Virus Diseases
The relationship between wine grapes and virus diseases is similar to that between humans and health problems such as the flu, chickenpox, and even AIDS. Visible effects of viral diseases range from decreased grapevine growth and performance to impaired quality of vital characteristics (berry weight and color and sugar content, etc.). Studies in viticultural regions around the world show that these negative impacts, if left unchecked, translate into diminished or even destroyed economic returns and consequent loss of income.
Nearly 60 different documented viruses infect grapes worldwide, which is far greater than the number of viruses documented in any other single perennial crop. It is therefore important to tailor management decisions to protect against grapevine vulnerability to a broad range of viruses with distinct epidemiological characteristics. Some viruses are spread by insect vectors and others by soil-inhabiting nematode vectors. Since wine grapes are propagated by vegetative cuttings to maintain varietal integrity and clonal identity (trueness-to-type), all these viruses can be spread from place to place through the distribution of infected vegetative cuttings (see Sources of virus-tested planting materials).
Major and minor virus diseases
Viruses infecting grapevines can be broadly divided into 2 groups based on their economic significance and geographic distribution. The first group consists of major diseases (also called traditional diseases) that are of great economic concern to viticulture and the global wine industry. These include grapevine leafroll disease (GLD), rugose wood (RW) complex, and nematode-borne viruses. The second group consists of minor diseases that are of less economic significance and/or limited geographic distribution. The majority of grapevine-infecting viruses fall into this latter category. It should be noted, however, that all viruses, irrespective of their economic significance, are considered equally important with respect to quarantine regulations and maintaining the sanitary status of a vineyard.
Although the biological and molecular characteristics of many viruses associated with specific grapevine diseases are well documented, the causal agent(s) of some grapevine diseases remain unknown. Such diseases are called “virus-like” since the symptoms mimic those caused by viruses. As with virus diseases, virus-like diseases can be transmitted by grafting. Thus, viruses and virus-like agents are also known as graft-transmissible agents (GTAs).
Grapevine leafroll disease. Among the virus and virus-like diseases infecting grapevines worldwide, GLD is considered the most destructive. It occurs widely in almost all grape-growing regions (including the entire Pacific Northwest) and accounts for about 60% of the global grape production losses due to virus diseases.
GLD is one of the most complex viral diseases known to infect plants. Virologists have identified 9 grapevine leafroll-associated viruses (GLRaVs) in wine grapes showing GLD symptoms. They are numbered GLRaV-1 through -9 in order of their discovery. GLRaVs have a great deal of diversity in their genome organization and gene content and may infect a grapevine separately or as mixed infections of different combinations. Consequently, it is not clearly established whether the induction of GLD is due to a single GLRaV infection or requires a mixture of GLRaVs. GLRaVs can also occur as mixed infections with other groups of viruses infecting grapevines. Mixed infections often result in synergistic effects leading to more severe damage to grapevines than single infections.
Of the 9 GLRaVs, 6 (-1 to -5 and -9) have so far been found in Washington vineyards. Among these, GLRaV-3 is the most common and widespread. All 6 GLRaVs occur in mixed infections of different combinations in GLD-infected vines in Washington State vineyards.
Rugose wood (RW) complex (or trunk disease). RW diseases can be differentiated into 4 different types of disorders based on their effects on woody grapevine cylinders when grafted onto specific Vitis indicator hosts:
- Rupestris stem pitting disorder
- Kober stem grooving disorder
- LN 33 stem grooving disorder
- Corky bark disorder
Similar to GLD, the etiology of RW complex is multifaceted and not yet completely understood. At least 4 viruses have been found associated with different types of RW diseases. They are grapevine rupestris stem pitting-associated virus (GRSPaV), grapevine virus A (GVA), grapevine virus B (GVB), and grapevine virus D (GVD). Recent studies at WSU indicate the occurrence of GRSPaV, GVA, and GVB in different wine grape cultivars in Washington vineyards. These viruses do not show obvious symptoms in own-rooted wine grape cultivars, which can therefore serve as RW complex carriers. GRSPaV, GVA, GVB, and GVD may also occur as mixed infections among themselves and with different GLRaVs.
Nematode-borne or nepovirus diseases. Nepoviruses can also significantly impair wine grape production. Transmitted by soil-inhabiting nematodes that feed on grapevine roots, symptoms due to these viruses vary depending on the type of virus, cultivar, and climatic conditions.
Among the nematode-transmitted viruses, grapevine fan leaf virus (GFLV) is the most widespread around the world. Besides GFLV, other important nepoviruses include tobacco ringspotvirus, tomato ringspotvirus, tomato blackringvirus, Arabis mosaic virus, and peach rosette mosaic virus, all of which occur throughout Europe and the U.S. (Northeast, Midwest, and California). Many of them are transmitted by the longidorid nematode (dagger nematode) species in the genus Xiphinema, which has been found in Washington vineyards. However, there is no documented evidence of nepoviruses here. To maintain this clean record, it is very important to use nepovirus-free propagating materials for planting new vineyards and exercise extreme caution while bringing planting materials from regions endemic to these viruses. Since nematodes and nepoviruses have a broad host range, it is also important to test soil for nematodes before new plantings or replacing an existing crop with new grapevines.
Stasis
As of this writing (December 2007), there is no evidence of any other viruses or virus-like diseases besides GLD and RW complex in Washington vineyards. However, rapid expansion of wine grape acreage and changing viticultural practices pose the risk of introducing new or exotic virus diseases, viroids and phytoplasmas, as well as disorders of unknown etiology, which means prevention requires the utmost vigilance.
If you find any suspicious symptoms, contact the grape virologist at WSU.
Grapevine Leafroll Disease
Written by Naidu Rayapati, Sally O'Neal, and Douglas Walsh, this publication presents the latest research findings and recommendations regarding Grapevine Leafroll Disease for wine grape growers and certified nurseries of Washington state. Download this free publication.
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