News from the battle front...
Native grapevine
may be secret weapon
in vineyard war

© 1999 Streamline Publications

If you are a grape grower you know all about this assault team responsible for Pierce’s Disease. You know that your vines are under merciless attack and you may not be able to save them from this silent army and its airborne troopers. You also know that Pierce’s Disease is essentially immune to standard botanical cures.

But you also know that industry and university researchers are feverishly searching for new ways to combat this remarkable disease.

Wine growing regions such as Napa, Sonoma and Mendocino counties, where woods, lakes and streams are abundant, are especially susceptible. The disease is carried by an insect that thrives in these riparian areas. The blue-green sharpshooter—a leafhopper—carries Xylella fastidiosa bacterium, passing it among grapevines. Commercial varieties of table and wine grapes that are cultivars of European-originating Vitis vinifera are vulnerable to Xylella fastidiosa.

The bacterium infects the xylem of the vine—the tissues that carry water throughout the plant. The result is that nutrients and water are cut off and the grapevine starves and dies.

A Wily Foe

Control of the sharpshooter has proven to be problematic. Unlike other disease carriers, such as mosquitos, that can be dealt with using pesticide sprays and by denying them a breeding habitat, the sharpshooter has resisted traditional vector control efforts. Some insecticides showed promise in killing the sharpshooter but in doing so have also posed a danger to fish in rivers and streams common to the sharpshooter habitat. Grapevines have also been killed by the insecticide sprays meant to save them.

The cure involves finding a method that will nullify the disease in some nontraditional way—eliminating the bacterium threat itself, rather than the carrier.

The New Arsenal

Antibiotics have shown promise in killing the Pierce’s bacterium but difficulty injecting these drugs into the hard grapevine wood has limited their use.

Dr. Richard Peterson, owner of Folie a Deux Winery in the Napa Valley, has developed a procedure to deliver tetracycline to grapevines. In Peterson’s experiments a hollow plastic screw containing the drug is inserted into the trunk of the grapevine. Dr. Peterson’s technique overcomes the difficulty of delivering the drug into the very tough wood of the vine but would require labor-intensive individual treatment of virtually all vines subject to attack by the disease. Testing continues.

Meanwhile another investigation is examining how increased levels of nutrients and micronutrients in the vines might immunize Vitis vinifera vines against Pierce’s Disease. Field testing has begun on the best ways to inoculate the plants.

This approach is attractive since it avoids using antibiotics, but will likely still require some mechanical form of inoculation.

Chill Out

Researchers under Professor Alexander Purcell at the University of California Berkeley have found that if infected vines are pruned severely then frozen, the X. fastidiosa bacterium is sometimes killed. Investigation in the laboratory continues on considerations of how freezing causes the vines to react to the disease rather than how the cold temperatures affect the bacterium itself. The method—freezing individual vines—is not likely to be practical for grape growers to employ, particularly in the more temperate growing areas.

The technique, at this time, must be considered a laboratory tool that may reveal weaknesses in the bacterium and suggest other cures.

Home Remedy?

These proposed methods are innovative and may ultimately prove successful in the short term, but another possible cure addresses the problem permanently. University of California Davis professor Andy Walker is exploring Muscadine rotundifolia grapevines, a southeastern American native that is naturally resistant to X. fastidiosa.

Muscadine grapes are not known for quality wine potential or flavor. Their lack of desirability gave impetus in early years for table grape, raisin and wine growers to import the Vitis vinifera cultivars. Professor Walker is reportedly making progress in creating a genetic fix to the Pierce’s Disease problem by breeding the native plant’s disease resistance into the affected vines.

Using modern genetic screening techniques, it may be possible to bypass field hybridization testing—which could take 30 years—cutting the cross breeding project to 10 year’s duration.

The Moral of the Story

Everyone has heard the predictions of what loss of South American rain forest could mean—elimination of plant species that could add to the world’s pharmacy of drugs; the undiscovered cure for cancer or AIDS or the common cold. Reason enough to protect these many species native to the rain forest.

We face that same challenge in the United States as our own native species are crowded out by invasive exotics or paved over before their genetic and curative benefits are known. Muscadine grapes are not in danger of extinction but they serve as a powerful indicator of the potential value of native species.

We owe it to ourselves to be biological packrats, to preserve our ecology, because you never know when we might need to borrow a cupful of native genes.

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