Last summer Italian officials destroyed dozens of groves of prized and economically vital olive trees with chainsaws in an attempt to stop the spread of Xylella fastidiosa, a bacterium that has been blamed for the death of up to a million olive trees in the nation’s southern region. But new studies released recently reveal that some of the destruction could have been avoided. One study shows that certain varieties that were felled are resistant to the bacterium even when they are near infected trees. Another specifies how the disease kills the trees, indicating that many might have been spared by extensive pruning or netting to protect them from insects that carry the bacterium. Had authorities been armed with this information last year, hundreds of trees might have been saved. Instead, angry olive growers in the Puglia region are still battling the government and the European Union over what to do about Xylella.
A landmark study released by the European Food Safety Authority (EFSA) on March 22 has identified just how Xylella kills trees, which was previously under great speculation. Because the ongoing Puglia outbreak, which began in 2013, was the first recorded incidence of the bacterium in olive trees worldwide, no viable research had been conducted. But now experts can say with certainty that Xylella travels toward the roots and branches from the point of infection, rather than starting at the root and traveling through the tree.
This insight suggests that growers or officials in Puglia could contain the disease by extensive pruning, rather than simply clear-cutting all trees in the vicinity of infected ones, which Italy insisted on after being pressured by the E.U., which was looking on warily. Local farmers have argued all along that pruning the trees could stop the disease, a practice that has fended off other scourges in centuries past.
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The EFSA study also determined that the Xylella strain that is affecting olives in Puglia does not infect grapes, stone fruits, citrus or oak trees, as some researchers had suspected. More importantly for olive farmers, the scientists who carried out the study pinpointed varieties of olive trees most at risk. “Amongst cultivars tested, Cellina di Nardò clearly resulted as the most susceptible, according to the study.” The Leccino variety, they said, is the most resistant against infections. Growers who have resistant varieties might avoid eradication, and those whose trees have been killed might make wiser choices if they replenish their groves with new seedlings. Many farmers in Puglia grow more than one variety based on soil conditions, personal preference or family heritage, with the most popular being Ogliarola, Cellina di Nardò, Coratina, Frantoio and Leccino.
Even with the good news, the current findings will do little to stop the ongoing battle between environmentalists, growers and scientists in the region who cannot reach an agreement about what to do to stop the disease’s spread. In early 2015 the E.U. ruled that Italy must destroy both sick and healthy trees to create a buffer zone between the severely infected areas in the province of Lecce in the heel of Italy’s boot and the region to its north where the bulk of Italy’s lucrative commercial olive oil is produced. Part of the motivation was to deny the bacterium a pathway to the rest of Europe. It could take another year for the E.U. to consider the new studies and possibly modify its demands.
Late last summer, after many groves were razed, environmental groups and farmers whose trees were marked for destruction went to a regional Italian court to try to save the trees. Its decision granted dozens of groves a stay of execution, ruling that the destruction of the trees—infected or not—was unconstitutional. A higher court upheld the measure in February even though that defied the E.U.’s position that destruction was necessary to stop the bacterium from spreading north.
Just as the EFSA study came out, a separate European pest study showed that the meadow spittlebug, the main insect that carries Xylella, was hatching two weeks earlier than the year before. That gave new urgency to a compromise plan introduced by the Department of Agriculture for the Region of Puglia. It called for implementing certain farming practices that can kill the bugs and their nests, including spraying insecticides on trees’ infected areas and clearing underbrush to eliminate the bugs’ nests.
The plan also calls for cutting down and burning infected trees in “hotspots”—single trees or isolated groups of them infected with Xylella in areas otherwise free of the pathogen. Special consideration would be granted to the region’s famous 100-year-old “secular” trees and 1,000-year-old “millennial” trees that have been carbon-dated and are considered official national landmarks. Those trees, even if infected, would be spared from destruction as long as the owners pruned them and netted them with mesh to contain insects.
Even though some olive tree strains may resist infection, other new research warns that weaker trees could still spread the bacterium to neighboring countries. In a study published in the March 16 edition of Biological Invasions, Luciano Bosso, an environmental scientist at the University of Naples–Federico II, says “Xylella fastidiosa bacteria is likely to spread throughout the Mediterranean Basin, affecting crops and native plants in Portugal, Spain, Italy, Corsica, Albania, Montenegro, Greece and Turkey as well as all countries of northern Africa and the Middle East.” The issue, he argues, is just how much destruction the bacterium will be allowed to do or if good farming practices such as killing insects and aggressive pruning will be enough to contain any new outbreaks.
The only good consumer news is that none of the new work indicates that the bacterium in any way affects the olive oil from trees. Infected branches do not produce olives, however, so the farther the disease spreads, the fewer olives will be produced. Forecasts note that global prices for olive oil could rise as much as 20 percent in 2016. According to the International Olive Council, consumers paid, on average, 19.8 percent more for olive oil in 2015 than they did the year before.