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April 2018 WINES&VINES 85 WINE EAST GRAPEGROWING Infection by GLRaVs is limited to the phloem, 11 and results in the degeneration of primary phloem tissues in shoots, leaves, and clusters. The phloem, the thin layer of tissue between the outer bark and the cambium, con- ducts the products of photosynthesis (primarily sugars) from green tissues (mainly leaves) to the rest of the vine. The degeneration of this tissue can have profound effects on grapevine health and productivity. How GLRaVs are spread As is the case with so many plant pathogens, GLRaVs have been distributed across the world through increased trade and movement of plant material and other products. 2 In fact, infected nursery stock appears to be the prime method of transmission. The viruses can spread through the graft union; but thank- fully, they do not appear to be transmissible from one plant to another via pruning or con- tact, since their concentration, or titer, in the phloem is very low. 11 A nationwide study conducted in Canada i n 1 9 9 4 - 9 5 r a i s e d t h e p o s s i b i l i t y t h a t GLRaV-3 may be endemic to North America, as it was most commonly found in non-vi- nifera grapevines that had been cultivated on the continent for a long time. On the other hand, the fact that GLRaV-1 was found most often in vinifera varieties (whose ar- rival in Canada is much more recent) pro- vided evidence that it was imported with vines from Europe and Asia. 11 Once in the vineyard, GRLD can be spread short distances, from vine to vine, by phloem- feeding insects such as mealybugs and scale insects. The grape mealybug (Pseudococcus maritimus) is thought to be the primary vector species, 3, 15 together with Planococcus ficus and P. longispinus, but its presence and distribution in Pennsylvania has never been studied. Inter- estingly, no vector has yet been found for GLRaV-2 and -7. 15 Resistance to GLRaVs There are no known sources of resistance to GLRaVs among Vitis species, and these viruses have been found in many cultivated grape va- rieties from labrusca to interspecific hybrids and vinifera. As mentioned earlier, labrusca varieties such as Concord and Niagara can also become infected with GLRaVs, but the infections appear to remain latent or dormant 1 and do not result in visual symptoms of the disease. 19 Vinifera varieties, however, make up a grow- ing sector of the eastern wine grape industry and are severely affected by GLRaVs. Where infected labrusca vineyards border on areas of wine grape production, there is a real concern that these viruses will be vectored by insects into healthy wine grape vineyards. Management of GLRD For new vineyards, planting only certified virus- free stock is the first line of defense. Infection by GLRaVs is permanent; there is no way to rid vines of these viruses once they become infected. To manage and reduce the effects of the disease once in the vineyard, infected vines must be rogued and replaced by certified healthy vines. The National Clean Plant Network (nation- alcleanplantnetwork.org) has a list of national clean plant centers that can be contacted for certified material. When a symptomatic vine has been identified as positive for GLRD, re- moval of each adjacent vine within the row is also recommended, as it may take from one to three years for the visible symptoms to be ex- pressed. Inspection of vineyards should be done annually between véraison and harvest, when symptoms are most vividly displayed, especially in red varieties. 12 Since certain species of phloem feeding insects can vector these viruses, insecticides that target mealybugs and soft scales can pre- vent vine-to-vine spread within the vineyard (grapevine leafroll disease fact sheet). 2 Studies have shown that applications of insecticides such as spirotetramat (Movento) can signifi- cantly reduce mealybug counts and can help to control spread. 10, 16 Grape mealybug overwinters on the vine and first instar crawlers are thought to be the most mobile life stage, and hence most apt to spread the virus. Since this insect can have two generations per season, there are two crawler stages that need to be targeted for manage- ment. 3, 18 Studies in New York showed that insecticide applications should target overwin- tered and early season mealybug crawlers by combining delayed dormant applications of horticultural oils, applied in late April, before green tissue, followed by acetamiprid in early July, when the first of the new summer genera- tion of crawlers was observed, or by applica- tions of spirotetramat after bud break and again in early July. KEY POINTS When individual grapevines start to display red leaves with green veins late in the summer and into the fall, it is a sign that grapevine leafroll disease (GLRD) has infected that plant. Other symptoms include cupped leaves and, on white- fruited varieties such as Chardonnay, leaves that are yellowish with green veins. Grapevine leafroll disease is primarily caused by five species of grapevine leafroll-associated vi- ruses; the most common species is known as grapevine leafroll-associated Virus-3, or GLRaV-3. This disease is relatively new in Pennsylvania and other northeastern states, because until re- cently the grapes grown in the region were pri- marily labrusca or hybrid varieties. With more vinifera now being planted, growers are begin- ning to see more evidence of GLRD. Researchers at Penn State University have started a study to determine the extent of GLRD in the state's vineyards and how it impacts veg- etative growth and vineyard yields. The sampling study will continue so growers can recognize the disease and its impact on the wine industry, and also help them deal with the effects of the vi- ruses on productivity and fruit quality. A vineyard row with healthy vines interspersed with vines with symptoms of grapevine leafroll virus.