Members of the Regional Agency for Irrigation and Forestry Activities (ARIF) cut down the olive trees with Xylella disease in June 2020 in Ostuni, Italy. Credit: Donato Fasano/Getty Images.
Xylella fastidiosa is an invasive pathogen that can infect at least 595 plant species. It was discovered in Europe in 2013, when an outbreak started in the olive tree population of Apulia, and then spread to France, Spain and Portugal. It causes the so-called Olive Quick Decline Syndrome (OQDS), that makes leaves, twigs and branches dry out and scorch, quickly killing the plant. The name fastidiosa (‘fastidious, troublesome’) comes from the difficulty in cultivating it in a lab, explains Maria Saponari, senior Researcher at the Institute for Sustainable Plant Protection for the Italian National Research Council in bari, who took part in the study. “While bacteria like Escherichia coli are easy to grow in vitro in 24-48 hours, it is hard to extract Xylella from the plants,” she says. This is why at the beginning of the outbreak it was hard to prove that the bacterium was the cause of the trees’ death.
Between 2013 and 2017, the scientists collected twig samples from more than 70 trees with OQDS, and used a new protocol to extract their bacterial DNA, focussing on its variability. “The more differences in DNA sequences, the longer Xylella would have been in Italy, because it would have had more time to produce mutations while adapting to the new environment and the new host species,” says Saponari. This DNA was also compared to four Costa Rican samples from coffee plants.
The results1 confirmed the idea that the Italian pathogen could come from Central America. There were only small differences between Costa Rican and Apulian samples, and even fewer differences within the Italian population: “This suggests that the pathogen has arrived in Italy recently with a single introduction from Costa Rica, because there is only one genetic population,” says the researcher. Considering the mean mutation rate of these bacteria, the researchers were also able to confirm 2008 as the most likely year when Xylella was introduced in Italy. This is consistent with Apulian farmers’ first reports of infected trees in 2010, since the disease incubation period can last more than two years.
The researchers also conducted experiments where they inoculated the bacteria in coffee plants, and spread the infection to olive trees in a controlled way using spittlebugs (the natural vector for Xylella), thus proving that Xylella could have been transmitted from one species to another.
The differences between the Costa Rican and Italian genomes, although small, are relevant. “The Italian strain lost some genes and acquired some others, potentially correlated to the adaptation to the Apulian olive trees,” Saponari points out. These genes could become new targets to fight the disease, for example by modifying the bacterium so that it can no longer infect olive trees. In order to confirm this idea, scientists would need to create a mutated strain of Xylella, with silenced or added genes. But such studies will be difficult to perform in Italy, due to a lack of facilities with the necessary quarantine structures to manipulate the pathogen, Saponari says.
The disease still causes trouble in Apulia, even if the epidemic is slowing down compared to levels between 2015 and 2018 around the cities of Lecce and Brindisi. “In recent years, we found outbreaks in the Bari area to the north, but the spread of the epidemic is lower, thanks to containment measures and to the fact that this area is more diversified, with different crops and landscapes to stop the spread,” explains Saponari.
