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California’s grape growers have a tough row to hoe. The root-sucking
phylloxera parasite is threatening their vines, and the public has reportedly
gone off wine. To cap it all, a tiny sap-sucking insect called the sharpshooter
has returned to spread disease among the vines.

Sharpshooters are leaf-hoppers that suck nutrients from the water-transport
systems of plants. The worst of them is the blue-green sharpshooter, Graphocephala
atropunctata. They are voracious feeders, guzzling hundreds of times their
body weight in plant fluid each day. As they feed, they inadvertently inoculate
the plants with a bacterium that lives in their gut. The bacterium, Xylella
fastidiosa, multiplies in the plant’s water conduits and clogs them. The
leaves turn brown and the plant eventually dies from dehydration.

This condition, known as Pierce’s disease, wiped out thousands of hectares
of grapes in southern California earlier this century and prevents wine
growers from moving into the southeast of the US. The Napa valley north
of San Francisco, California’s prime grape growing region, has always suffered
from small outbreaks of Pierce’s disease. But it is generally confined
to the edges of vineyards, next to rivers or areas of natural scrub or
woodland harbouring plants that are host to the sharpshooter.

For the past two years, however, the disease has been on the rise in
Napa county, and is cropping up in previously trouble-free areas. This may
be because recent wet springs, and the resulting lush vegetation, have produced
a population boom of sharpshooters. But it might also be an unforeseen consequence
of the epidemic of phylloxera that has swept through California. Terrified
of losing their livelihood to this root-sucking insect, growers are ripping
up their vines and planting new stocks that are resistant to it. Unfortunately,
young vines are particularly susceptible to Pierce’s disease.

It is even possible that a new strain of Xylella fastidiosa may have
arisen, according to Alexander Purcell, an entomologist at the University
of California at Berkeley. Cold weather is probably critical in killing
off the bacterium each winter, says Purcell. ‘If you got a strain that would
survive the winter better you’d be in trouble.’

Such changes do happen. A similar disease, also caused by Xylella,
arose suddenly among citrus trees in Brazil, says Purcell. In 1987, farmers
noticed three trees with blotchy leaves; today more than a million are infected.
‘That was definitely a new strain,’ he says.

If a region is prone to Pierce’s disease, there is little to be done,
short of continuously replanting the infected parts of the vineyard or clearing
away nearby vegetation. All attempts to breed resistant varieties of wine
grape have failed so far.

For now, while phylloxera threatens all the Napa valley’s vineyards,
Pierce’s disease has hit only 5 per cent of the vines. Experts suggest
that the sharpshooter will never do as much damage as phylloxera, although
they cannot be certain. ‘The disease has been present in Napa county for
a very long time, and historically it has had these cycles of severity,’
says Ed Weber, a farm adviser with the University of California. ‘My sense
is that in a year or two we will see the incidence taper off once again.’