Ministry of Agriculture

Grape Virus Diseases

What is a virus?

Grape virus diseases are caused by microscopic particles that are composed of genetic material (DNA or RNA) inside a protective protein coat. Viruses are much smaller than bacteria and fungi, and require very specialized tests for their detection. Once inside a living plant, viruses have the ability to multiply by taking over plant cells and reprogramming them to make more virus particles. In grapevines the virus will spread systemically to all parts of the vine (roots and vegetative parts). Once a plant is infected, it will remain so for life.

General Virus Symptoms

Viruses cause a wide range of symptoms, ranging from no visible symptoms to plant death. Many grapevine viruses cause a general decline in vigour and productivity and delayed maturity of fruit. Other symptoms may also be present on leaves, stems or fruit.

It is possible for a virus to infect a grapevine without the plant showing any obvious symptoms (called a latent virus or sleeping virus). The degree of virus symptom development and effect on a plant is influenced by the virus strain, plant variety and age, environment, and time of year. A previously latent virus may become more virulent (or more severe) at some time due to changes in the environment or through propagation, often onto a different rootstock. The symptoms may be subtle, requiring proper experiments to demonstrate the effects of the virus. For example, the involvement of a virus may not be obvious in an increased susceptibility to winter injury or a gradual decline in yield.

Symptoms such as graft rejection, rapid decline of vines, severe stunting, late blossom and late or poorly ripening fruit are more obvious. Foliar, fruit or cane symptoms of virus diseases are also frequently visible. These may include any one of the following: red foliage (on red varieties), short internodes, mottling of leaves, fasciation of canes, double internodes, excessive growth from secondary buds, straggly bunches with both large and small berries, rolling of the leaves in the fall, wood pitting and grooving, etc.

grapevine corky bark grapevine rupestris stem pitting
Grapevine corky bark disease foliar symptoms on LN33 indicator (Grapevine virus B).
Photo courtesy Canadian Food Inspection Agency.
Grapevine rupestris stem pitting disease.
Photo courtesy Canadian Food Inspection Agency.

Importance of Virus Diseases

Virus diseases can have a serious impact on vine health, yield and quality of the fruit. Symptoms are not always severe or obvious, but even a small decrease in yield will add up over time causing significant economic losses. Decreased yields of 5 to 10% are not uncommon for grapevine viruses, and losses can be much higher. Viruses may also influence wine quality by causing delays in sugar accumulation, poor acid development, and poor colour development.

The difficulty of detecting virus infection can lead to rapid dissemination of virus-infected material through propagation. When infected cuttings are used for propagation, whole vineyards can become infected.

How Viruses are Spread

Humans are the most effective transmitter of virus diseases through movement and propagation of virus infected rootstocks, cuttings and finished plants.

Most viruses also have other methods of spreading, although not all are known. Insects or other organisms that spread viruses are known as “vectors”. Many serious grapevine viruses are classified as “nepoviruses” which are transmitted by certain species of nematodes. Examples of nepoviruses include Grapevine fanleaf virus, Arabis mosaic virus, Tomato black ring virus, Tomato ringspot virus, Tobacco ringspot virus, Peach rosette mosaic virus and many others. These viruses can cause significant economic losses. The nematode feeds on the roots of infected vines and retains the virus for several months. The disease is spread as the infected nematode feeds from grapevine to grapevine. Infected nematodes may be spread on the roots of nursery plants and through soil water (irrigation, seepage, flooding). Fortunately plant protection measures that require treatment of imported planting stock have prevented the most damaging virus-vectoring nematode of grapevines, Xiphinema index, from being introduced into British Columbia. However, other nematodes found in British Columbia, such as Xiphinema americanum, are known to vector nepoviruses. Some of these nepoviruses may also be spread through seeds of infected weeds in the vineyard. They may then be picked up by the nematodes and moved from the weeds to grapevines.

Other viruses are transmitted by soil fungi or insects such as aphids, leafhoppers, psyllids and mealybugs. Mealybugs and soft scale insects are known to be vectors of some of the grapevine leafroll-associated viruses, Grapevine virus A (causing Kober stem grooving disease, formerly Grapevine stem grooving virus), and Grapevine virus B (causing grapevine corky bark disease).

There are also virus-like diseases for which the method of transmission (apart from propagative) is not known or well documented. This group includes many damaging diseases such as Grapevine Enation, Grapevine Yellow Speckle, and Grapevine Shoot Necrosis. Little is known about some of these viruses beyond the symptoms they cause. This lack of knowledge forms a serious obstacle to effective disease management.

