A chance to fight Pierce's disease, one of the wine industry's most deadly foes, at its origins has come a step closer after scientists found common weeds nestling in vineyards were helping the sickness to spread.
Scientists in California found that 27 out of the 29 commonly found weeds they tested contained the bacteria Xylella fastidiosa that causes Pierce's disease.
Researchers examined previously untested plant species in California's San Joaquin Valley after a series of outbreaks of the deadly disease.
Those most frequently affected were Sunflower, cocklebur, annual bur-sage, morning glory, horseweed, sacred datura, poison hemlock and fava bean, according to the research published in this month's edition of Plant Disease magazine.
Pierce's disease, spread by leaf-hopping insects called sharpshooters, cripples grape vines by their blocking water passages, leaving them literally to die of thirst.
The most significant US losses occur in California's Napa and San Joaquin Valleys, where severe epidemics can require major replanting. It is seen by some as the most deadly disease faced by the wine industry, even more potent than phylloxera, which almost wiped out Europe's vineyards in the 19th Century.
The new research could open up new ways to tackle the disease at its root cause, before the bacteria has left the weeds.
"Currently, Pierce's disease is controlled by reducing populations of the insect vector, either through insecticide sprays or habitat modification to remove insect breeding host plants," said Christina Wistrom, a staff research associate at the University of California.
"Our study reinforces the need for weed control in irrigation ditches and roadsides adjacent to vineyards, in regions with chronic Pierce's disease and established populations of sharpshooters, especially in warm weather."
But, the scientists warned wineries that a delicate approach was needed to defeat the bacteria on weeds.
Wistrom said plants could not simply be identified as hosts or non-hosts of the Xylella fastidiosa as its presence varied with different species and different environmental conditions.
"Weed species in vineyards must be evaluated on an individual basis to determine their potential contribution to Pierce's disease," she said.
Pierce's disease, tends to be restricted to areas of North America where winters are milder.
California's Department of Food and Agriculture (CDFA) recently got approval to bring in Anagrus epos wasps from Minnesota to take down the disease-carrying sharpshooters.
The wasps, which have no sting and are described by the CDFA as sharpshooters' natural enemy, can survive winter weather and, when spring comes, seek out sharpshooter eggs and lay their own inside them. The emerging wasps then eat through the sharpshooter eggs on their way out.
"We have spent the past five years working to limit the spread of the glassy-winged sharpshooter," said Bob Wynn, state co-ordinator for the Pierce's Disease Program.
"This discovery [wasps] will aid our efforts to control this pest until researchers can arrive at a long-term solution to the problem of Pierce's disease."