In Italy in particular, it is known for attacking olive trees and has spread particularly in Puglia in recent years.
But how did Xylella arrive in Puglia ? Its history is relatively recent: it is thought that the Xylella bacterium arrived in Italy in 2008 via an insect hosted on an ornamental plant from Costa Rica.
The first problems began only five years later when the disease began attacking olive trees in Puglia: in October 2013, it was the first time the bacterium was reported in the European Union.
It is presumed to have arrived in Puglia in 2008, specifically in the southern Salento area, with the first reports in October 2013.
Identified as a quarantine pathogen, all prophylaxis for its containment has been implemented.
Since then, its presence has also been reported in France, Spain and Portugal and controls are underway to prevent the spread of the bacterium.
It is known that several species of piercing-sucking insects that feed on xylem sap are vectors of the bacterium.
Xylella is polyphagous , meaning it feeds on a variety of plants, including many common cultivated and wild plants.
In August 2023, EFSA co-organized the fourth European Conference on Xylella , bringing together hundreds of plant health specialists from around the world in Lyon, France. Experts discussed the state of the art in research to control this bacterium, which continues to threaten the European economy and environment.
What is Xylella fastidiosa?
As mentioned above, Xylella fastidiosa is a bacterial plant pathogen transmitted by insects that feed on the xylem sap .
What plants does Xylella attack? It is associated with serious diseases in a wide range of plants worldwide: in addition to the well-known disease that attacks olive trees, it causes Pierce's disease in grapevines, citrus variegated chlorosis, phony peach disease, coffee leaf scorch, olive rapid decline syndrome, and other diseases that affect common trees such as plum, almond, oak, and oleander.
Where is Xylella found?
Diseases caused by Xylella occur in tropical, subtropical, and temperate areas.
The bacterium is widespread in many areas of North, Central and South America and has reached Europe in recent years.
Xylella was not present in Europe and, being a very dangerous bacterium, measures for its eradication were immediately put in place.
Official investigations conducted by EU Member States so far confirm that its presence is limited to Italy (southern Puglia), France (Provence-Alpes-Côte d'Azur and Corsica), Spain (Balearic Islands) and an isolated case in a greenhouse in Germany (Saxony).
How do plants become infected?
Xylella fastidiosa colonizes the xylem bundles within plants.
The main function of the xylem vessels is to transport water from the plant's roots to the leaves , so when bacteria colonize the plant, the xylem vessels become blocked and the plant slowly dies.
To reach a new host, it must be transported and transmitted by a vector: all insects that feed on xylem sap and have piercing-sucking mouthparts are potential vectors, until proven otherwise.
The most common vectors of Xylella worldwide are:
- sharpshooter leafhoppers Cicadellidae
- spittlebugs (Aphrophoridae)
- Cercopidae leafhoppers
- the buzzer Philaenus spumarius : currently the only confirmed vector in Puglia
How to recognize Xylella on olive trees
The symptoms observed on olive trees range from the browning of the leaves to the drying out of more or less extensive portions of the tree, up to the complete death of the tree itself.
The speed of disease expression depends on:
- weather conditions
- phytosanitary state of the plant before contracting bacteria and spread of the vector (quantity of leafhoppers present in the area with the plantations)
Olive trees can become infected with Xylella fastidiosa and remain asymptomatic for long periods of time.
Which plants are vulnerable to attack by the bacterium?
EFSA maintains a database of known hosts, which currently lists 563 plant species , both naturally and experimentally infected, belonging to 82 botanical families . However, not all listed plants are susceptible to all "types" (subspecies or strains) of Xylella fastidiosa, and even if affected, plants will not necessarily become symptomatic.
Furthermore, even plants belonging to the same species can express different levels of susceptibility depending on the variety and growing conditions.
Identification of asymptomatic host species is critical as they can act as asymptomatic carriers of this bacterium to new, more susceptible areas and crops.
Is there more than one “type” of Xylella fastidiosa?
Yes, there are five currently recognized subspecies of Xylella (which in turn have their own variants):
- annoying
- morus
- multiplex
- pauca
- sandy
In Europe, the fastidiosa subspecies, associated with Pierce's disease of grapevine and almond leaf scorch in the Americas, has been detected in the Balearic Islands in Spain, in isolated plants in a greenhouse in Germany (Saxony), and has been intercepted on some shipments of imported coffee plants.
The multiplex subspecies has been reported in France (Corsica and Provence-Alpes-Côte d'Azur) and in the Balearic Islands.
The subspecies pauca , very common in South America, has been detected in Italy (Puglia), the Balearic Islands and in an isolated outbreak in France (Menton, Provence-Alpes-Côte d'Azur).
Additionally, another species of Xylella, Xylella taiwanensis , is present in Taiwan, where it causes leaf burn on pear trees.
The subspecies X. fastidiosa is also present in Taiwan.
The Frantoio D'Orazio did not suffer any damage because it is not located in the area where Xylella has spread (lower Salento).
However, good agricultural practices are in place throughout Puglia to control and contain the bacterium , preventing it from spreading to unaffected areas such as the one surrounding the Frantoio D'Orazio.
