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Published:  23 July, 2008

California's wine industry is facing one of the worst crises in its history, in the form of Pierce's Disease. Monty Waldin reports on the progress of this vine-killer, and the growing opposition to potential control measures

The foundations for the California wine industry were laid only 20 miles from Temecula when, during the late 18th century, the first significant Mission vineyards were planted at San Juan Capistrano. Today Temecula, an American Viticultural Area (AVA) in Riverside County with over 15 wineries and 1,214 hectares (ha) of vineyards, has become a very public host to the Glassy Winged Sharpshooter, an insect which could destroy California's wine industry within a decade. Dr Matthew Blua, an entomologist at the University of California, Riverside has conducted extensive field work into the spread of Pierce's Disease in Temecula. UC Riverside is located fortuitously close to Temecula, considering its entomology department is the largest in the state, and that it contains one of only three research departments in California with specialist knowledge of Pierce's Disease. Blua says that finding a "control" - or non-infected - vine in the Temecula AVA is "hard, if not impossible". He points out that this is as much to do with the existing level of Xylella fastidiosa infection there as with the reluctance of some local producers to allow researchers like him into vineyards to monitor control plots, for fear of scaring shareholders if the findings are unwelcome. Efforts at tracking the disease in the table and wine grape vineyards in the southern part of Kern County, the area which looks sure to become the next Pierce's "hot spot", have also been hampered by a siege mentality among growers. Blua stands by a comment printed by the Wine Spectator last year, in which he said that what he had seen in Temecula was as close to a "worst case scenario" as the local wine industry could imagine. The Temecula Valley Winegrowers Association (TVWA) website ( states that its members "produce some of the finest wines and winegrapes in the world", but local industry figures were unwilling to discuss Pierce's during a visit I made to Temecula in April. My impression was of a local industry keen to show that it was business as usual, not least because income from wine tourism generated by winery restaurants, gift shops and the renting out of wedding gazebos is so fundamental to an AVA within such easy reach of both Los Angeles and San Diego. Yet behind the scenes the wine industry is searching for local, state and federal funds to offset the cost of replanting an entire AVA, estimated at $30-50 million.

The culprit The insect primarily responsible for the current spread of Pierce's is the Glassy Winged Sharpshooter. Brown, with transparent wings and half an inch in length, it feeds on the sap of vines, causing only minor physical damage. Physiologically, however, its effect is devastating, as the insect carries Xylella fastidiosa, a bacterium that shuts down the xylem - or water-conducting tissues - causing vines to die from lack of nutrients and water within two to three years. Affected vines show symptoms from mid-summer onwards, usually a drying or scorching of the leaves. Pierce's Disease is not new to California, for it destroyed 14,000ha of vineyard around Anaheim, in Southern California, in the 1880s. Further outbreaks occurred in the 1930s and 1940s. Pierce's Disease was known as Anaheim, Mysterious or California Vine Disease until 1892, when it was named after Newton B Pierce, the first professionally trained plant pathologist to work in California. So why is a well-documented and only sporadically important disease causing talk in the California wine industry of "meltdown"? After all, the state has 303,000ha of wine grape vineyards and Temecula's share of that is just 1,214ha, or less than half a per cent. The answer lies partly in the fact that Pierce's has no cure, and that its current major vector is the Glassy Winged Sharpshooter. Previous outbreaks of Pierce's were spread by another sharpshooter leaf hopper (Cicadellidae, sub-family Tettigellinae), the Blue-Green Sharpshooter. This insect, which is native to California, has caused minor infestations of Pierce's Disease in Northern California wine counties such as Napa, Sonoma and Mendocino for as long as vines have been planted there. But its population levels are necessarily restricted because its favoured habitat is riparian (riverbank-based) and it spreads Pierce's Disease much more slowly than the Glassy Winged Sharpshooter, partly because it is capable of flying only comparatively short distances in a lifetime (less than 100 metres), but mainly because of how it feeds. The Blue-Green Sharpshooter feeds only on the tender, green parts of the vine furthest from the trunk. And because the Xylella fastidiosa bacterium travels only slowly within plant tissue, infected parts can thus be pruned out in winter, making the vine disease-free again for the following growing season, and beyond. In complete contrast, the Glassy Winged Sharpshooter is not native to California but to the southern states of America. It was carried north and west on ornamental plants supplied to nurseries for homes, gardens and landscaping. It is thus an "exotic" pest, with no indigenous natural predators to check its population growth. The Glassy Winged Sharpshooter produces two population generations a season, whereas the Blue-Green Sharpshooter produces one. Also, the Glassy Winged Sharpshooter has a wider range of host plants than the Blue-Green Sharpshooter, and is a comparatively more aggressive flier, travelling up to 600 metres in a lifetime. This means that whereas Pierce's Disease infection caused by the Blue-Green Sharpshooter appears solely around the outer rows of vineyards, the so-called "edge effect", damage from the Glassy Winged Sharpshooter penetrates the heart of a vine plot. And most significantly of all, the Glassy Winged Sharpshooter usually feeds on tissue much closer to the tougher, woodier parts of the vine (rather than merely on tender vine leaves) where it can inject the Xylella fastidiosa bacterium directly into the vine's xylem, infecting it permanently. So, whereas the Blue-Green Sharpshooter spreads Pierce's from riparian habitat to vineyard, with vineyard infection easily controllable through pruning in a "primary spread", the spread from Glassy Winged Sharpshooters occurs from vine to vine within the vineyard in a "secondary spread". Woodier vine tissue becomes an inoculum to "clean" sharpshooters who, after feeding, then spread the disease rapidly. Diseased vines should be removed from a vineyard immediately, so it was a shock to see certain Pierce's-infected vineyards in Temecula last April being made ready for cropping later in the year. Growers appeared to be making a bit of extra money in the short term by cropping the vines one final time before they died, even though keeping infected vines would endanger the health of neighbouring vines.

