It is on a coffee plant that came from Costa Rica in 2008 that the bacterium Xylella fastidiosa made its first appearance in Italy, to later adapt to olive trees in Puglia and eventually kill millions of plants. To confirm the hypothesis of the origin of the killer of olive trees, a study conducted by researchers in Italy, France and the United States was published in the scientific journal “Nature”, in which the Italian researcher Maria Saponari from Cnr-Ipspi of Bari participated, certain genetic traits that may have helped the spread of the bacterium is also slow. “Xylella fastidiosa is an invasive pathogen that can infect at least 595 plant species – explains Saponari – was discovered in Europe in 2013 and causes the so-called complex of rapid drying of the olive tree (Codiro), which causes leaves, twigs and branches to dry up, quickly kill the olive tree “plant”. The annoying name derives from the difficulty of growing it in the laboratory, unlike bacteria such as Escherichia coli; therefore, at the beginning of the epidemic, it was difficult to prove that the bacterium was the cause of the trees’ death.
Between 2013 and 2017, researchers collected twig samples from more than 70 infected trees and used a new protocol to extract bacterial DNA, focusing on its variation. “The more differences we see in the sequences – the researcher specifies – 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 the new host species.” And this DNA was also compared with four Costa Rican samples of coffee plants, which 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.
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. By inoculating the bacterium into coffee plants and spreading the infection to olive trees in a controlled manner through insects called “sputkins” (the natural vector of Xylella), the researchers were able to show that it can pass from one species to another.
The differences between the Costa Rican and Italian genomes are significant, although small. “The Italian tribe has lost some genes and acquired others that are potentially related to adaptation to Apulian olive trees,” Saponari points out. Genes that 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; studies according to the researcher, difficult to perform in Italy, due to the lack of facilities with the quarantine facilities required to manipulate the pathogen.
To date, there are still problems in Puglia, although the epidemic is slowing down compared to the levels reached between 2015 and 2018 around the cities of Lecce and Brindisi. “In recent years, we have found outbreaks in the Bari area in the north – Saponari concludes – but the spread of the epidemic is lower, thanks to containment measures and the fact that this area is more diversified, with different crops and landscapes.”