Early results in a 5-year study suggest that managing riverbank vegetation near vineyards may be a potent control of Pierce's disease, a bacterial scourge of grapevines now ravaging some prized vineyards in Napa and Sonoma counties.

Transmitted by the blue-green sharpshooter, the bacteria (Xylelle fastidiosa) multiply in the plant xylem, blocking this vital water conducting system and causing leaves and fruit clusters to dry up and die, often within 2 years. There are no effective treatments or resistant rootstocks.

The sharpshooter overwinters and breeds in riparian (river bank) areas; it transmits the bacteria to grapevines from a wide array of wild plants, all symptomless carriers. UC Berkeley entomologist Alexander Purcell and forest ecologist Joe McBride are running experiments in Napa Valley to see if they can reduce sharpshooter by changing riparian habitat.

“The early results are very encouraging: we have seen a 70% to 99% reduction in insect activity,” says Purcell, who is an international authority on Pierce's disease. “We have removed the main breeding hosts of the insect such as blackberry, elder-berry and wild grape, and replaced them with native riparian species that are not breeding hosts — such as California bay, black walnut and ash. In most cases we are increasing plant diversity.”

The investigators have planted saplings in two locations and recorded sharpshooter activity for 1 and 2 years; a third location was added this year.

“The results are still preliminary,” Purcell cautions. “Before we could recommend this as a control method, we would need to determine if reductions can be maintained over several years as the planted trees mature, and if there will be any adverse effects on wildlife and stream ecology. Wild-life effects are now under study by UC Berkeley entomologist Don Dahlsten.”

While the removal and replanting of riparian vegetation is expensive, it is less costly than Pierce's disease losses in many North Coast vineyards near riparian habitats, says Purcell.

“A UC Berkeley study of the economics of Pierce's disease showed that growers were losing money trying to grow grapes within 150 to 200 feet of riparian woodlands,” Purcell notes. “This raises the possibility that buffer planting of other crops might be used to protect grapes in extremely high risk areas.”

In a related study by Purcell, McBride and UC Davis scientists Bruce Kirkpatrick and Roy Sachs, conifers and hardwoods are being planted to create 20-foot-wide buffers between the riparian habitat and vineyards. Earlier studies in the Santa Cruz Mountains showed sharp-shooters didn't feed in or penetrate conifer stands.

Pierce's disease has damaged the state's vineyards since 1880, decimating Southern California vineyards in the 1880s, and Central Valley vineyards in the 1980s. In more recent decades it has been a “quiet” disease, causing occasional outbreaks across the state. However, since 1991 its incidence has been rising rapidly in the North Bay counties. It has recently spread at an epidemic rate in Napa Valley and has appeared for the first time in Mendocino County and parts of Sonoma County.

“Conservatively, it took a $24 million toll in Napa County in 1996, based on a grower survey by the Pierce's Disease Task Force,” says Purcell. The task force was organized by the wine industry and provides research funding and educational efforts.

“Pierce's disease has always occurred at hot spots close to riparian areas,” says Ed Weber, viticulture farm advisor in Napa County. “The news is that we are seeing the disease striking farther and farther away from riparian areas. Usually the disease goes through epidemic cycles, increasing for a few years then returning to normal levels. In Napa, we've been waiting for 6 years for Pierce's disease to return to normal levels, but it seems to just keep getting worse.”

Why is the incidence of Pierce's disease on the rise? Weber speculates there are five possible reasons: more vineyards, planted to meet rising demand for quality wines; more habitat for sharpshooters, such as ornamental landscapes; inadvertent spread of infected insects by workers or managers; reduced insecticide use, and more young vines, which are more susceptible to the disease. Napa growers have recently replanted many vineyards on rootstock resistant to phylloxera, a vine-destroying insect that also posed grave threats to North Coast vineyards.

Napa's recent warm winters may be contributing to the epidemic. The disease does not occur in areas with cold winter climates.

Current control methods include pesticide spraying of riparian vegetation bordering vineyards, and the vineyards themselves. Dimethoate insecticide can only be applied with a special permit. Imidacloprid is registered for use on grapes but is not specifically approved for use against Pierce's disease vectors. —Ed.