Spreading its wings According to the California Farm Bureau, the Glassy Winged Sharpshooter was first collected near Irvine in 1989, but was not recognised as a newly-introduced species until later. From 1990 to 1994, the Glassy Winged Sharpshooter was mistaken for a similar California species, the Smoketree Sharpshooter, which also transmits Pierce's, and which is also a minor contributor to the spread of the disease in Temecula. The Glassy Winged Sharpshooter was first discovered in Temecula in 1996 and has now affected 11 counties within California's blanket South Coast and Central Coast AVAs: Fresno, Kern, Los Angeles, Orange, Riverside, Sacramento, San Bernardino, San Diego, Santa Barbara, Tulare and Ventura. Sharpshooter adults and/or eggs have been discovered in several other counties; some insects have even been detected as far north as Oregon. It would appear the pest gained a foothold in Temecula partly because the local climate is not dissimilar to that of the Glassy Winged Sharpshooter's native southeastern US, and because of the presence of extensive citrus plantations, which act as a breeding habitat. Sightings of the Glassy Winged Sharpshooter in areas where California's blue-chip vineyards are concentrated, namely the North Coast AVAs of Napa and Sonoma, have so far been restricted to larvae on ornamental plants shipped to nurseries. The level of Pierce's Disease within vineyards there has remained steady for a generation, and has been the work of the Blue-Green Sharpshooter alone. Infection appears confined to vineyards near rivers and streams and some mountain vineyards, such as Spring Mountain. However, Matthew Blua believes the Glassy Winged Sharpshooter could breed in Napa, despite its relatively cool climate.

