Grapevine Leafroll Disease Spreads Rapidly

Experts share latest facts about vectors like mealybug, and prevention
  
Davis, Calif. -- The rapid spread of grapevine leafroll disease in California in recent years, particularly in Napa Valley vineyards, has raised many questions along with some answers among plant pathologists and researchers, but there is still much to be learned. Ongoing studies are progressing, and experts agree that preventing leafroll and controlling its spread require the use of clean, certified vine material; monitoring vines for symptoms and identifying leafroll through testing when symptoms are observed; preventing the movement of infected vine material into non-infected vineyards; and controlling movement of mealybugs that can vector the virus.

A daylong grapevine leafroll symposium with presentations by experts from throughout the world was held last year at UC Davis. With ongoing concern about the disease in California, Grapevine Leafroll Disease Symposium 2.0 was held June 2 at the University of California, Davis, organized and moderated by UCD plant pathologist and Foundation Plant Services (FPS) director Dr. Deborah Golino. It focused on leafroll in California's North Coast, along with research updates and advice on preventing its occurrence and spread.

The disease causes problems including delayed fruit ripening/maturity, lower sugar content, reduced anthocyanins, increased acidity, reduced yields, poor wine quality, reduced economic returns, and ultimately demise of vine viability causing the need to remove and replant individual vines or complete vineyards.

Visible symptoms include: leaf reddening and curling in red grape varieties; or leaf discoloration, chlorosis and curling in white varieties. However, these can also be symptoms of other diseases, pathogens, and nutritional or environmental problems, so vines should be tested to confirm the presence of leafroll.

 In an overview presentation, Bob Martin, plant pathologist with the U.S. Department of Agriculture's Agricultural Research Service based in Oregon, said, "Grapevine leafroll disease is the most severe disease in grapevines worldwide, and causes about 60% of all grapevine losses that are caused by viruses." He said leafroll spread has also been observed recently in other U.S. grapegrowing regions such as the Finger Lakes in New York, and in Washington, Oregon, and Idaho.

There are now 10 known types of grapevine leafroll associated viruses (GLRaV). The most recent type was identifed in the grape variety Carnelian by UCD plant pathologist Dr. Adib Rowhani, who discussed what is known about this and lesser known virus types. In California, GLRaV-3 is the most common and most severe virus type. Up until the 1980s, leafroll was thought to be transmitted only through grafting of infected vine material (unhealthy or non-certified scions, or illegally imported budwood) onto rootstock.

Mealybugs as disease vectors

UC Berkeley entomologist Dr. Kent Daane said worldwide there are 15 known leafroll insect vectors, of which eight are mealybug species and four are soft scale species. Whiteflies and aphids are also known vectors of some GLRaV types. Lab testing has shown that GLRaV-3 can be transferred by five mealybugs found in California: grape, vine, obscure, longtailed, and citrus.

The grape mealybug (GMB) has long been found in California vineyards, is likely native to North America, and is considered the vector for the recent spread of leafroll in North Coast vineyards. It is common in coastal and Central Valley vineyards, and often tolerated by growers in low numbers as long as it does not cause economic damage to grape clusters. However, even low numbers of GMB can effectively spread leafroll from vine to vine, and growers should consider treatment and control of GMB at any population level when leafroll is also known to be present.

UC Berkeley researcher Rodrigo Almeida discussed his lab research on the vine mealybug (VMB) and its ability to vector GLRaV-3. He said the first instar, or "crawler" stage of VMB is much more efficient at transmitting leafroll virus than adult stages. A VMB crawler can acquire and transmit leafroll virus within one hour of feeding on infected vine material and transfer it to new vine material for up to four days thereafter.

Almeida said research will continue on the ecology of leafroll disease to look at the relationship between populations of mealybugs and virus levels to try to determine transmission rates and identify times of the year when the risk of spread is greatest, in order to better target control.
 
Although VMB has not been linked with the rapid spread of leafroll in North Coast vineyards, it is cited as the vector for leafroll spread in South Africa. Daane said, "I think the VMB will become a bigger player in California as time goes on. So far we've looked at it for different economic factors, but low levels could also cause leafroll problems."

