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84 WINES&VINES April 2018 G rapevine leafroll disease (GLRD) is nothing new to most of the world and has had the attention of scientists and grapegrowers for a long time. As early as 1853, the disease was observed in French vineyards and was called rougeau (red leaf), for obvious rea- sons. By 1936, the causal organism was thought to be viral in nature. 2 However, the disease is relatively new to the grape and wine industry in the northeastern United States, partly because vinifera grapevines, the species most dramati- cally affected, were not widely planted in the region. As the acreage of vinifera in Pennsylvania continues to expand and become a larger part of the "bottom line," encounters and frustrations with GLRD will likely increase. The symptoms and causes of GLRD The most obvious symptoms of the disease consist of cupping and loss of chlorophyll in the leaves during late summer and fall. For example, on red-fruited varieties such as Cab- ernet Franc, leaves of infected vines can display a distinct red coloration of the interveinal tis- sue, while veins remain green. 11 On white- fruited vinifera varieties like Chardonnay, symptoms are less noticeable and leaves are cupped and tend to look yellowish in the in- terveinal tissue. 5 Discoloration generally affects older leaves first. 2 These symptoms are not necessarily diagnostic of the disease, as they may be due to other causes such as nutrient deficiencies, water stress and even crown gall, which many growers in Pennsylvania have become familiar with since the very cold win- ters of 2014 and 2015. More significant, and perhaps less recog- nized, are the detrimental effects of GLRD on yield and fruit quality in the region. Compared to healthy vines, those affected by GLRD are known to have reduced yield, vegetative growth and even lower cold hardiness, which is a factor of critical importance for varieties grown in the region. GLRD can also delay fruit maturity, negatively affect grape chemistry at harvest (lower soluble solids, higher titratable acidity) and reduce color development in red vinifera grapes—all factors that may adversely impact perceived wine quality. GLRD is caused by viruses in the family Closteroviridae, which have relatively long (as viruses go), rod-shaped particles that can be viewed with transmission electron microscopy. Currently, there are about five distinct species of grapevine leafroll-associated viruses (GL- RaVs) found in cultivated grapes: GLRaV-1, 3, 4 in the genus Ampelovirus; -2 in the genus Closterovirus, and -7 in the genus Velarivirus (International Committee on Taxonomy of Vi- ruses (ICTV), 2016). Grapevine leafroll associ- ated viruses-5, 6, 9, 10 and 11 are also reported in the literature, but the phylogeny clusters them as close relatives of GLRaV-4. The most common species found in grapes is GLRaV-3, which is thought to be endemic to North America. 2 The severity of the GLRD ef- fects can vary from one season to the next and is dependent on a number of factors including grapevine variety, scion/rootstock combina- tion, virus strain, climate, soil, cultural prac- tices, stress factors, etc. The effects of these viruses on hybrid grape varieties and Vitis labrusca are generally con- sidered to be less serious than those on vinifera, but the effects are also less well-known and studied on those species. As the symptoms of GLRD can mimic other maladies of the vine, analysis of grapevine tissues in the laboratory is the only way to confirm the presence (or absence) of these viruses. Grapevine Leafroll Disease in the East How big a threat does it pose to the eastern wine grape industry? By Bryan Hed, Cristina Rosa, Michela Centinari and Joseph Walls III Red leaves with green veins are signs of infection with grapevine leafroll virus.