A plant disease spread by sap-sucking insects has been devastating olive and fruit orchards across southern Europe, but scientists are inching closer to halting its spread with the help of insect repelling clays, vegetative barriers and genetic analysis.
In the late summer harvest of 2013, olive farmers in the Puglia region of southern Italy noticed that the leaves on several of their trees were turning brown and their shoots withering. The problem spread from one orchard to another, as more olive farmers found their trees were drying out and beginning to die.
Genetic testing confirmed them to be infected with Xylella fastidiosa, a bacteria originally found in America. Soon outbreaks appeared throughout the Mediterranean, even briefly as far north as Germany in 2016.
The bacteria is mainly spread by sap-sucking insects known as spittlebugs and sharpshooters. As the insects feed, the bacteria is able to infect the vessels that transport water and nutrients around the plant, known as the xylem. As the bacteria destroy the xylem, it slowly chokes the plant.
‘We are dealing with a very severe situation in southern Italy,’ said Dr Maria Saponari, based at the Institute for Sustainable Plant Protection in Bari, Italy. Europe’s researchers were caught off-guard by the epidemic, she explained. ‘When the bacterium was discovered here, there wasn’t any research centre in Europe working specifically on this pathogen. We were starting from zero.’
The disease can infect a wide range of plants, including shrubs like the myrtle-leaf milkwort and rosemary, oak trees, and important crops like lavender. Food crops including cherry trees, plum trees and olive trees are among the species considered to be at high risk.
In particular, the outbreak has amplified problems in Italy’s strained olive oil sector. In 2018, the country reported a 57% drop in its olive harvest compared to 2017 – a 25-year low. Researchers blamed a frosty spring followed by a summer drought, which weakened the olive trees and left them even more susceptible to infection.
The intense summer weather in southern Italy may also have made it easier for the disease to spread among olive trees as insects carrying the bacteria sought out food in the dry conditions. ‘Here in summer, olives are the only green plants that we see,’ said Dr Saponari. ‘Olive canopies, for them, (are) like a refuge to survive.’
While the disease has been found in a number of EU countries, it appears ‘the strains that have been imported in Corsica or in Spain are much less aggressive than the strain spreading in Puglia’, added Dr Saponari.
In response, Dr Saponari is leading one of several Europe-wide projects seeking ways to curb this new threat to Europe’s olive crops, and monitor its spread. Her XF-ACTORS project is examining olive trees’ genetics to see if some of the plants have natural resistance to Xylella fastidiosa that can then be used to breed crops that are more resilient against the disease.
[Photo by Sjor via Wikimedia commons, licenced under CC BY-SA 4.0]