Olive groves in Spain and Italy provide some 70 percent of the world’s olive oil, but over the last few years, a deadly disease has infected thousands of olive trees, causing them to wither and die. The current outbreak of Xylella fastidiosa, or Xylella, one of the most dangerous plant bacteria, was first seen in 2013 in Puglia, the boot heel region of Italy that is home to some 60 million olive trees. This outbreak caused havoc. Thousands of trees have been uprooted in an attempt stop the spread of Xylella and create buffer zones between healthy and infected crops. Millions more have succumbed to the infestation. The disease, which affects fruit bearing trees, has since spread to both France and Spain. Wild and cultivated olive trees, vines, and almond trees are the main hosts.
The disease caused by the bacteria, which the Italian named Quick Olive Decline Syndrome, poses a major threat to both the Italian and Spanish economies. It has also led to confusion and panic amongst olive growers as they have watched their precious trees — some of which are hundreds of years old — die. It’s easy to understand why: Traditional family- growers were suddenly faced with an epidemic, one that scientists could not initially explain, a situation that seriously threatened their livelihoods.
Xylella strains can cause different symptoms in different species, but essentially the bacteria prevent infected plants from feeding. It is spread by sap-sucking vectors such as spittlebugs and leafhoppers, which attach to the plant’s xylem, the mechanism that transports water from a plant’s roots all the way to the leaves. When these plants become infected the water supply leading to the leaves gets cut off. The leaves then turn brown, followed by wilting of the foliage, and the plant eventually dies. There is no cure to date for this lethal disease, which can affect more than 300 species, ranging from oak and elm trees to citrus and almond trees. Strains of the bacteria have cropped up everywhere from California to Iran.
Though recent outbreaks are alarming, Xylella has been around for more than a century — it has been tied to diseased peach trees in the southern United States as early as the 1890s. One of the first reported outbreaks took place in Minas Gerais, a citrus growing region in southeast Brazil back in 1987. It started with a few infected orange trees, but the malady escalated, and by 1992 the disease caused by the bacteria — known as citrus variegated choruses — had affected two million citrus trees. The bacterial disease also decimated plants in Southern California, where the glassy- winged sharpshooter caused oleander leaf scorch, killing oleander shrubs. In 2015, Xylella began spreading to other European nations, appeared in Corsica as well as in mainland France. Germany was also affected: authorities reported cases of the disease in potted plants in 2016. More outbreaks involving wild and cultivated olive trees were reported in Spain that year.
Since the first 2013 outbreak in Puglia, health experts have set up international research programs to investigate the disease and to share their knowledge. A three-day scientific conference was held in Mallorca, Spain in November 2017 covering a variety of issues related to the pathogen including early detection and recent legislative measures. Sharing of knowledge amongst scientists is vital as Xylella fastidiosa diseases do not have the same epidiology, and as symptoms can take some time to present after infection.
“Dispersal of [Xylella] disease over an area takes at least a few years to get going to a noticeable incidence” says Alexander Purcell, a global expert on the history and etiology of Xylella fastidiosa. Though the recent outbreaks are the first confirmed incidences of the disease in Europe, Purcell believes that Xylella fastidiosa has been in the Spain’s Baleares Islands for at least 20 years. “Three genotypes (genetic strains) have been found. Given the few numbers of isolates of [Xylella] that have been sequenced so far, it is possible that even more genotypes are present. This means there have been multiple introductions, which is possible but unlikely to have been made over just a few years.”
The professor emeritus from the Department of Environmental Science, Policy and Management at the University of California, Berkeley, who has been studying the bacteria for 40 years, says that evidence that Xylella may have been around in Europe for so long provides some hope regarding the current outbreaks: “Now that we realize that Xylella has been present for at least a couple of decades in Corsica and Spain, there is hope that the spread of Xylella will not be as fast as first expected.”
Still, Purcell says that Xylella is on the rise globally, and that warming climates will increase the total area vulnerable to diseases caused by the bacteria, making research all the more urgent. He says the more scientists working on various aspects of Xylella research, the better chances are for significant improvement in treating infected plants. As he puts it, “Most scientific ideas are not successful. That’s why lots of different ideas have to be tested to find one or a few that succeed.”
Effective treatments would be more than welcome in Corsica, which has not yet faced the same level of devastation seen in Italy, but which has a deep agricultural history. Growers fear for their millennium-old olive oil culture and their olive tree plantations, some with trees that are more than 2,000 years old, as well as for impacts on the island’s ecosystem. “The culture of the olive tree in Corsica dates back to antiquity,” says Sandrine Marfisi, president of Corsica’s Olive Growers Federation. “It evolved with the different invasions of the island by the eastern peoples of the Mediterranean basin. It is an integral part of the biodiversity of the island: It participates in the fight against erosion, the maintenance of water resources, and the regulation of carbon dioxide with its forests. It is a firewall when the orchards are maintained; it makes it possible to fight against the desertification of the villages of the interior by creating jobs in the rural environment.”
There are certain advantages to living on an island when it comes to fighting the bacteria, including monitoring and restricting exactly what comes into Corsica. “We know that we have the Xylella strain Multiplex and maybe recombination of the disease, but we live on an island and can protect ourselves from new strains which come from outside,” Marfisi says. “Five percent of the plants sold on the island are home produced and all the rest comes from abroad,” she says, noting that some residents are pushing for a total ban on the importation of six plants that the EU deemed more likely to have Xylella. She believes that stricter importation would reduce the risk of introducing new strains of bacteria already detected in neighboring Italy and Spain.
Marfisi says growers regularly treat their orchards against the olive fruit fly, the principal pest affecting olive tree. Treating for the flies also diminishes the insect vectors responsible for transmitting Xylella. She pointed out, however, that it would be almost impossible to treat the olive trees in Corsica’s vast areas of remote shrub land (the maquis). The maquis covers about 20 percent of the island.
What’s more, organic farmers aren’t keen to spray pesticides — they don’t want to intentionally release toxic substances into the atmosphere and into their olive products. Many olive oil farms in Corsica strive to get organic certification. In Italy, some olive growers have been turning to more natural methods to improve the health of their trees, and hopefully decrease their vulnerability to the bacteria. These include pruning dry branches, fertilizing the soil with cow manure, and cleaning infected branches with copper sulphate. Others have been looking for resistant olive varieties. One of the best practices, Marfisi says, would be to reduce the plant cover in Corsican orchards. She explained, “less grass would reduce the habitat for the larvae of the insects, vectors of the disease.”
As scientists have not yet found a way to cure the diseased plant, the next-best option would be to develop an early detection method for Xylella right when it strikes, which would allow growers to deal with infected plants before the disease spreads. So far, such detection has proved challenging, as it can take up to a year for infected trees to exhibit symptoms. But, as The Guardian reports, a newly developed technique using “hyperspectral” cameras detects subtle changes in the color of olive leaves before other disease symptoms are apparent, and could offer an early warning system to government officials and farmers alike when crops are infected.