Once on the decline in California vineyards, grapevine leafroll disease is staging a comeback. The disease typically cuts yields 10 percent to 20 percent and fruit from infected vines is low in sugar, poorly colored and late ripening.

The disease is associated with at least nine different plant viruses that are transmitted to healthy grapevines by grafting or by several different mealybugs and soft scale insects.

Until nine years ago, the incidence of leafroll virus had been declining for half a century, reflecting the use of clean certified grapevine nursery stock. But in 2000, several growers in the Napa Valley reported seeing an increase of infected vines.

Deborah Golino, Foundation Plant Services director and Cooperative Extension plant pathology specialist at UC Davis, and her colleagues are learning more about the disease as they seek to stop its spread.

That work began when they identified a block in a Napa Valley vineyard that was infected with grapevine leafroll associated virus-3 (GLRaV-3). It is one of the most common viruses associated with the disease and is transmitted by five known mealybug species.

When researchers began mapping spread of leafroll disease in 2002, it had infected 22 percent of the Napa Valley block and was confined to the edges next to an older vineyard that had been removed due to severe leafroll disease.

By 2006, incidence of the disease had increased to 66 percent of the block, moving mostly down rows. Since then, growers have reported a similar spread of the disease in several other California vineyards.

Even though grape mealybugs were in the infected block, the current rate at which the disease is spreading seems unusually high, Golino says. She offers several possible reasons:

• A new insect vector may be transmitting the virus more efficiently.

• A new virus or strain of virus may be present and is more readily transmitted by grape mealybugs.

• Growers may be using more susceptible rootstocks.

• Alternate hosts such as wild Vitis or non-Vitis species may be serving as virus reservoirs.

In a two-year trial, Golino and colleagues found some rootstocks are more susceptible to virus infection than others. They grafted commonly used rootstocks onto Cabernet Sauvignon vines that were infected with one or more viruses. Regardless of rootstock, Brix levels were significantly lower in all the virus-infected vines except those with rupestris stem-pitting associated virus (RSPaV).

In contrast, the effect on yield depended largely on the combination of rootstock and virus. The lowest yields occurred on vines infected with multiple viruses and grafted rootstock Kober 5BB rootstock. Although St. George was not the highest yielding rootstock, it was less affected by any of the viruses.

The situation calls for good management practices, says Bob Martin, plant pathologist with the USDA’s Agricultural Research Service, Corvallis, Ore. His advice:

• Start with clean rootstock and scions and scout for mealybugs. Because disease symptoms vary widely between different grape cultivars and different rootstocks, have plants and each scion/rootstock combination tested for presence of the virus.

• Keep good records of planting, scouting, testing, and treatments.

• Remove infected vines if disease incidence is low and vectors are absent or under control. If not, replanting is probably the best option.

More information about the disease is available at UC Integrated Viticulture Online — www.iv.ucdavis.edu Click Videotaped Seminars and Events and select Grapevine Leafroll Symposium 2.0 and Grapevine Leafroll Disease: An Increasing Problem for California Vineyards.