Daane also expressed concerns about a new species, Gill's mealybug, found in California in 2003, which feeds on a wide range of host plants including ornamental plants, pistachios, almonds, and has been found on grapevines in El Dorado County. "The concern is that it can be found on many types of ornamental plants that are moved to all areas of the state," Daane said.

Mealybugs can be spread by birds, wind that blows the insects or infested leaves to other areas, or by vineyard equipment, mechanical harvesters, or field crews on their tools or clothing. As a result, leafroll virus more commonly spreads along the same vine rows rather than between rows, once in a vineyard. Daane speculated on the possiblilty of root-to-root tran smission in adjacent vines by underground mealybug crawlers, or perhaps even by the phylloxera louse, which could be subjects of future research.

Sonoma County Cooperative Extension farm advisor Rhonda Smith discussed the results of a study done during two recent harvests to determine if mealybugs could be spread as a result of winery pomace waste management. The study showed that a percentage of VMBs on infested clusters could survive whole cluster pressing at the winery on the stem, skin and seed pomace removed from the press.

The study also evaluated survival of the VMB in pomace compost piles and showed that VMB could survive up to four weeks in an uncovered, untreated compost pile. If the pile was completely covered with clear plastic for at least one week, nearly 99% of the VMB did not survive. Smith concluded, "Untreated (fresh) winery press pomace waste should not be placed in vineyards." This precaution is necessary for white grape pomace, not for red wine pomace that has been through fermentation.

In short, taking all the same precautions to control mealybugs and prevent their spread will also help prevent the spread of leafroll.

Other leafroll hosts

Golino presented initial results of a new study started in fall 2008 that collected plant samples of other vegetation located in and near North Coast vineyards with known infestations of leafroll to determine if these plants were also hosts of leafroll or other viruses.

Testing was completed on 231 samples that included 83 species of non-Vitis plants and weeds, along with samples of native wild Vitis, escaped vinifiera seedlings from nearby vineyards, and hybrids of vinifera and native Vitis that had crossed. All the non-Vitis weed/cover crop species tested negative for all viruses, but some Vitis vines did show presence of viruses. Golino summarized, "Our preliminary results show that the native variety Vitis californica is not a significant reservoir for leafroll virus, but it can have Rupestris stem-pitting virus. Escaped vinifera vines and hybrid vinifera/native vines can be a potential leafroll virus reservoir (GLRaV-2 and 3)."

Rootstock factors

One reason cited for increased leafroll incidence in recent years, is that rootstocks used as alternatives to AxR-1, which was removed and replaced due to phylloxera infestations, are not as leafroll- resistant as AxR-1. Some rootstocks are considered more resistant to viruses, such as St. George, Ramsey, and 110R. UCD professor and vine breeder Dr. Andy Walker said rootstocks have not been bred yet specifically for tolerance to leafroll viruses, but he is breeding and evaluating seedlings that are crosses of leafroll-resistant and nematode-resistant rootstocks that include St. George x 5BB, and St. George x Freedom.

Golino noted that severity of leafroll symptoms in individual vines can be highly variable, based on different scion/rootstock combinations, and because there are different strains of virus even within individual types of GLRaVs. Combinations of different types of GLRaVs (or strains), or in combination with other viruses, and some that may not yet be identified, could also be factors in leafroll spread.

Golino expressed frustration she's experienced over the years from encounters with some growers who think that "a little virus" in the vineyard can be tolerated, and even some winemakers who believe a little leafroll can be a positive factor for wine quality. She noted that healthy Cabernet Sauvignon should have leaves with a golden color in the fall, rather than the bright red color that is appearing more commonly in Napa Valley vineyards. Golino summarized, "A virus-infected vine is more susceptible to fungal pathogens as well as other stresses that can affect vine health and grape quality."

Within the next few weeks, a videotaped version of the symposium will be posted and accessible to view through the UC Integrated Viticulture Online website at http://groups.ucanr.org/iv/. Last year's leafroll symposium is currently accessible at the site.