Xylella Fastidiosa

Xylella fastidiosa, a threat to olive trees and hundreds of plants

Xylella fastidiosa is a phytopathogenic quarantine bacterium that can infect more than 600 plant species, affecting major agricultural crops such as grapevines, citrus fruits, almonds and olives, as well as herbs and ornamental, forest and wild plants. There are currently no curative measures against this bacterium. To avoid the spread of the disease, therefore, contaminated plants must be grubbed up and destroyed, and the insect vectors controlled. Here is an update on the bacterium, and on what the ANSES Plant Health Laboratory (LSV) is doing to help combat this pest.

What is Xylella fastidiosa?

By colonising the xylem of a plant and producing aggregates or biofilms, Xylella fastidiosa prevents raw sap from rising within the xylem vessels.

This bacterium is transmitted by xylem-feeding insect vectors. About thirty species of leafhoppers feed on rising xylem sap and can therefore potentially transmit the bacterium in France. Four species are much more common in or near cultivated areas and one of them, Philaenus spumarius, the meadow spittlebug, is already a proven vector in Europe.

Xylella  fastidiosa attacks around 600 plant species belonging to more than 80 different botanical families: grapevines, citrus, almond, olive, cherry and other fruit trees, coffee, avocado, alfalfa, oleander, oak, maple, etc.

There are currently no curative measures for combating this bacterium. European regulations designed to prevent its introduction and spread in Europe advocate the grubbing up and destruction of contaminated plants, as well as increased surveillance.

Within the Xylella fastidiosa species, five subspecies have been identified so far: fastidiosa, multiplex, morus, pauca and sandyi. They do not all attack the same plants and have different degrees of virulence.  

What are the symptoms?

The symptoms caused are non-specific: wilting, necrosis and scorch of leaves or twigs. They may appear late (latency phenomenon), or not at all (asymptomatic plant). They can be unevenly distributed in the plant, affect fruit production and be followed in the most serious cases by the death of the plant.

Depending on the plants, the following can be observed:

  • leaf scorch and, in the more advanced stages, desiccation of branches;
  • leaf chlorosis;
  • dwarfing of the plant;
  • drooping appearance and shorter internodes;
  • yellowing and reddening of the leaves.

Xylella fastidiosa in Europe: different situations depending on the countries

An initial outbreak of Xylella fastidiosa on olive trees, oleanders and almond trees was reported in the Puglia region of Italy in 2013. The situation evolved very rapidly into an epidemic, expanding exponentially in olive groves. The strain responsible belongs to the pauca subspecies of X. fastidiosa (Xylella fastidiosa subsp. pauca De Donno).

As a result of reinforced surveillance implemented by French State services, the ANSES Plant Health Laboratory (Bacteriology, Virology & GMO Unit in Angers) confirmed detection of the first French case in Corsica in 2015, following a strong suspicion of Xylella fastidiosa on a myrtle-leaf milkwort shrub (Polygala myrtifolia) in a commercial area in the municipality of Propriano. This surveillance plan deployed throughout the country revealed that the bacterium was present in Corsica and the Provence-Alpes-Côte d'Azur (PACA) region in 2015, and probably earlier.

The situation is not comparable to that in Italy, where the disease is due to the rapid spread of a single very aggressive strain on olive trees. The contamination in French regions seems to be due to several different introductions during the 1980s. The strains of Xylella fastidiosa isolated in France mainly belong to the subspecies multiplex, divided between sequence types ST6 and ST7.

Then in September 2019, for the first time in France, in the PACA region (Menton and Antibes), ANSES confirmed the presence of the bacterium on two olive trees, tested positive by the approved laboratory:  one of them was contaminated with the pauca subspecies, whose profile is equivalent to that of the strain found in southern Italy, while the other was contaminated with the multiplex subspecies. More recently, since the summer of 2020, new outbreaks were detected in the Occitania region, first in a nursery on lavender plants, and then in the environment on lavender, rosemary and broom, with the list getting longer as investigations progressed. Thus, in October 2021 in the Gard département, a Spanish broom (Spartium junceum) plant was found to be positive for the bacteria.

Since 2016, Spain has reported the presence of several subspecies of Xylella fastidiosa on a wide range of host plants in the Balearic Islands and then on the mainland, with the subspecies multiplex (sequence type ST6) causing enormous damage in almond orchards in the province of Alicante. The same subspecies has also been identified in Portugal, although the genotype is different (ST7) from the one present in orchards in south-eastern Spain.

Territorial surveillance and the Agency's role

Two units of ANSES's Plant Health Laboratory (LSV) are involved. The Bacteriology, Virology & GMO Unit followed a validation process to develop the official method for detecting Xylella fastidiosa, which is used by the approved laboratories for territorial surveillance. The Entomology & Invasive Plants Unit has drafted a protocol for identifying potential vectors, for implementation of the vector surveillance plan. These units also organise training and monitoring of approved laboratories and are in charge of carrying out confirmatory analyses. Within the Lyon Laboratory, the Epidemiology  & Surveillance Support Unit contributes to epidemiological surveillance by compiling and analysing data on the occurrence of outbreaks. It produces updated maps of the distribution of the disease, which are published on the Ministry of Agriculture's website and used by the epidemiological surveillance platform for plant health (ESV).

And finally, in the framework of the ESV, coordinated by the DGAL, INRAE and ANSES, the LSV is an active member of the working group for surveillance of Xylella fastidiosa. It is also the co-coordinator of this working group which supports surveillance in areas with existing outbreaks and provides early detection of new introductions of the disease.

Developing methodologies and research: the Agency's involvement

The LSV is continuously working to improve the sensitivity and reliability of methods for detecting the bacterium on host plant species and its insect vectors, and has explored the possibility of using new molecular techniques such as digital PCR.
In partnership with INRA and as part of H2020 European research programmes, the LSV is working to standardise the methods used to characterise the strains found in France and Europe, and is taking part in a study of the spatial structure and dynamics of the populations, in order to identify and characterise the invasion routes of the Xylella fastidiosa bacterium. Lastly, the LSV has carried out a study to improve knowledge of the list of potential vectors found in crops or in the environment and their distribution in France, with the cooperation of various national partners.