Europe Updates Xylella Risk Assessment

An update to EFSA's 2015 assessment concludes the plant disease has not been eradicated, but control measures have contained its spread.

Jun. 11, 2019
By Isabel Putinja

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The European Food Safety Authority (EFSA) has pub­lished an update to its 2015 assess­ment of the risk of Xylella fas­tidiosa in the European Union (E.U.).

The update broadly con­cludes that there is no known way to erad­i­cate this plant dis­ease, which is spread by pests, such as the olive fruit fly, but that con­trol mea­sures have been shown to be effec­tive in con­tain­ing its spread.

This was a com­plex sci­en­tific chal­lenge with many areas of uncer­tainty, but we have dis­tilled some impor­tant con­clu­sions that will assist risk man­agers, risk asses­sors and researchers.- Stephen Parnell, chair of Xylella fas­tidiosa work­ing group

Experts of the EFSA Panel on Plant Health addressed spe­cific ques­tions, includ­ing the short and long-range spread of Xylella fas­tidiosa and its impacts, the period of time dur­ing which the dis­ease is asymp­to­matic and how to reduce the risk of the infec­tion spread­ing.

This was a com­plex sci­en­tific chal­lenge with many areas of uncer­tainty, but we have dis­tilled some impor­tant con­clu­sions that will assist risk man­agers, risk asses­sors and researchers,” Stephen Parnell, the chair of the panel’s Xylella fas­tidiosa work­ing group, wrote in an arti­cle on the EFSA web­site.

See Also:Xylella fas­tidiosa News

Computer sim­u­la­tions devel­oped by its experts revealed that south­ern Europe is most at risk of infec­tion through some sub­species of the bac­terium, such as Xylella fas­tidiosa subsp. mul­ti­plex, could also cause infec­tions in north­ern Europe.

Through these sim­u­la­tions, the experts were also able to sim­u­late how Xylella fas­tidiosa could prop­a­gate under dif­fer­ent con­di­tions and over short as well as long dis­tances, and reveal the effec­tive­ness of emer­gency phy­tosan­i­tary con­trol mea­sures if put in place quickly.


Computer sim­u­la­tions are at the core of this sci­en­tific opin­ion,” Parnell said. The mod­els we have devel­oped are robust and, impor­tantly, flex­i­ble so they can be adapted to explore a wide range of dif­fer­ent sce­nar­ios and sup­port emer­gency plan­ning.”

Regarding the period of time the dis­ease is asymp­to­matic, this can vary depend­ing on the sub­species of the bac­terium and the species of plant affected.

For exam­ple, stud­ies exam­ined by the experts revealed that an olive plant infected with the sub­species Xylella fas­tidiosa subsp. pauca is asymp­to­matic for approx­i­mately 10 months and has a 95 per­cent chance of devel­op­ing symp­toms within a period of four years. This long incu­ba­tion period means that visual inspec­tions are not effec­tive in detect­ing infec­tions and that other meth­ods such as sam­pling and diag­nos­tic test­ing are required.

Measures that have been taken to pre­vent fur­ther infec­tion have included cre­at­ing con­tain­ment zones around infected areas, the destruc­tion of dis­eased plants and pest con­trol.

The effec­tive­ness of buffer zones was found to be rel­a­tive, while bio­log­i­cal con­trol mea­sures only tem­porar­ily reduce but do not elim­i­nate the risk of infec­tion. However, early detec­tion is cru­cial for effec­tive erad­i­ca­tion and to con­trol the spread of the dis­ease. Pest con­trol is also impor­tant: insec­ti­cides used in Italy such as acetamiprid and deltamethrin had a 75 to 100 per­cent effec­tive­ness rate.

The study lists a num­ber of plant dis­eases caused by this plant pathogen trans­mit­ted by insects. These include Pierce’s dis­ease, which attacks grapevines; olive quick decline syn­drome, which dec­i­mates olive groves; cit­rus var­ie­gated chloro­sis, affect­ing cit­rus plants; almond leaf scorch, which infects almond trees; and other leaf scorch dis­eases.

Xylella fas­tidiosa was first detected in olive groves in Puglia, south Italy in 2013. New infec­tions of the bac­terium and its sub­species were sub­se­quently detected in plants in other parts of the E.U., notably on the French island of Corsica and the Provence-Alpes-Côte d’Azur region of south­ern France in 2015. In the past four years, sev­eral more cases were reported in Spain, the Italian region of Tuscany, and in Portugal’s Porto dis­trict.


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