Unlike many other crop species, wine grape cultivars are vegetatively propagated to maintain quality characteristics or varietal integrity (trueness-to-type). Thus, distribution of infected vegetative cuttings is the most significant means of spreading viruses and virus-like agents around the world.
Wine grapes are grown on their own roots in eastern Washington, so using cuttings from sources with compromised sanitary status will introduce virus and virus-like diseases into new plantings here. In western Washington, grapevine cultivars are propagated by grafting onto suitable rootstocks to protect from phylloxera and nematode-borne virus infections and promote early ripening. In these cases, virus introduction into new plantings can occur if either the rootstock or scion is infected with a virus. If a new vineyard is planted with infected cuttings or grafted with budwood from infected vines, the vines will grow with infection and suffer chronic and cumulative losses over the life of the vineyard. In addition, infected vines in a new vineyard can serve as a source of inoculum for secondary spread by resident vectors (either insect or nematode) and infect other vineyards if used as cuttings for new plantings.
Viruses associated with diseases like grapevine leafroll disease (GLD) can be transmitted by insect vectors such as mealybugs (Pseudococcidae) and scale insects (Coccidae). In California, for example, 5 different species of mealybugs (Obscure mealybug, Pseudococcus viburni; Citrus mealybug, Planococcus citri; Vine mealybug, Planococcus ficus; Longtailed mealybug, P. longispinus; and Pink hibiscus mealybug, Maconellicoccus hirsutus) were reported as vectors of different GLRaVs. Fortunately, grape mealybug is the only known insect in Washington State vineyards capable of transmitting GLRaV-3 under laboratory conditions.
Among all mealybug vectors, vine mealybug is noteworthy because of its high efficiency in spreading GLD. Unlike grape mealybug, which has 2 generations per season, the vine mealybug has up to 9 generations per season, equipping it with the ability to dramatically increase the spread of disease within a vineyard during the season. Vine mealybug is a quarantine pest in Washington, which is indicative of the concerted effort needed to prevent its establishment in state vineyards. The status of scale insects in Washington vineyards and their ability to act as vectors for GLD is currently not known.