Treatments ‘do not eliminate Xylella from olive trees but can reduce disease symptoms’

Treatments being tested on olive trees in Apulia reduce the symptoms of disease caused by Xylella fastidiosa but do not eliminate the pathogen from infected plants.

This is the main conclusion of an assessment carried out by EFSA into the effectiveness of treatments for X. fastidiosa in olive trees. The findings confirm experience from other parts of the world, where X. fastidiosa is also causing enormous damage and no treatment has been found to eliminate the pathogen from plants grown outdoors.

EFSA’s plant health experts evaluated research being carried out in Apulia by two groups – one from the University of Foggia, the other from the Consiglio per la ricerca in agricoltura e l’analisi dell’economia agraria (CREA) in Caserta – and also examined treatments used elsewhere to control bacterial infections in plants including olive, citrus, apple, pear and grapevine.

The Foggia researchers treated infected olive trees with several bioactive compounds following severe pruning. They reported vigorous new growth of branches carrying symptom-free leaves within five months of the treatments being applied. They also reported sizeable olive harvests from the treated trees.

The CREA group also reported positive results after using a commercial product containing zinc, copper and citric acid – all the treated plants survived both the summer of 2015 and the subsequent winter.

However, EFSA’s Panel on Plant Health (PLH) says it would be premature to draw conclusions on the long-term effectiveness of such treatments from these experimental results. Only experiments conducted over several seasons can provide reliable information on the long-term effectiveness of these treatments, the Panel says in its Statement.

The researchers told EFSA that they also considered the results to be preliminary, pending repetition for at least one more season. They added that the objective of the tests was to assess the ability of the treatments to suppress disease symptoms rather than to eliminate the pathogen in infected olive trees.

The PLH Panel was unable to conduct a review of the experimental data, as the researchers in Apulia could not share the information due to patent/copyright issues. However, the Panel emphasises that the extensive research already conducted on diseases in other crops, such as Pierce’s disease in grapevine, shows that such treatments can improve the health of plants – particularly when combined with crop management practices – but cannot cure them or stop them from being infected by Xylella.

The Panel experts note the possible benefits of the treatments in prolonging the life of olive trees, particularly those in the Apulian containment zone where X. fastidiosa CoDiRO is present and widespread, and support the need for further studies in this regard. 

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