The presence of grapevine leafroll viruses in California vineyards is well known, but the extent that vineyards in other parts of the country may be affected is only beginning to be determined, according to Wines & Vines.
Cornell University’s Cooperative Extension recently released the results of a survey that began in 2006, designed to gather data about the presence of leafroll viruses in the Finger Lakes region of New York.
The New York survey looked at 95 vineyard blocks in 25 different vineyards throughout the Finger Lakes region. Up to 20 leaf samples were collected from each vineyard to find the percentage of positive samples and to map the location within the vineyard of any infected vines.
A total of more than 1,000 leaf samples were tested in Marc Fuchs’ laboratory at the New York State Agricultural Experiment Station in Geneva for the presence of three grapevine leafroll associated viruses: GLRaV-1, GLRaV-2 and GLRaV-3.
The results showed that 65 of the 95 blocks (68%) had at least one positive GLRaV test. Of those blocks testing positive, 27 showed low to moderate (less than 20%) levels of infected vines, while 38 vineyard blocks showed high to extremely high levels (20 to 100%) of positive samples. Thirty blocks (32%) were free of these three viruses. Vinifera blocks showed 70% infection and, while fewer hybrid blocks were sampled, those blocks had 57% positive testing.
The impetus for the survey came from a Lemberger vineyard with symptoms of leafroll viruses on most of the vines. Researchers made a special effort to collect samples in Lemberger vineyards where leafroll viruses were expected to be found, but then found it in blocks of different varieties within the same vineyard.
Further testing on the initial vineyard revealed that two of the grapevine leafroll associated viruses, GLRaV-2 and GLRaV-3, were present on that site. In addition, nearby blocks also had GLRaV-1 present, the first time this form of leafroll was reported in New York.
While all leafroll viruses are spread through infected budwood cuttings and graft unions, mealybugs and soft scale insects have been identified as vectors that can spread leafroll viruses to clean vines. Consequently, the researchers in New York also looked for grape mealybugs and soft scale insects. Samples of these possible vectors were collected at 31 blocks in 25 vineyards. Some of the same vineyards used in the leaf sample survey were included, but not all of them. Six sites were free of soft scales or mealybugs. While 25 sites (81%) had either soft scales or mealybugs, 22 of those blocks had very low numbers.
Once a grapevine is infected with a leafroll virus, it cannot be cured. Consequently, it is important to prevent infection initially and to keep the viruses from spreading.
The economic impact of leafroll viruses in New York is amplified by the fact that growers are facing a second replacement of grapevines. Many growers lost vines in the freeze during the winter in 2004, and many of the replacement vines that were ordered were not clean vines but came with leafroll viruses.
Notes Martinson: “This means growers are facing an enormous economic loss, since they went without a crop after the 2004 freeze and now have to plant again.”
It should be noted that viruses like those described do not impact wine quality at all; they simply impair a vine’s ability, over time, to produce fruit.
