Plant viruses that have plagued wineries in Napa and Sonoma counties the last two years have started threatening some table and raisin grape vineyards in the Central Valley, the U.S. Department of Agriculture reported Thursday.
Although the diseases so far have affected a tiny percentage of the region's grape acreage, they potentially could devastate vast areas unless growers take precautions, the agency said.
"This is a serious problem that could affect them very directly," said Deborah A. Golino, an authority on grape diseases with the USDA's Agricultural Research Service in Davis, Calif.
Grapes are a $1.6-billion industry in California, which has about 685,000 acres planted in wine, table and raisin grapes.
Golino, who first diagnosed the virus problem two years ago in the premium-wine region of Napa and Sonoma, said the problems can be avoided.
Table and raisin grape growers who are replacing aging vineyards need to use only new plantings--known as root stocks--certified to be virus-free and ensure any buds grafted onto those root stocks are also tested for disease, she said.
This could prevent the need to rip out infected vineyards and start again, at enormous cost in cash and lost production. Replanting an acre of table or raisin grape vines costs $6,000 to $7,000 and two or three years of lost harvest.
The viruses that just started showing up in the Central Valley first cropped up in Napa and Sonoma in the fall of 1991 after wineries there replanted thousands of acres infested with a root-eating pest known as phylloxera (pronounced fill-OX-era.)
Some of the new root stocks proved to be susceptible to viruses that stunted the vines' growth, curled their leaves and, in extreme cases, killed them. Viticulturists who spotted these problems in newly replanted vineyards called them to Golino's attention.
The viruses in question cause grooves or discoloration at the point where grafting is done. They do not cause ill effects in human beings who consume wine or grapes produced from infected vines, Golino said.
By spreading the word early in Napa and Sonoma, Golino said, farm officials were able to avert what could have been an economic disaster.
The situation is potentially much worse in the Central Valley, she added. Last summer, John Coelho, a Fresno County grower, was forced to tear out 40 acres of a type of grape known as Zante currant after the vines became infected. He said he lost $50,000 to $75,000.
Although the viruses have not been nearly as severe or widespread as the phylloxera problem in Napa and Sonoma, the outbreak nonetheless has set beleaguered growers and vintners there on edge.
"People are nervous," said Daniel Roberts, viticulturist at Sterling Vineyards in Calistoga. "People are having a hard enough time getting loans."
To keep the viruses at bay, Sterling is having vines tested for viruses at $20 to $30 per vine before it uses them for grafting. For its massive phylloxera replanting effort, the Robert Mondavi Winery used primarily new vines from nurseries that had been certified as virus-free.
At this point, the virus problem is "a very low risk" for table and raisin growers, said Jim Wolpert, extension viticulturist for UC Davis. But, he acknowledged, "we're kind of in a profound state of ignorance right now."