Control and Prevention of Viruses

  • Prevention of virus diseases is critical, as there are no cures for virus-infected vines other than vine removal.
  • Plant only fully virus-tested vines to reduce the risk of introducing virus diseases into your vineyard.
  • If an insect vector is known to spread a virus, controlling the insect may help to limit or reduce the rate of spread within a vineyard.
  • Canada has specific grapevine import regulations that help to protect grape growers from the introduction and spread of virus diseases and other pests which are not yet established in British Columbia. Selected grapevine varieties/clones and rootstocks from Canadian-approved nurseries in France and Germany are currently approved for importation into Canada. Under this program tests for regulated viruses/pests of grapevines are carried out both in the exporting country and in Canada, prior to the approval for the importation of any variety and rootstock. Canada has different import requirements for grapevines from the U.S, because of the similar pest situation and the proximity of the two countries. For more information on importing grapevines, contact the Canadian Food Inspection Agency (CFIA).
  • Note that important viruses such as Arabis mosaic virus, Grapevine fanleaf virus and the viruses causing grapevine leafroll disease have been removed from the federal quarantine pests list. Although CFIA-approved foreign certification programs are supposed to produce plants free of these viruses, their removal means that foreign nurseries approved to ship grapevines into Canada no longer have to prove their vines are free of these viruses. Grapevines infected with these viruses have been imported since this de-regulation. It is recommended that growers importing vines request that they be tested for these viruses in the country of origin. These viruses cause serious economic diseases.

Clonal Selection

Elimination of viruses from grapevines is possible, but requires a combination of heat therapy and micro-shoot tip tissue culture. This process is both expensive and time consuming, as it must be verified that the virus was successfully removed. Such vines then become the foundation for propagation programs designed to produce “clean” vines for industry. Vines that are to be subjected to virus elimination should be chosen carefully, so that only the very best varieties or clones are selected. The CFIA Laboratory at Sidney, British Columbia (Centre for Plant Health) provides testing, virus indexing and therapy for viruses of grapevines, tree fruits and other crops. These tested mother plants may then be placed in a Generation 1 (Nuclear) collection (repository) at the Sidney Lab or become mother plants for growers or nurseries. Propagative material from these mother plants may be used to produce other plants for certification under the Canadian Export Certification Program for the eventual establishment of healthy vineyards.

Major Grape Virus Diseases

There are over 70 different viruses and viroids that infect grapevines distributed throughout the world. Several are known to exist in Canada already. With increasing international trade and travel it is expected that more of these viruses will arrive.

A national grapevine virus survey in 1994-1995 found vines infected with Arabis mosaic virus, Grapevine fanleaf virus and Grapevine leafroll-associated viruses 1 and 3 in both Ontario and British Columbia. Leafroll 3 was the most commonly found virus, but the other viruses were present at very low levels in British Columbia. For example, out of 1485 B.C. samples, only 1 positive was found for fanleaf virus, and only 5 positives for arabis mosaic. Although the survey was not detailed enough to determine incidence, the low number of positives is an encouraging indication that B.C. remains largely free of these damaging viruses, and it is worth-while to continue to prevent introduction through careful screening of nursery stock. However, additional grapevines infected with these viruses have been imported since 1995.

Grapevine Leafroll-Associated Viruses

Grapevine leafroll is probably the most wide-spread virus disease of grapevines world-wide. There are currently 5 different viruses associated with leafroll, including:

  • Grapevine leafroll-associated virus 1 (GLRaV-1),
  • Grapevine leafroll-associated virus 2 (GLRaV-2),
  • Grapevine leafroll-associated virus 3 (GLRaV-3),
  • Grapevine leafroll-associated virus 4 (GLRaV-4), which includes several different strains
  • Grapevine leafroll-associated virus 7 (GLRaV-7),

GLRaV-3 has been the most commonly found leafroll virus in Canada, however GLRaV-1 and GLRaV-2 are also present. In the national grapevine survey conducted in 1994-1995, GLRaV-1 was present in 1.0% of samples tested, whereas GLRaV-3 was present in 10.5% of samples tested nationally. Preliminary results from current surveys of symptomatic vines in conducted by Agriculture and Agri-Food Canada, Summerland, in 2011-2013 indicated that GLRaV-3 and GLRaV-2 are the predominant leafroll viruses in B.C. Smaller amounts of GLRaV-1 and GLRaV-4 were also found.

Symptoms: Leafroll virus has impacts on both vine health and grape quality. Growth and yield may be reduced by 10-40%. The virus reduces yield by inhibiting cluster formation and development. Infected vines have an increased sensitivity to environmental stress. Quality impacts included delayed maturity of grapes, a 25-50% reduction in sugar content, poorly coloured fruit, and modified aromatic profiles of the must.

Leafroll symptoms are highly variable between cultivars. Typical leaf symptoms include reddening of the leaves between major veins in red varieties, and yellowing of the leaves between major veins in white varieties. In both cases, a key diagnostic symptom is that primary veins usually remain green in colour. Leaves become thick, brittle, and roll downwards in some cultivars. Symptoms typically appear around veraison, beginning with older leaves and gradually moving upwards in the canopy as the season progresses.

grapevine leafroll disease grapevine leafroll disease
Grapevine leafroll disease.  Photos courtesy Canadian Food Inspection Agency.

