The second symposium that focused on the leafroll-associated virus in grapevine was held on June 2, 2009 at UC Davis. The American Vineyard Foundation provided much of the funding for the research being reported, as well as providing funding for the symposium itself. Convened by Deborah Golino, director of Foundation Plant Services at Davis, this symposium was intended to focus on local efforts being made in understanding the spread of the disease in vineyards. Because leafroll virus is spread primarily via mealybug, this symposium was largely focused on that pest.
Bob Martin, of the USDA-ARS based in Corvallis, Oregon stated that 60 percent of virus-caused grapevine losses are due to the leafroll complex of viruses. It doesn't take very much thought to understand how important this disease is to our industry. Important in a negative way. Leafroll virus causes delayed maturity, retardation of sugar accumulation, reduction in anthocyanin content, increases in fruit acidity, lowered yield and wine quality, and an overall reduction in return to the grower. The sometimes-found opinion that "a little virus is good for wine quality" is nonsense and needs to be purged from our thoughts. Virus does reduce yields, but not in a way that increases wine quality. And as the virus has been seen to spread rapidly within and among vineyards, a "little" virus can easily turn into a virus outbreak in a matter of a few years.
The Stress of Strains
The leafroll disease is really a class of virus-caused disorders that have similar symptom expression. Symptoms are diverse and may be confused with other disorders. Leafroll generally appears on older leaves first, in which leaves of red varieties turn deep purple-red, usually leaving the veins green. But not always. Leaves of white varieties do not turn color, but in most varieties, the leaves curl downward. But, again not always. Sometimes leaf curling is not that strong. Red leaf symptoms may be confused with phosphorus deficiency, which also appears on the basal leaves first. Mite damage also causes leaf reddening, but usually on the upper leaves initially. Other mistaken symptoms can arise from gopher or vole damage. So, be careful before pulling the leafroll alarm. Checking for pests and nutrient status is a wise way to avoid mistakenly thinking that your vineyard is infested with leafroll.
There is not a single virus strain that causes disease, but a whole series of leafroll-associated virus strains that cause disease. There are even strains within strains! Lucky us. And while there are about 10 known strains, I have no doubt that more will be identified going forward. And I have no reason to doubt that the virus will constantly mutate, causing strains to continue to evolve. This doomsday scenario means that we will be living with this disease and will need to be ever-vigilant in obtaining clean plant material and preventing its spread from known diseased vineyards via control of its vectors.
The most severe strains of leafroll are LR1 and LR3, of which LR3 is the most commonly found strain. While the LR2 strain is primarily transmitted by grafting, most other strains are transmitted by mealybug species and by scale insects. Mealybug insects are, by far, the most common means of spread in vineyards. Martin stated that there is no evidence that the disease spreads through root grafting (i.e. contact between neighboring vines). While it is possible to transmit in this way, it is not a significant cause for spread of the disease.
Adib Rowhani, from the USDA-ARS at Davis, noted that LR2 is the so-called "Red Globe Virus," which affected primarily that table grape variety, though it has also been detected in other varieties. Unlike the other strains, it is vectored only by aphid. So, it is not as significant a threat for spread as the other strains (unless phylloxera, an aphid insect, can vector it). LR7 is from an unknown virus genus, and its mode of spread is not yet known. Other strains, including LR4, 5, 6, 9, 10 and 11 are all transmitted via mealybug, according to Rodrigo Almeida, of UC Berkeley's Department of Environmental Science, Policy and Management. Hence the intense focus on mealybug research.
Rootstock Effects: Some Blush More Readily
Golino stated that earlier acceptance of "a little" leafroll virus was probably more reasonable during days of own-rooted and AxR#1-grafted vineyards, for which the virus does not express itself as much. So, while many vineyards were riddled with disease, little economic or quality consequences arose, and the problem was paid little heed. With the replant of vineyards on alternative rootstocks during the 1990s, vineyards with leafroll disease now seem as common as those that remain green well into fall. St. George rootstock seems to be one in which leafrolll symptoms are not expressed to a great degree. Some productivity loss has been seen even with that roostock, but to a lesser degree than many other rootstocks. Other seemingly "tolerant" rootstocks include O39-16, 101-14, Freedom, Harmony and Ramsey, the latter three of which are not commonly used in the coastal regions of California. 420A seems to express fewer symptoms than other rootstocks, but symptoms do appear, nonetheless.
Sensitive rootstocks include 5BB, 5C, 1103P and 3309C. Of those, 5BB and 3309C seem to exhibit the most striking leaf symptoms and hyper-sensitivity to the leafroll-associated viruses. Golino also noted that 110R exhibited moderate leaf symptoms.
Because vineyards on own roots, AxR#1 and a few other rootstocks may be riddled with virus but may not exhibit strong symptoms, taking budwood cuttings from apparently clean vineyards is not a good idea without testing them first. Once a vine is infected with the virus(es), the disease becomes quickly systemic. So, any propagated tissue will contain the virus. Have vineyard-sourced grafting material tested for leafroll virus before using it (or before harvesting it for that matter--why waste your time?).
