The role of ANSES in combating Xylella fastidiosa
Published on 22/10/2019
Plant health, Xylella fastidiosa
Xylella fastidiosa is a phytopathogenic bacterium that can affect plants in key agricultural sectors: grapes, citrus, Prunus, olives, alfalfa, oak, maple, horticulture, forestry, etc. There are no curative measures against this bacterium, except for grubbing up and destroying the contaminated plants and controlling the insect vectors. X. fastidiosa was first detected in Europe in 2013 in Italy, and was detected in Corsica in July 2015. More recently, isolated outbreaks have been reported in a greenhouse in Germany and also in Spain (Balearic Islands and the Alicante region). Here is an update on the role of the ANSES Plant Health Laboratory in combating this bacterium.
Xylella fastidiosa: defining characteristics
Xylella fastidiosa is a bacterium that affects the xylem, preventing the plant from feeding by impeding the movement of rising sap. The symptoms of contagion are not specific (wilting, leaf scorch), which makes it difficult to detect. There are currently no curative methods for combating this bacterium. The European Council Directive 2000/29/EC, on protective measures against the introduction and spread of the bacterium in Europe, advocates the grubbing up and destruction of contaminated plants.
Xylella fastidiosa, transmitted and mediated by insect vectors that feed on rising xylem sap (including leafhoppers and spittlebugs, xylem-biting and sucking insects), can affect more than 300 plant species belonging to 60 different botanical families: grapevines, citrus, fruit trees (Prunus, olive trees, etc.), coffee, avocado, alfalfa, oleander, oak, maple, etc.
There are currently six known subspecies of Xylella fastidiosa: fastidiosa, multiplex, morus, pauca, sandyi and tashke. They do not attack the same plants and have different degrees of virulence.
They produce different symptoms:
- leaf scorch (oleander) and in the more advanced stages, parching of twigs (random distribution in the crown), followed in the most serious cases by the death of the plant (olive, almond, oak, elm, sycamore, etc.),
- leaf chlorosis (on coffee, orange): on orange trees, infection also causes the production of smaller fruit,
- defective lignification and persistence of petioles after the fall of the leaves in grapevines,
- dwarf disease in alfalfa, together with a blue-green colouring of the leaves,
- a bushy appearance and shorter internodes in peach trees,
- yellowing and reddening of leaves in grapevines
Xylella fastidiosa in Europe: different situations in different countries
An initial outbreak of Xylella fastidiosa on olive trees, oleanders and almond trees was reported in October 2013 in the Puglia region, in the south of Italy. The situation evolved very rapidly into an epidemic, expanding exponentially in olive groves. The strain responsible belongs to the subspecies X. f. pauca.
As part of reinforced monitoring by state services, in July 2015, the Regional Food Service (SRAL) in Corsica informed the ANSES Plant Health Laboratory in Angers of a strong suspicion of Xylella fastidiosa on a myrtle-leaf milkwort shrub (Polygala myrtifolia), in the municipality of Propriano. This case was confirmed by ANSES two days later.
A vigilance plan was set up in 2015 and is still being applied throughout France. In 2017, the plan revealed that the bacterium is present in Corsica and on the Cote d'Azur. It does not seem to be an epidemic, as in Italy, where the disease is due to the rapid spread of a single strain. Unlike the situation in Italy, the contamination of these French regions seems to be due to at least two different introductions at least 25 years previously. The strains of X. fastidiosa isolated in France belong to two genetic lineages of the sub-species multiplex, whereas the Italian strain belongs to the subspecies pauca.
New outbreaks were detected in Europe in 2016. One, in Germany, is extremely localised. It concerns ornamental plants grown in a greenhouse in Saxony. The strain belongs to the subspecies X. fastidiosa fastidiosa. The others, in Spain, are much more disturbing. The presence of X. fastidiosa was reported in Mallorca in November 2016. Since then, the bacterium has been detected in three Balearic Islands and in the region of Alicante on the mainland.
Territorial surveillance and the Agency's role
The Plant Health Laboratory, which is the National Reference Laboratory (NRL) for this bacterium, has developed an official method for the detection of X. fastidiosa, which is used for territorial surveillance by the accredited laboratories.
The Plant Health Laboratory also organises training and conducts assessments of the accredited laboratories. It is responsible for carrying out confirmation analyses when new outbreaks are detected. The ANSES Lyon Laboratory compiles and analyses the data concerning the occurrence of outbreaks so as to provide updated maps of the distribution of the disease, available on the website of the Ministry of Agriculture.
Developing methodologies and new research: the Agency’s involvement
The Plant Health Laboratory works to improve the sensitivity and reliability of methods for the detection of this bacterium on the host plant species and in the insect vectors by introducing new molecular techniques such as digital PCR.
In collaboration with INRA and in the framework of the H2020 European research programme, ANSES is working to characterise the strains present in France and Europe and the geographical distribution of the populations and their dynamics, in order to identify and characterise each introduction of X. fastidiosa. Work on the identification and the distribution of potential vectors is also under way, in collaboration with various partners in France.
The article has been added to your library