Transmission: Long distance spread occurs primarily by movement of propagation material from infected plants. Several mealybug and soft scale insect species are known to transmit leafroll associated viruses 1, 3 and 4. Leafroll vectors that are present in B.C. include the common grape mealybug (Pseudococcus maritimus), the cottony maple scale and the European fruit lecanium scale. There are no known vectors for GLRaV-2 or GLRaV-7.

Management:

  • Source vines that have been tested free of leafroll viruses, if available.
  • Monitor for and manage leafroll vectors including mealybugs and soft scales.
  • Monitor for leafroll symptoms in late summer. Since symptoms are often variable and unreliable, have suspect vines tested to confirm the presence of leafroll.
  • If a small percentage of vines in a block are known or suspected to be infected, it is best to remove them to limit spread within the block.
  • If a high percentage of vines are infected, consider whole block removal.
  • Take measures to prevent moving mealybug and scale insects from infected to healthy blocks by working infected blocks last and cleaning equipment between blocks.

Grapevine Fanleaf Virus

Fanleaf degeneration disease is thought to be the most serious grapevine virus disease. The severity of symptoms varies by cultivar.

Symptoms: Infected leaves are often malformed with abnormally gathered primary veins, giving the leaf the appearance of an open fan. Other symptoms may include a yellow mosaic pattern on leaves or bright yellow bands along major veins. Fan-shaped leaves may or may not be present with mosaic or veinbanding symptoms.

grapevine fanleaf grapevine fanleaf
Grapevine fanleaf virus symptoms - yellow mosaic pattern on leaf. Photo courtesy Canadian Food Inspection Agency. Grapevine fanleaf virus symptoms - bright yellow vein banding on leaf.  Photo courtesy Canadian Food Inspection Agency.

Affected vines tend to be smaller than healthy vines. Sensitive varieties show progressive decline, low yields (up to 80% losses) and low fruit quality. The productive life of the vineyard is shortened and winter hardiness is decreased.

Transmission: Fanleaf virus is transmitted by the nematodes Xiphinema index and X. italiae, which have never been found in Canada. There are no natural weed hosts. Long distance spread occurs primarily by movement of propagation material from infected plants.

Arabis Mosaic Virus

Symptoms: The most common symptoms include leaf mottling and flecking, stunting and leaf deformation including enations. Many infections are latent and plants do not show symptoms. Symptoms can vary greatly by variety, rootstock and environment conditions. Yield losses of up to 50% may occur through reduced growth, dieback and severe dropping of fruit.

Arabis mosaic virus Grapevines with yellow leaf mottle due to Arabis mosaic nepovirus, cv. Tamiioasca romineasca.

Photo courtesy A. Eppler, Justus-Liebig Universität, www.ipmimages.org

Transmission: Long distance spread occurs primarily by movement of propagation material from infected plants. Arabis Mosaic Virus is transmitted by the nematode Xiphinema diversicaudatum, which has been found in scattered locations in Canada. This virus can also infect many other herbaceous and woody hosts such as raspberry, strawberry, rhubarb, cherry, peach, and plums.

Rugose Wood Complex Viruses

The rugose wood complex is caused by several viruses, including Rupestris stem pitting associated virus (RSPaV), Grapevine virus A (GVA) and Grapevine virus B (GVB).

RSPaV is a common and widespread virus of grapevines. It is not regulated in Canada or the USA. Both GVA and GVB are regulated diseases in Canada, and have been found in only a few localized areas. GVB has not been found in B.C. to date (2014).

Symptoms: GVA is associated with Kober stem grooving disease. Affected vines may show swelling at the graft union and fail to thrive. GVB is associated with corky bark disease, which may cause an incompatibility to develop at the graft union. Leaf symptoms resemble those of leafroll virus but are more severe, and severe stunting of the vine may occur.

Rupestris stem pitting causes a slow decline in growth, resulting in vines that are smaller than normal with reduced yields. No leaf discoloration is observed. The disease affects only grafted vines; ungrafted vines may be infected, but usually do not show symptoms. Small pits may develop in the wood on rootstocks, in particular V. rupestris and American rootstocks. The severity of the disease is more pronounced in vines that are also infected with other viruses of the rugose wood complex, including GVA and GVB.

Transmission: Spread is mainly through propagation. There is no known insect vector for Rupestris stem pitting virus. GVA and GVB can be transmitted by some species of mealybugs. The cottony maple scale (Neopulvinaria innumerabilis) can transmit GVA.

Management: Disease management depends upon use of certified healthy, virus-free stock.  Remove vines showing suspicious symptoms.

 

References

Updated September, 2014

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