Stephen Jordan, University of Wisconsin-Madison
Symptoms differ depending on the variety of affected grape and are best observed in late summer and fall. On red-skinned varieties of Vitis vinifera, leaf tissue between the veins turns deep red to purple, with downward curling or cupping of the leaf margins. On white varieties, the leaf tissue will turn yellow with curling or cupping of the leaf margins. For both symptom types, the veins will remain green. Rootstock, American native, and hybrid varieties can be infected, but typically do not show symptoms. Vines with leafroll disease are less vigorous than healthy vines and may be less likely to recover from winter injury due to reduced carbohydrate stores. Leafroll affected vines can have yield losses of 30-50%, delayed and uneven ripening of fruit, a reduction in brix and berry color, and an increase in titratable acids. Phosphorus and potassium nutritional deficiencies  mimic many of the symptoms caused by grape leafroll disease. For this reason, confirmation of leafroll infection prior to management decision-making is recommended.
Currently, there is no chemical control for grape leafroll disease. Chemical control of potential insect vectors of the viruses that cause leafroll disease may help limit spread of the disease within a vineyard.
Grape leafroll disease symptoms are associated with at least ten different viruses that are referred to as Grapevine Leafroll-Associated Viruses (GLRaVs). The distinct viruses are named GLRaV-1 through GLRaV-10, based on the order of their discovery. The most common means of spreading leafroll-associated viruses is through vegetative propagation and grafting. GLRaVs can be moved across long distances in planting and propagation materials. In addition, two insect vectors, mealybugs and soft scales, have been shown to transmit GLRaVs between vines, and in some cases, between nearby vineyard blocks. Confirmation of a suspected leafroll infection can be made by a commercial or university laboratory. When submitting samples for testing, it is advisable to contact the testing facility prior to sampling to obtain proper collection, storage, and submission procedures.
Mealy Bugs and Grape Leafroll Disease, University of California
Grape Leafroll Disease, Washington State University
Grape Leafroll Disease, Cornell University
Grape Leafroll Disease video, Washington State University
Reviewed by Damon Smith, Oklahoma State University and Michelle Moyer, Washington State University