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 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 infected for life.
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 and cause significant economic losses. Decreased yields of 5-10% 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.
There are over 50 different viruses and viroids that infect grapevines distributed throughout the world. Several are known to exist in Canada already.
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 B.C.
Leafroll 3 was the most commonly found virus, but the other viruses were present at very low levels in B.C. For example, out of 1,485 B.C. samples, only one positive was found for fanleaf virus and only five 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 worthwhile to continue to prevent introduction through careful screening of nursery stock.
However, additional grapevines infected with these viruses have been imported since 1995. With increasing international trade and travel, it is expected that more of these viruses will arrive.
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 as well as 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. This is 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) 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, such as:
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.
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 and peach rosette mosaic virus. 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 B.C. However, other nematodes found in the province, 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.
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 protect grape growers from the introduction and spread of virus diseases and other pests that are not yet established in B.C.
Selected grapevine varieties/clones and rootstocks from Canadian-approved nurseries in France and Germany are currently allowed to be imported 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 deregulation. It is recommended that growers importing vines request that they be tested for these viruses in the country of origin as these viruses cause serious economic diseases.
Removal 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 in Sidney (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.
Decline of young vines may have several causes, including diseases, nematodes, environmental damage such as winter injury, and cultural factors. Often, more than one factor or disease is involved.
Vine decline symptoms can be difficult to diagnose accurately. B.C. grape growers interested in diagnosis of decline problems should contact the provincial plant health laboratory or the Summerland Research and Development Centre for more information.
General symptoms of grapevine declines caused by fungal pathogens include delayed and stunted growth, short internodes, yellowing and premature loss of leaves, tendril dieback, trunk dieback, dead arm and cankers.
Discolouration may be observed in the wood when vines are cut open. Decline may be rapid, causing plant death within 2-3 weeks, or slow, resulting in reduced vigour and yield over a period of years.
A 2007-2009 survey conducted by Agriculture and Agri-Food Canada investigated decline problems in Okanagan vineyards (O'Gorman, Haag & Sholberg). The survey confirmed the presence of several fungal pathogens causing vine decline symptoms.
Diseases detected included:
This was the first time that these diseases, excpet for roesleria root rot, were diagnosed in B.C. vineyards.
There are many grapevine pests and diseases that are not yet established in grape growing areas in B.C. Some of these are downy mildew, black rot, phomopsis, flavescence dorée, Pierces’ disease, anthracnose, rust and dead arm.
If you suspect a new disease, contact the B.C. Ministry of Agriculture to confirm identification or send a sample to the Plant Health Laboratory in Abbotsford:
