Researchers have long known about grapevine leafroll disease, which has been present in California vineyards since early last century. But until the 1990s, field spread of leafroll was not a problem, and the disease could be controlled by planting virus tested, certified stock. It is not a coincidence that problems arose at the same time as when growers lost AXR rootstock due to phylloxera, and new detection technology was developed, says Deborah Golino, the director of Foundation Plant Services at the University of California-Davis. But in recent years the problem has become much worse, and Golino fears it may explode.
“I think this is going to be a really huge problem,” says Golino.
To understand why Golino thinks the disease may become such a scourge, it’s best to go back to the fall of 2002, when she and Napa County farm adviser Ed Weber were doing field work in Napa County. (Weber died on the last day of 2007, and a publication documenting this research has been dedicated to his memory.) The two were contacted by a viticulturist, who showed them a vineyard in the Oakville area which showed symptoms of the virus. Golino and Weber embarked on a five-year mapping study, which became the first documented spread of leafroll disease in the U.S.
In 2002, mapping showed that leafroll symptoms were present in 23% of the vines in the 7.2-acre vineyard. It spread rapidly through the years. By 2004, 46% of the vines suffered from leafroll disease. By that time, says Golino, the fruit from the healthy and diseased vines differed so greatly in fruit quality and ripening patterns that the vineyard had to be harvested twice, once for the high-quality healthy fruit, and once for the fruit from the virus-infected vines.
“Research shows that most leafroll isolates not only affect yield (by up to 40%), but reduce pigmentation and slow the production of sugar,” she says. “The fruit becomes flavorless, with poor sugar and color.”
The fruit from the diseased vines fell below standard, and could not be bottled as a more expensive reserve wine that the vineyard traditionally produced. Golino does not know how much the disease has cost the grower, but says that to this day the vineyard has to be picked twice. The grower is now faced with replanting the block, which is only 15 years old, as more than two-thirds of the vines exhibit symptoms of the disease.
The disease spread so quickly in the Napa vineyard that Golino believes something fundamental changed in the vineyard environment. Weber’s theory was that grape cuttings were smuggled into the state, bypassing quarantine, and that the cuttings carried a transmittable form of the virus that was new to California. Golino concurs with her late colleague’s theory, noting that several recent problems could be traced to smuggling, including plum pox in the Eastern U.S. and the vine mealybug, which was brought to California’s Coachella Valley on table grape cuttings from Israel. “One of the biggest threats to agriculture in the U.S. is the importation of exotic pests and diseases,” she says.
As with some other diseases, leafroll disease somewhat resembles a nutritional deficiency. Besides leafrolling, there is stunting, reduced vigor, etc. It’s more difficult to identify on white grape varieties, says Golino, because loss of color isn’t as apparent. It’s difficult to characterize in any case because the many strains of the virus differ in their effects, and different rootstocks and scions aren’t affected the same. Because of that, she cautions growers not to trust their eyes. They should either contact their local farm adviser or consultant or have the vines tested.
“Visual diagnosis can be tricky,” she says. “A laboratory diagnosis is needed, and even that can be difficult to do.”
Unfortunately, a confirmation of the disease doesn’t leave a grower with too many choices, says Golino. “Unlike a fungal or bacterial disease, there’s no treatment for a viral disease,” she says. “Once they’re infected, they stay infected.”
The only answer for growers lies in replanting, which means it’s crucial to make sure that only certified rootstock is used. If a grower uses his own scion wood, says Golino, that wood must be thoroughly tested to make sure the disease isn’t present. In addition, if a grower does replant, she recommends that the entire area be replanted, not just certain spots. She noted that one grower replanted with a new Cabernet Sauvignon block, but left about a dozen old Zinfandel vines with which he felt a sentimental attachment. That large Cabernet block, which had been certified clean, was soon infected.
That example points out how difficult it is going to be for growers to protect their vineyards, says Golino. Because the disease can spread so easily, it’s going to be critical for growers to keep an eye on neighboring vineyards.
“Make sure you have good neighbors,” she says with a rueful chuckle. “Things have changed, and growers will have to pay more attention.”