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Natural Born Killers: Pierce’s disease

California's sunny climate makes it one of the best places to make wine, but the huge industry is currently under threat from a small insect and the deadly Pierce's disease it carries. Viticulturalist Dr Richard Smart reports.

Winemakers and grape growers in California are worried. The cause of their concern is an apparently insignificant insect called the glassy-winged sharpshooter (GWSS), which was accidentally introduced to the region from the south-eastern United States. This pest has the charming habit of sucking sap from grape vines, but it’s not the sap loss which is causing vintners to lose sleep – it’s the fact that the insect spreads the deadly Pierce’s Disease (PD), arguably the most feared disease of the grape vine world.

Named after pioneer researcher Newton Pierce who studied an outbreak of the disease in Southern California in the 1880s, PD is caused by a bacterium called Xylella Fastidiosa, which spreads through the vine and kills it within two years of infection. It is highly prevalent throughout the south-eastern United States, in a zone extending from North Carolina down to Florida and into Texas, and is already present in Southern California and in isolated pockets in the North Coast vineyards of the Napa and Sonoma Valleys.

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The insect

So far, only limited areas of vines have been affected and these are located near creeks, because the insect that has traditionally spread PD, the blue-green sharpshooter, lives in creek-side vegetation. The blue-green sharpshooter is a small insect, just a millimetre long, and can fly only a limited distance – 20 metres or so – which means vine damage is limited to a strip along the creek edges. But its close relative, the GWSS, is a different matter entirely. At over a centimetre long, this insect can fly further, is quicker and could be described as the B-52 of PD carriers. It will attack vine canes even when dormant and feeds on a much larger number of plants, including oak, elm, ash, eucalyptus and apricot. In the last few years, vineyards south of Los Angeles, around Temecula, have been destroyed and the GWSS is travelling around the state, being carried on ornamental plants which are grown in infected areas. All grape-growing counties in California are judged to be at risk from the spread of the GWSS and it is now present in 12 counties, including Butte, north of Napa Valley.

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Great concern over Pierce’s Disease

The wine industry is not alone in its concerns about Pierce’s Disease, and both the federal and state governments have responded quickly, the former providing $22 million dollars and the latter $15 million for the fight. Much of this money has been used to try and prevent the spread of the GWSS, but the hope is that a cure can be found for the disease and money has also been allocated to research.

Tom Shelton, CEO and president of Joseph Phelps Winery in the Napa Valley, and a former president of the Napa Valley Vintners Association, says the problem is ‘potentially devastating’ for the industry and the North Coast in particular, since the disease already has a foothold here.

The authorities are making every effort to stop the introduction of the GWSS to Napa and other counties, including inspecting ornamental nursery shipments, but there is no guarantee this will be successful in the long term and Shelton says that the industry must instigate a public relations campaign to warn locals about the impact of the disease. Bob Steinhauer is another industry leader who is worried about the spread of PD. He is senior vice-president, vineyard operations, with Beringer, a Napa-based company with vineyards there and in the southern Santa Maria region, and he is optimistic that a solution to PD will be found. However, he is the first to admit that he doesn’t know where it will come from, although proposals to raise industry funding are due to go before the state legislature in 2001.

Is there a solution?

Dr Terry Lee is vice-president, research and technical services, for the E & J Gallo winery, and a member of several committees set up to study the problem, as well as the California secretary of Agriculture’s Task Force. He suggests that some industry personnel ‘have their heads in the sand’ over the issue, but the ‘serious’ viticulturalists are very concerned. At a forum he attended at the University of California, Davis, late last year, research strategies and the problems at Temecula were discussed, including the reduction in sprays on citrus plantings which may have contributed to the grape problem. Although local community pressure may preclude widespread use of insecticides, with increased citrus spraying in 2000 the number of GWSS in nearby vineyards did fall dramatically.

It is clear that scientists in California are facing substantial pressure to come up with a solution, but their task is not straightforward. After all, the disease has been studied for more than 100 years, but no simple solution such as grafting to rootstock has been found. There are some grape vines in the East which are tolerant to PD, but in the main the wine quality is not to the same standard as for European vines. Professor Andy Walker of the Department of Oenology and Viticulture, University of California, Davis, is one of the scientists involved in the research. He believes that the only hope for a permanent cure lies in the genes of the vines from the Eastern states, but he thinks that conventional plant breeding offers little cause for optimism because a new variety with a new name would be created. Genetic engineering incorporating new genes into existing varieties could provide an answer, but there is no guarantee that existing names would be retained and consumer resistance to wines made from GM grape vines is likely. In any event, with 10 to 20 years’ work involved, this would not be a quick solution.

This situation is paradoxically similar to what happened in Europe with phylloxera some 120 years ago. This pest was introduced into Europe from the Eastern USA on nursery plants. Vineyards in the south of France were decimated and for a while there was debate among scientists as to the cause. There was even greater debate among the vine growers as to the best solution, be it flooding vineyards in winter, injecting carbon bisulphide or using native American vines as rootstocks.

But what is the future of California’s wine industry? Right now, no one knows, but there is definitely an undercurrent of concern that the GWSS may spread throughout the grape-growing regions of the state, bringing Pierce’s Disease with it. According to Dr Sandy Purcell of Berkeley University, the big unknown is the extent to which the GWSS will establish itself in natural vegetation and from there move into vineyards. It is all too easy to be pessimistic about problems such as Pierce’s Disease, but while the future might appear grim, it is important to realise that the disease is limited to only a few regions, and its incidence fluctuates from year to year. Andy Walker says that in places where the blue-green sharpshooter is behind its spread, PD is now declining.

And what do other countries think? In Australia the local grape industry has acted to restrict the importation of California table grapes for fear of introducing PD, and many other countries will be considering how effective their quarantine measures are. Perhaps grape growers who are accustomed to smuggling cuttings (so-called ‘samsonite varieties’) will think twice before taking vines from California. And some growers in Chile, Argentina and Australia have been heard to wonder aloud about the prospects for growth of their own vineyards should Californian production decline. Mother Nature is constantly making the winemakers’ life a misery, but what is unfolding in California right now has the potential to be one of the largest disasters ever to affect agriculture in world history. For the moment, though, PD has been contained, government and industry are joining forces to tackle the problem as effectively as possible and it is too premature to write California off. Let us hope, for the sake of our future enjoyment of Californian fine wines, that this gloomy scenario does not transpire.

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