Scientist, this is how the “killer” Xylella came to Puglia

The mystery of the landing in southern Italy of the “killer” Xylella who killed millions of Apulian olive trees has been revealed: the pathogen – which has caused a real pandemic such as the coronavirus – phe would have arrived in Puglia in 2008 transported from a Costa Rican coffee factory. According to a study done by researchers in Italy, France and the United States – published in the prestigious scientific journal ‘Nature’ and where he participated the Italian researcher Maria Saponari from Cnr-Ipsp in Bari The Xylella fastidiosa bacterium first came to Italy in 2008, on a coffee tree, and then adapted to olive trees in the southern region of Puglia, eventually killing millions of plants. The study also highlights some genetic traits that may have helped the bacterium to spread. “Xylella fastidiosa – explains Saponari – is an invasive pathogen that can infect at least 595 plant species. It was discovered in Europe in 2013, after the start of an epidemic among olive trees in Puglia, and then spread to France, Spain and Portugal. Causes the so-called Rapid Olive Drying Complex (CoDiRO), which causes leaves, twigs and branches to dry out, which quickly kills the plant.

The annoying name stems from the difficulty of growing it in the laboratory, explains Maria Saponari, a researcher at the Institute for Sustainable Plant Protection at the National Research Council of Bari, who participated in the study. “While bacteria like Escherichia coli are easy to grow in vitro in 24-48 hours, it is very difficult to extract Xylella from plants,” he says. Therefore, at the beginning of the epidemic, it was difficult to prove that the bacterium was the cause of the trees’ death. The study reconstructs researchers between 2013 and 2017 collecting twig samples from more than 70 trees with CoDiRo and using a new protocol to extract bacterial DNA, focusing on its variation. “The more differences we see in the DNA sequences, the longer Xylella must have been in Italy, because that means it had more time to produce mutations because it adapted to the new environment and host species,” Saponari continues. This DNA was also compared with four Costa Rican coffee plant samples.

The results confirm the idea that the Italian pathogen comes from Central America. There were only small differences between the samples from Costa Rica and Puglia, and even fewer differences within the Italian population. “This suggests that the pathogen arrived in Italy recently with a single introduction from Costa Rica, as we only see one genetic population,” says the researcher. Given the average mutation rate of these bacteria, the researchers were also able to confirm 2008 as the most likely year for the introduction of Xylella in Italy. This would be in line with the first reports of infected trees from Puglia farmers in 2010, as the incubation period of the disease can be more than two years. The researchers also conducted experiments in which they inoculated the bacterium into coffee plants and spread the infection to olive trees in a controlled way using insects called “sputtachine” (the natural vector of Xylella), which shows that the bacterium can pass through. one species to another. The differences between the Costa Rican and Italian genomes are significant, although small.

“In recent years there have been outbreaks north of Bari but the epidemic is lower due to containment measures”

“The Italian tribe has lost some genes and acquired others, potentially related to adaptation to the Apulian olive trees,” Saponari points out. These genes can become new targets for fighting the disease, for example by modifying the bacterium so that it can no longer infect olive trees. To confirm this idea, researchers would need to create a mutated Xylella strain, with silenced or added genes. But such studies will be difficult to carry out in Italy, due to the lack of facilities with the necessary quarantine facilities to manipulate the pathogen, warns Saponari.

The disease is still creating problems in Puglia, although the epidemic is slowing down from the levels reached between 2015 and 2018 around the cities of Lecce and Brindisi. Finally, Saponari notes that “in recent years we have found outbreaks in the Bari area in the north, but the spread of the epidemic is lower, due to containment measures and the fact that this area is more diversified, with different crops and landscapes slowing the spread”.

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