by Tamara Scully
Although there are over 70 different viruses worldwide, which can impact the productivity, vigor and food quality of grapes, there are just three, which are making the most impact in Northeast vineyards. Marc Fuchs, Associate Professor, Cornell University presented on the topic for a recent webinar, “Viral Diseases Transmitted Through Nursery Stock in the East: Leafroll, Tomato Ringspot & Red Blotch.”
When the Northeast was rapidly expanding its wine grape production several decades ago, clean, virus-tested plant materials were not available in the amounts needed to supply the demand, which lead to untested materials being widely planted. Insect vectors then spread viruses present in the plant materials.
Top Three
Leafroll is the most wide spread virus of grapes in the Northeast. Symptoms include early leaf reddening on red grape cultivars, while white cultivars show a lightening in color. Leaves will also begin to curl down and cup. But symptoms are not conclusive, and can be hard to detect.
Research studies on Cabernet franc, as well as some on Pinot noir, have shown that impacted grapes can have a reduced Brix level compared to healthy samples, and that the fruit ripening, measured by Brix, is delayed by two or three weeks comparative to healthy vines. Grapes from infected plants are lighter in color, and vine productivity is reduced.
The studies demonstrate “leafroll will affect food chemistry,” Fuchs said, and infected grapes will make a “poor quality beverage”.
Leafroll in the northeastern grapes is caused by five different viruses, three of which are transmitted through many species of mealybugs or soft-scaled insects. There are three insect vectors identified to be responsible for primarily transmitting leafroll virus in the Northeast region; brown scale, cottony maple scale and grape mealybug. Scouting for these pests, and removing any vines showing symptoms of the virus immediately, is needed to help curtail the virus.
Tomato ringspot is “a very destructive virus,” Fuchs said. Own-rooted vines are most susceptible, with several rootstocks providing some protection against the disease, so grafting onto these can provide a degree of protection.
Transmitted by three species of soil-dwelling dagger nematodes, which pierce the roots of the vine like a needle as they feed, the virus causes “dramatic yield reductions.” Chlorotic leaf distortion is a symptom of this disease, too.
The most recent virus causing destruction is red blotch. Again, chlorotic changes in leaves — including red spots on green leaves of red grapes, and yellow areas, which may become necrotic on the leaves of white grape vines, can occur. Red blotch affects grape color, grape quality and vine vigor.
Red blotch was identified in 2011, and is known to be transmitted by the three-cornered alfalfa treehopper, Fuchs said. For red blotch, sourcing locally produced Northeastern planting materials can reduce the risk of transmission, as vineyards here are less likely to harbor red blotch.
Disease symptoms are not reliable means of disease identification, since leafroll and red blotch share very similar symptoms.
These three diseases have “tremendous economic significance” to the grape industry. There are no known cures for the diseases, and chemical controls are expensive, may not be efficient, and carry environmental concerns, Fuchs said.
Loss of income over the life span of the vineyard is due to reduced yield, as well as penalties for poor quality grapes. Studies show that the economic loss to vineyards, due to these viruses, can range from approximately $10,000 per acre upwards to $60,000 per acre.
There are no cultivated or wild grape varieties known to have resistance or immunity to these diseases, Fuchs said. Breeding programs are “basically not an option” for control.
There is no known mechanical transmission of these viruses. This means that pruning, suckering, trimming, spraying, harvesting and other vine management activities themselves will not move the virus from place to place in the vineyard. However, vectors can be moved through soil, plant materials or equipment, and potentially carry the virus with them.
Equipment can carry infected materials from one vineyard to another. And soil erosion or moving soil from place to place, can be a means of transmission for tomato ringspot, as soil may contain its nematode vector. The wind can disperse mealybugs and other vectors from infected areas, spreading the viruses.
“There are so many factors which will influence whether a virus is impactful, or the degree of it being impactful, and this can vary from location to location,” Fuchs said. “It doesn’t mean that if you don’t see symptoms that there will not be any yield or quality depression.”
Hybrid grape cultivars often show little or no expression of leafroll, and studies vary as to whether the disease impacts the sugar content of hybrid grapes. Whether showing symptoms or not, hybrids can serve as a source of the virus, however, spreading it to more susceptible cultivars through the insect vectors.
Based on other prevalent grape viruses, Fuchs believes that a vector would be equally able to move a virus from a plant with an active red blotch virus as from one with a latent virus.
“We do not have information to see whether there is a relationship between virus titer (for red blotch) and efficiency of transmission” via a vector, Fuchs said.
If planting clean stock in the vicinity of infected vineyards, growers need to be extremely diligent about scouting and removing vines showing signs of infection.
Growers should be “very selective in sourcing planting material,” which is the best defense against issues. The National Clean Plant Network, which tests plant materials, is “paramount to eliminate the presence of viruses in propagation materials, ” Fuchs said.
The Eastern Clean Plant Center is at Cornell University’s New York State Agricultural Experiment Station in Geneva, NY. Here, virus tested clean plant material is planted and propagated in a foundation block. Second generation planting blocks increase the stock. Ongoing quality control and monitoring occur as plants make their way from the foundation plantings to nurseries, for distribution to vineyards.