Andrew Walker of UC Davis discussed initial efforts at breeding leafroll-resistant rootstocks. He mentioned that what was previously thought to be scion/rootstock incompatibility was probably virus disease and that Redglobe virus was not discovered until that variety was grafted onto 1103P rootstock. Walker is embarking on a LR-resistant rootstock breeding program using St. George rootstock as a starting point. While St. George is tolerant of leafroll, it has little resistance to nematodes. He is crossing nematode-resistant rootstocks like 5BB and Freedom with St. George and currently has about 200 seedlings from those crosses. It will take years before a commercial rootstock is developed, but work is underway.
White, Fluffy and Tiny-- Mealybugs Seem an Unlikely Villain
It seems like while the big flyers grab all the headlines (like glassy-winged sharpshooter and light brown apple moth), the lowly little mealybug has recaptured our attention because of its casual association with the leafroll virus complex. Casual because it's not really trying to harm the grapevine--it's just doing its thing and sucking a little phloem sap from the plant. But with the sugar-laden juice comes a virus particle or two (or thousands), which reside in its system for about four days, during which time the little "belch" it gives off during its next feeding includes the virus, wherein the virus replicates and infects yet another plant. Both the acquisition time of the virus and transmission time to transfer it to a new host plant are less than one hour. Almeida noted that most transmission of virus occurs during September through November.
Before the arrival of the Vine Mealybug, itself a serious pest of grapevine, the "local" mealybugs were not given much attention, being relegated to minor pest status. We now know that their presence is not a minor issue.
Kent Daane, of UC Berkeley, has become a "rock star" in his research efforts geared towards mealybug pests, and provided a wealth of information to the symposium on the waxy little buggers. The Grape Mealybug (GMB), is probably native to North America and is similar in appearance to the alien Obsure Mealybug (OMB), which probably originated in South America. GMB can be differentiated from OMB by its osteolar fluid (a small drop of fluid excreted under duress). GMB bas an orange osteolar fluid while OMB's fluid is clear. Poke at the insect to find out which one it is.
The Longtailed Mealybug differs in its appearance, in that it has tails. The tails are not part of its body, but are features of its waxy excretion. So, recently-molted insects may not have the tails attached. That species originated from Australia and New Zealand. The newest one to grace our U.S. shores is the Vine Mealybug, which is thought to have originated from the Mediterranean. A serious pest in its own right, it is also an effective vector of the leafroll-associated viruses. Other mealybug species are known, but are less common or not found in grapevines.
Daane has done much research into the life cycles and control of the mealybug pests. Control of each species is different and has been evolving due to new research efforts, largely in Daane's lab. To keep up with current knowledge, attend local meetings and check UC's IPM website (www.ipm .ucdavis.edu).
Monitoring and detection of mealybugs is essential. Pheromone-lure traps are the best way to monitor, but it is only effective during periods of flight (males only). Daane says that October is the peak period of flight for the insects and the traps last for about eight weeks. There has been some success in using dogs that can sniff the pests. While positive discovery of pests have been made using the trained dogs, the time required for thorough investigation is about 2 to 3 hours per 1.5 acres, which is tiring for both dog and human. It's probably best to use the traps for primary identification of mealybug presence, while the dogs may be used to identify hot spots.
Once the pest has been discovered, systemic pesticides are proving to be the best control: i.e., Admire, Assail, Applaud, Clutch or Movento. Lannate and Lorsban are also effective.
Quite unfortunately, organic methods have shown limited efficacy. Oils can kill crawler stage insects but cannot kill adults. Biocontrol methods show potential, but don't provide complete control. The Mealybug Destroyer beetle, Cryptolaemus montrouzieri, may not be effective if mealybug populations are low. The parasitoid wasp Anagyrus pseudococci seems to have more promise, as it has similar seasonal patterns of activity.
A method that combines biocontrol with mating disruption can be about as effective as chemical control, according to Daane. Mating disruption involves the use of synthetic insect pheromones that confuse male insects and prevent them from finding female partners. Cruel, but effective.
Rhonda Smith, of UC Cooperative Extension, discussed the results of a trial she did a few years ago, investigating survivability of Vine Mealybug during winery processing. Many vineyards have spread raw pomace in their vineyards without considering that the pomace may include live insects, including mealybugs. Her work was painstaking and very thorough--imagine counting thousands of insects on clusters that had been placed in fine-mesh bags! Smith found that VMB survived just fine going through the press. Therefore, white grape pomace can be assumed to have a population of VMB if the vineyard from which it came supported such a population. But, can it survive if the pomace is piled up before application to the vineyard? Yes, and no. VMB kill is successful only if the piles were covered during composting. The covers raise the temperature of the piles and kill nearly all of the insects. Poorer kill was found in "stemmy" piles. Bottom line is to thoroughly compost all winery waste at high temperature before application to a vineyard. Compost piles should be covered in plastic.
Two things were clear from this symposium: 1) don't assume that the plant materials you are getting are clean (test them) and 2) all mealybugs are important pests (monitor and control them). wbm
