LEAF ROLL virus is a vine disease caused by any one or some combination of nine known virus strains. The disease is chronic, but not fatal to the vine. Infected vineyards decline in fruit quality and quantity, the results of delayed and uneven fruit maturation with reduced sugar content and color and subnormal berry and cluster size, hence lowered crop yields.
The symptoms are leaves that curl downward from the shoot and leaf edges that curl under with color becoming red (black grape varieties) or yellow (white varieties) between the veins. Symptoms display from mid-to-late season and often seem more apparent in red than in white varieties. Not all infected vines show symptoms to the same degree, which may be somewhat dependent upon rootstock, variety, clone and other factors. The majority of infections in Cabernet Franc, Pinot Noir, and Chardonnay, for example, show classic symptoms; very few infections show obviously in Riesling; one or more rootstocks may be immune.
Most leaf roll viruses are introduced by humans propagating infected vine cuttings, but infected nursery stock has also shown to be a problem. Some of the nine virus types can also spread through vineyards by species of mealy bug, which can migrate on clothing, tools, equipment, and birds. Other insect species are also possible carriers.
The spread of LRV through many California vineyards seems to be accelerating in recent years, for any number of possible reasons, including increased traffic and mechanization in vineyards, low priority and attention to mealy bug eradication, and possible recent infection of stocks of plant material formerly thought to be clean. Chances are that improved testing for viruses that can now be completed in a matter of days, rather than weeks or months, has revealed more widespread infection than was previously thought to exist.
The sole method known to rid existing nursery stocks of virus infection is by "shoot tip culture". The growing shoot tip (meristem) has a distinct tendency to be virus-free, even in otherwise infected plants. A tiny bit of the shoot tip material is removed and cultured to re-grow new vines, but this process is both time-consuming and costly. Plants grown from this material continue to be quarantined and repeatedly tested for viral infections until certified free and clean.
Heat treating the vines can shorten the process somewhat by producing more culture material. Viruses become inactive or dramatically slowed in vines that are grown in temperature conditions of 100-105°F. The vine essentially "outgrows" the virus, so more of the the shoot tip is theoretically "safer". Some viticulturists suspect that heat treating causes genetic mutation and variation from the original plants, so extended testing to ensure genetic stability and quality, ironically, can add more time and expense to this treatment.
Rombaugh, Lon. Grape Grower: a guide to organic viticulture, 2002, (Chelsea Green Publishing: White River Junction, VT), Pp 105-109
Greenspan, Mark. "Leafroll Redux: The Problem is Spreading", Wine Business Monthly (March 15, 2009)
Greenspan, Mark. "Leafroll Virus: Spreading Like Wildfire?", Wine Business Monthly (March 15, 2008)
Carol, Brenda. "Grapevine leafroll virus increasingly a problem in California vineyards", Western Farm Press (July 10,2008)