French scientists have found the bacterium Xylella fastidiosa, the source of Pierce’s disease in grapevines, on the island of Corsica. The discovery is raising fears of a Mediterranean epidemic, coming less than two years after another strain of the pathogen was found in diseased olive trees in southern Italy.
On the morning of July 20, experts from FREDON, the French plant protection network, took samples from myrtle bushes with brown and dried-out leaves in the Corsican seaside community of Propriano. Experts had already sent 160 plant samples for analysis this year in mainland France, but all had returned negative. This felt urgent, however, they said. Samples were flown to Angers that afternoon, and within two days tests came back positive for xylella, setting in motion emergency measures to contain and exterminate the bacterium.
“This pathogen is known across the Atlantic as the agent of Pierce’s disease, which struck California vineyards hard in the 1990s,” said Stephane Le Foll, French minister of Agriculture and Forestry, when the results were announced. “The steps for eradication started today.”
The French government has a dedicated xylella hotline for a good reason: The bacterium poses a threat to more than 200 types of plants, including grapevines and citrus, olive, peach, almond, avocado, maple and oak trees, as well as coffee plants and rosemary. The bacterium spreads easily and quickly, entering the plants with the help of insects, and can move across oceans on infected plants.
Different subspecies of xylella attack different plants, travel on different insects and cause different diseases, none of them good. Government agents are particularly worried about the wine trade. The strain that causes Pierce’s disease began to spread rapidly through California vineyards in 1996, thanks to the arrival of the glassy-winged sharpshooter, a non-local insect.
In the nearly two decades since, scientists and growers have worked hard to contain the insect and the disease, a very expensive fight. Infected vines are essentially choked—a gel forms in their xylem, preventing water from reaching their leaves.
Xylella was first detected in Europe in October 2013 in Italy’s Puglia region, where it was attacking olive trees. Since then more than 600,000 acres of olive groves have been cordoned off, and millions of trees have been killed by a wasting disease dubbed Olive Quick Decline Syndrome.
For winemakers, it’s important to remember that the strain of xylella that caused Pierce’s disease in California is not the same strain found in Italy. Scientists at U.C. Berkley have said that the strain in Italy originated from contaminated coffee plants imported from Costa Rica. So far, no signs of the disease have appeared in vineyards, even those directly adjacent to contaminated olive orchards.
The strain in Corsica is different from that in Italy, however. As Le Foll explained, “The bacteria present in the samples from the Propriano region belong to the subspecies multiplex, totally different than the subspecies pauca identified in Italy.” A Corsican government spokesperson told Wine Spectator that experts are currently determining the plants vulnerable to the multiplex strain. They have not yet identified the insect that spreads it.
So far three populations of xylella, all on myrtle bushes, have been identified on Corsica in various areas. All of the bushes come from Italian suppliers for a plant nursery in Tuscany. The French have stepped up inspections of plants brought to the island.
But now that xylella has arrived, it must be dealt with. Dr. Marcello Nicoletti in Rome, who has been studying ways to combat and eradicate the bacterium, says the French need to take a multipronged approach. “The only way to face the xylella challenge is integrated pest management, tackling all the elements at once,” said Nicoletti. He advised soil treatment to sustain the plants, an insecticide to kill the bugs, an anti-fungal treatment and natural, low-cost antibiotics to nurse the trees back to health.
In Corsica, a dedicated team for fighting plant epidemics has arrived. European protocol requires the burning of the contaminated plants. The zone around the population of xylella will be sprayed with the insecticide Deltamethrin.
The French fear that if measures to eradicate the bacteria are not taken soon enough, there will be no getting rid of xylella. “It won’t be a matter any more of eradicating it, but of managing the economic and environmental impact on production,” said Le Foll.