The are many viruses that cause diseases in grapevines, and due to the woody nature of much grapevine tissue, it is often difficult to purify such tissue. New vineyards planted from infected cuttings or budwood will be diseased from the time they are established. That is why a programme such as the Foundation Plant Services (FPS) at UC Davis is so vital to the wine industry. FPS is a self-supporting service department in the College of Agricultural & Environmental Sciences and is very instrumental in the detection and elimination (using heat treatment techniques) of grapevine diseases.
Grapevine leafroll is probably the most wide-spread virus disease of grapevines and there are currently 9 different viruses associated with leafroll. Leafroll spreads slowly from vine to vine and impacts both vine health and grape quality, in some instances reducing yields as much as 50 percent or even more, depending on the severity of infection. Typical leaf symptoms include reddening of the leaves between major veins in red varieties. Thus, we can assume that the vivid dark red of the vine in the above photograph, and the reddening of its neighbouring vines on the Silverado Trail, indicates that it is indeed a red variety.
A green leaf is green because of the presence of a pigment known as chlorophyll. When chlorophyll is abundant in the leaf's cells, as they are during the growing season, the chlorophyll's green colour dominates and masks the colours of any other pigments that may be present in the leaf. Consequently, the leaves of summer are characteristically green, but in the autumn and at harvest time the entire vine takes on a reddish cast. It's very pretty, but it's very unhealthy.
When a virus is present it disrupts the normal physiological function of the vine's cells. A healthy vine will efficiently remove pigments leaving the dying leaf to become yellow, then brown, followed by the detachment of the petiole from the shoot and leaf drop. The red pigment in the leaf, anthocyanin, is usually the most difficult pigment to remove when the vine's phloem has been compromised by a virus. Exactly how leafroll affects the anthocyanin pathway through the vine's vascular system remains a mystery and is the subject of ongoing research. Curiously, a way of checking if a particular vine is infected with leafroll virus is to graft a bud from the suspect vine onto a healthy Cabernet franc vine. This vinifera cultivar is very sensitive to the disease, showing strong symptoms sometimes within as little as 18 months of grafting.
Because leafroll virus does not kill vines, but instead causes reductions in yield, maturity and quality, infected vines are rarely removed from otherwise healthy and productive vineyards. The only effective control of leafroll, or any other grape virus disease, is to ensure that healthy propagating material from virus-tested stock is used to plant vineyards.
Red coloured foliage, although the result of disease in a vine, really is very attractive. I, along with the valley's tourists, really enjoy the show.