This is a typical question we hear at the winery during the fall harvest. Yes they are beautiful, but we do not want these pretty vines in our vineyard. Why ? Because these vines are sick. They are unable to ripen the grapes to the 22-26% sugar we need to ferment a wine to 13-14% alcohol. The process of photo synthesis has been short circuited by Grapevine Leafroll Virus. Without photosynthesis, the vines cannot produce sugar and the leaves cannot retain their green chlorophyll.
Leafroll Virus is found the world over and is common on grafted vines, but with proper heat treatment in the nursery it can usually be controlled. With infected vines,sugar production is slowed and fruit maturity can be delayed for weeks. If the infection is limited, growers can live with a small loss. On the other hand, if the entire vineyard is a beautiful red color the grower should think about inviting artists and photographers to record the pretty picture, as sugar production / wine production will cease.
Leafroll spreads very slowly from vine to vine and is subtle in its effect on production, and some growers choose to live with this problem, but the usual solution is the eventual removal of the infected vines.