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Grape Virus DiseasesWhat 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 SymptomsViruses 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. Other symptoms may also be present on foliage, stems, leaves 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 environment. 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.
Importance of Virus DiseasesVirus 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 SpreadHumans 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
Clonal SelectionRemoval of viruses from plant material is possible, but requires heat treatment of the vines to “kill” the virus. 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 heat treatment 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 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 DiseasesThere are over 50 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 BC 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 BC 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 Fanleaf VirusFanleaf 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.
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. Grapevine Leafroll-Associated VirusesGrapevine leafroll is probably the most wide-spread virus disease of grapevines world-wide. There are currently 9 different viruses associated with leafroll, but Grapevine leafroll-associated viruses 1 and 3 are most commonly found. 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. Since then Grapevine leafroll-associated viruses 2, 4 and 5 have been detected in imported grapevines. Symptoms: Leafroll virus has impacts on both vine health and grape quality. Growth and yield may be reduced by 10-70%. 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, and poorly coloured fruit. 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. Leaves become thick, brittle, and roll downwards.
Transmission: Long distance spread occurs primarily by movement of propagation material from infected plants. Several mealybug and soft scale insect species have been shown to transmit leafroll associated viruses under experimental conditions. Rapid leafroll spread has been reported in European vineyards having significant insect vector populations. In California, a very low rate of natural spread within vineyards has been observed. The common grape mealybug (Pseudococcus maritimus) which occurs in the Okanagan is a vector of GLRaV-3. Arabis Mosaic VirusSymptoms: 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.
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. Rupestris Stem Pitting VirusRupestris stem pitting associated virus-1 is a common and widespread virus of grapevines. It is not regulated in Canada or the USA. A recent survey found this disease in 4.6% of the samples in Washington State. Symptoms: 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 infected with other viruses of the rugose wood complex, including Grapevine virus A (GVA) and Grapevine virus B (GVB). 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. Transmission: Spread is mainly through propagation. There is no known insect vector for Rupestris stem pitting virus. GVB can be transmitted by some species of mealybugs. Disease management depends upon use of certified healthy, virus-free stock. Remove vines showing suspicious symptoms. November 2006
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