, he said, reminding vineyardists to always use certified stock when planting a new vineyard.
The WSU lab can test samples for viruses associated with Rugose wood complex. Growers who see suspicious symptoms in the vineyard and wish to test the samples for viruses are encouraged to contact Rayapati at email@example.com for further investigation.