Disease distribution patterns indicate two common patterns of spread: 1) two or more infected vines directly adjacent to each other in the same row, and 2) a focus of infected vines at the edges of a vineyard with a gradient of diminishing incidence towards the middle or other side of the vineyard.

This latter pattern suggests that leafroll disease has been introduced into a vineyard from an established external source on one side of the vineyard. In most cases the number of infected vines in these gradients increases in the opposite direction of an adjacent older, infected vineyard. These vineyards probably serve as the source of the disease where mealybugs acquire the virus and then are spread to neighboring vineyards by some combination of wind, laborers, or machinery.

The introduction of leafroll disease from outside sources is particularly worrisome because it is difficult to control and has the potential to compromise the benefits of clean plant programs. Older infected vineyards are one obvious disease source; it is also possible that there are other yet unidentified outside sources.

rapevine leafroll disease is one of the most economically damaging and widespread diseases of Vitis vinifera throughout the world. To date, eleven different phloem-limited viruses, identified as Grapevine leafroll- associated viruses (GLRaVs) 1, 2, 3, 4, 5, 6, 7, 9, 10, 11, and Carnelian have been isolated from diseased grapevines and characterized. All GLRaVs can be graft-transmitted; GLRaV-1, -3, -5, and -9 are also transmitted by several species of mealybugs (Homoptera: Pseudococcidae) and soft scale insects (Hemiptera: Coccidae).

While the incidence of leafroll disease due to graft transmission has been reduced in propagative material through the use of certified clean planting stock, disease incidence associated with insect transmission still occurs in many grape growing regions.