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Fanleaf threat: A concerted effort is needed to prevent its spread.
Grapevine fanleaf, an infectious degenerative disease, is the oldest known viral disease of grapevines. It is believed that fanleaf virus originated from ancient Persia and spread to other grape-growing regions via transport of vegetative propagative materials. Consequently, the disease has become established as one the most serious and devastating grapevine virus diseases worldwide. The disease can cripple infected grapevines with misshapen leaves, short internodes, zigzag growth of canes, and poor berry set. The disease also has a detrimental effect on fruit yield and quality, and longevity of the vines. All cultivars of wine grapes (Vitis vinifera) are susceptible to the disease. Severe economic damage, with yield losses up to 80 percent, has been recorded in many sensitive cultivars.
To date, fanleaf disease has been observed in Washington vineyards in four wine grape cultivars (Pinot Noir, Merlot, Cabernet Franc, and Chardonnay). Since these cultivars were planted in different years in geographically distinct vineyard blocks by unrelated growers, it is likely that infected planting materials were introduced from multiple sources. In Pinot Noir, infected grapevines showed a broad range of symptoms consisting of fan-shaped leaves mimicking the "lady's fan" (and hence the name of the disease) with toothed margins, vein-banding and yellow mosaic symptoms (Figure 1). In Chardonnay, infected grapevines showed fan-shaped leaves and yellow mosaic symptoms, but not vein- banding symptoms (Figure 2). In both cases, infected grapevines produced small clusters with poor fruit set, irregular ripening and shot berries (Figure 3 a & b). Although fanleaf disease has so far been detected only in Washington vineyards, the virus is likely to be present in Oregon and Idaho vineyards, also.
Characteristic symptoms of fanleaf disease can be readily noticed in early spring, but not in summer and fall. Thus, unlike grapevine leafroll disease symptoms, which can be observed in infected grapevines in late summer and fall, monitoring vineyards during the spring would provide an indication for the presence of fanleaf disease. Lab-based methods like enzyme-linked immunosorbent assay (or ELISA) and reverse transcription-polymerase chain reaction (or PCR) are available for reliable detection of fanleaf disease.
Fanleaf disease symptoms sometimes mimic herbicide injury. It is advisable to get suspected samples tested for the virus to determine if the symptoms are due to virus or herbicide damage. One important thing to keep in mind is that fanleaf disease symptoms recur in successive seasons in the same grapevine, whereas herbicide damage symptoms may not be apparent the following season.
Unlike grapevine leafroll disease, fanleaf is a soilborne viral disease and is transmitted by the soil- inhabiting plant parasitic dagger nematode called Xiphinema index. The spread of the virus occurs by two principal modes: grapevine-to-grapevine spread in the vineyard by the nematode vector; and, long-distance spread via distribution of propagation material from infected grapevines. X. index is the only natural vector of the virus. Other dagger nematode species like X. pachtaicum and X. americanum present in the Pacific Northwest are unlikely to serve as vectors of the virus.
Several viruses transmitted by different nematode species, collectively known as nepoviruses, have been documented in many grape-growing regions around the world. Although none of these nepoviruses have been documented thus far in the Pacific Northwest region, utmost vigilance is required to avoid their introduction, since nematode species present in soils of the region can potentially spread some of these nepoviruses. Recently, tomato ring spot virus was observed in a vineyard in Oregon, and this virus can be transmitted by X. americanum.
Due to the vegetative propagation and perennial nature of the grapevine, co-infection of several viruses can occur in the same plant. Research conducted at Washington State University's Irrigated Agriculture Research and Extension Center in Prosser has shown that fanleaf virus can occur frequently as a mixed infection with other grapevine viruses. Figure 4 shows grapevine leaves with grapevine leafroll and fanleaf disease symptoms due to mixed infection of both diseases. It is therefore advisable to test grapevine cuttings for different viruses to ensure the cleanliness of planting materials.
Unlike fungal diseases, such as powdery mildew that can be controlled by timely and judicious applications of fungicides, there are no curative measures to treat fanleaf-virus–infected grapevines. Therefore, preventive measures have to be deployed for the management of virus diseases. Since there is no credible evidence yet of the presence of X. index in Pacific Northwest vineyards, measures to eradicate the nematode vector for preventing grapevine-to-grapevine spread of the disease have no beneficial value. Studies have indicated random distribution of virus-infected grapevines in Pinot Noir, Merlot, Cabernet Franc, and Chardonnay blocks, suggesting that virus dissemination has been through planting materials. This indicates that roguing and replanting with virus-tested cuttings will be an effective strategy in eradicating the disease from virus-infested vineyard blocks. Planting new vineyards with virus-tested planting materials is one of the most critical decisions that a grower will live with for the life of the vineyard and deserves careful thought in procuring the planting material from reliable sources like certified nurseries.
Grapevine fanleaf disease shows a wide range of symptoms, influenced by environmental factors, management practices, nature of planting (grafting onto a rootstock versus own-rooted vines), cultivar, and virus combinations. Thus symptoms expressed by infected grapevine cultivars in the cool-climate conditions of the Pacific Northwest may differ from other viticulture regions. It is therefore important to get suspected grapevine samples tested for the presence of grapevine fanleaf virus.
If appropriate measures are not in place, the virus could become a major threat to the wine grape industry in the Pacific Northwest because of the increased exchange of vegetative cuttings to meet the expansion of wine grape acreage. A concerted effort among researchers, industry stakeholders, and regulatory agencies is critical to curtail the spread of fanleaf virus among the three states in the region and also to prevent the introduction of other nepoviruses and/or nematode vectors into the region via planting materials imported from outside.
Contact Dr. Naidu Rayapati for more information (e-mail firstname.lastname@example.org, phone 509-786-9216, or cell 509-788-5350).
Research supporting this information was funded in part by WSU's Issue-focused Team internal competitive grant, Washington Wine Commission's Wine Advisory Committee, Washington State Department of Agriculture's Nursery Assessment Funds, WSDA Specialty Crops Block Grant Program, Washington State Commission on Pesticide Registration, U.S. Department of Agriculture's Northwest Center for Small Fruits Research, and USDA Viticulture Consortium-West.