Countermeasures and legislation In March 1999, state legislators acted to provide emergency funds to fight the pest. Bill number SB 671, drafted originally by state senator Chesbro to cover advertising restrictions on alcoholic beverages, metamorphosed into a $14 million aid package. Federal funds of more than $22 million were announced on 23 June 2000 by the then US vice president Al Gore and agriculture secretary Dan Glickman to control the Glassy Winged Sharpshooter in the short term, and to research permanent control measures in the long term. These funds were not designated to help the wine industry alone, for it is not yet the hardest-hit industry in the state - this dubious distinction falls to the nursery industry. But whereas quarantine restrictions have paralysed interstate nursery shipments, wine grapes from South Coast AVAs can still be moved to wineries within North Coast AVAs come crush time. State and federal funding has allowed the University of California, Davis to establish The University of California's Pierce's Disease and Emergency Response Task Force. Its first report, published in April 2000, identified six priority research areas, including: establishing and monitoring an information database; controlling the Blue-Green and the Glassy Winged Sharpshooter, and Pierce's Disease; establishing plant resistance to Pierce's Disease; and studying the movement, multiplication and pathology of Xylella fastidiosa. The report concluded: "It should be noted that the Task Force members are convinced that breeding resistance to the Xylella fastidiosa bacterium into plants will provide the only long-term protection [from Pierce's Disease] for grapes and other commodities. This will require time, fiscal resources and the application of genetic engineering and other biotechnology techniques." Pierce's Disease, as spread by the Glassy Winged Sharpshooter, could thus have created the apocalyptic scenario the wine industry needs to convince both Luddite wine producers and sceptical consumers of the need for genetically modified (GM) vines. However, those involved in GM vine research, including world-renowned geneticist Professor Carole Meredith of UC Davis, consistently maintain that a GM vine resistant to Xylella fastidiosa is at least 20 years away from commercial production. Current research at UC Riverside and UC Davis is focusing on identifying whether anything can be done to prevent the Glassy Winged Sharpshooter jumping from citrus fruits into neighbouring vineyards, possibly by locating a weak link in its dispersion cycle, in order to slow its spread. Winegrowers who planted vines in at-risk areas close to riparian habitats, and who are practising habitat rather than vineyard removal in an effort to halt the Glassy Winged Sharpshooter, have angered many local people. Wendy Blankenheim of the Mendocino Environmental Center asked in an article entitled Pierce's Disease - Plague or Paranoia whether "removing native riparian vegetation to slow a native insect from spreading a native bacterium to their non-native cash crops" could ever be justified, especially when financed by emergency tax funds.

Reaction to the proposals If the pest can be prevented from feeding, the spread of Pierce's Disease can be contained. Treatment with systemic insecticides like Admire, which is spread into the soil via irrigation water, stops the Glassy Winged Sharpshooter from feeding by affecting its mouth once it latches onto a vine (it takes several hours of feeding for the Xylella fastidiosa to penetrate the vine xylem). County agricultural commissioners have been authorised by the State to enforce the spraying of other chemicals, known to be neurotoxic to humans, under emergency legislation calling for the "mandatory abatement" of the sharpshooter. Action groups such as the Pesticide Action Network North America (PANNA) have worked with local groups to win opt-outs in Santa Cruz County, where the county has agreed that "no pesticide applications will be made on private property without the consent of the resident". A partial opt-out is on the cards in Sonoma, where local residents pointed out to winegrowers that although it was their civic duty under "mandatory abatement" to report sightings of sharpshooters, they would not comply if it would mean their home would then be sprayed with chemicals. In other words: "our health is more important than your vineyards". Organic groups, such as California Certified Organic Farmers (CCOF), the world's biggest third party organic inspector, has nearly a hundred vineyard members and has argued that mandatory abatement of the Glassy Winged Sharpshooter by the use of chemical pesticides infringes farmers' rights under the US Constitution to farm organically. CCOF says that any trace of chemical pesticides used by county agricultural commissioners to counter the pest which are compulsorily used on an organic vineyard, or blown onto one, invalidate the organic status of the vines. CCOF maintains that the Organic Materials Review Institute (OMRI) lists several plant-based treatments known to counter sharpshooters, and says that these should be part of any "mandatory abatement" policy, not known neurotoxic chemical materials whose success rate, it claims, is only around 60% anyway. Sharpshooters favour the underside of vine canopies and so are hard to hit with any kind of spray, organically approved or otherwise. Other potential control measures under review which are also acceptable to local residents and the organic community include the introduction of a natural predator of the Glassy Winged Sharpshooter, in the form of a tiny, non-stinging parasitic wasp, Gonatocerus triguttatus, imported from Mexico and bred at UC Riverside. The wasp lays its eggs inside those of the larger sharpshooter, preventing it from breeding. However, no one can say for sure whether the wasp may have an adverse effect on other agricultural crops grown in California in the future. UC Riverside researchers are also investigating the efficacy of natural barriers (habitat breaks) and insecticide-treated trap crops (designed to reduce the number of Sharpshooters that enter from adjacent vineyards). The Catholic missionaries brought to California the vineyards that have become an enduring symbol of a desirable, healthy and profitable lifestyle. But with the Mission vines came European diseases that decimated the indigenous human population. Now, in a reversal of fortune, the Glassy Winged Sharpshooter is becoming American nature's method of repelling a European invader, Vitis vinifera, whose very presence and viability in California have never been under such scrutiny.