Grapevines harbor over 60 virus and virus-like agents that cause a range of disease symptoms that can vary from mild causing little to no economic effect to very serious causing reduced yield, delayed ripening, and even vine death. In California there are several damaging diseases caused by virus and virus-like agents that are widespread in vineyards. Fanleaf degeneration, Leafroll Disease, Rootstock Stem Lesion and Corky Bark are the most common virus diseases in California. The virus that causes Grapevine Red Blotch Disease was found in 2012 and the incidence of this disease relative to other virus diseases is currently not known. For more information on specific virus diseases refer to Grape Pest Management, UC ANR 3343.
The intensity of virus symptoms depends on the cultivar of grape (scion and rootstock), and the weather during the growing season. Although some disease symptoms are diagnostic, the absence of symptoms is not a reliable indication that a plant is virus-free. Many grapevine viruses are latent (not showing symptoms nor causing disease) under certain circumstances. Using propagating wood that carries these latent viruses may lead to serious diseases or even vine death in a highly susceptible cultivar of rootstock or scion.
Vineyards planted or grafted with virus-infected material will often be impacted in the first year and any chronic effects of the viral infection are experienced over the entire life of the vineyard. To avoid losses from virus diseases, it is necessary to use clean stock in propagation that has been tested for known grape viruses.
Laboratory testing is an available tool to diagnose potential virus infected vines. Commercial labs offer serological (ELISA) and nucleic acid based assays (PCR) for identifying the presence of specific viral pathogens. The diagnostic lab should be consulted prior to collecting samples to avoid test results which are false negatives. Timing, type of material to be collected and sample handling is important.
The most important control of virus disease is the use of clean plant material for the propagation of vines to be planted in growers' vineyards and for budwood used for grafting mature vineyards. The most reliable way to avoid planting virus-infected vines is to use plant material that has been certified to have been planted, grown and distributed under the California Registration & Certification (R & C) Program administered by the California Department of Food and Agriculture (CDFA). Growers should insist that grapevine nurseries provide certified stock, which should be accompanied by an official tag issued by CDFA. Nurseries participating in the R & C program obtain their wood from the Foundation Plant Services at the University of California, Davis. UC Davis has a foundation vineyard for major grape cultivars and clonal selections. Before planting in the foundation vineyard, selections are tested for viruses using various methods including biological indicators, as well as ELISA and advanced PCR assays. The foundation vineyard is visually monitored in spring and fall and a portion is retested each year for viruses known to be spread naturally.
Natural spread of specific grapevine viruses can occur by insects and nematodes. Vectors can acquire the virus by feeding on vines infected with such viruses. To prevent vines from becoming infected in the field, control measures target the vectors. Grapevine leafroll associated virus-3 (GLRaV-3) is vectored by mealybugs and scale insects. As vectors, mealybugs typically carry GLRaV-3 in their foregut for short periods, losing the virus after each molt. The smaller stages (crawlers and second instars) are the most effective life stages in terms of their efficiency at acquiring leafrollvirus (feeding on an infected plant and picking up the virus) and transmitting it (placing the pathogen in another plant by feeding). Mealybug crawlers acquire the pathogen from an infected vine, disperse short or long distances by wind, feed for a short time and transmit the virus to clean vines. Disease symptoms may not be apparent until the season following the year vines are infected. This type of spread occurs even at low densities of mealybugs that would not be considered an economic problem if not for the potential for disease spread.
Leafroll infected blocks can be a source for mealybug vector and disease spread into adjacent clean plantings. Treatment of mealybugs in virus source blocks should reduce the number of infective vectors leaving the block. Treatment of clean blocks should target these vectors the same season they are first detected to reduce secondary spread to adjacent vines. Research has shown that secondary spread of virus is reduced when growers coordinate their efforts in area wide programs that reduce insect vector populations in combination with removal of GLRaV-infected vines. Vine removal as a control measure is most effective when disease incidence is low. See MEALYBUGS for recommended control practices.