Xylella: a little killer for a giant tree

29 Jun 2017 by Gloria Comadira

italian infected olive treeHundreds of thousands of olive trees have been destroyed within the past years in the South of Italy by the bacterium Xylella fastidiosa. It is impossible to know the exact number of infected trees but there are over 40.000 hectares affected by the disease. Spread towards the north of the country will be inevitable if decisive measures are not taken. This is a serious problem which is causing irreparable economic losses in the Italian agricultural sector and represents a serious threat to the EU as a whole. The alert started in the rest of Europe only after the recent outbreaks of the bacterium in some regions of France and Spain.

Xylella fastidiosa is spread by flying insects. They feed on the plant's xylem and can transmit the bacterium from an infected plant to a healthy plant. Once the bacterium enters the plant, it spreads through the xylem, clogging the sap transportation which causes the plant to slowly wither away until it dies. To this date, there are about 350 plants species known to be susceptible to this bacterium, including important species such as olive, citrus, grapevine, almond and cherry trees.

Currently, there is no curative treatment for the disease; the only available option is to control further dispersion by removing the affected plants. The method recommended by the European Normative is the feeling of the infected trees together with the adjacent ones in a radius of 100 m.  Unfortunately, in the south of Italy (Apulia) it is already too late for this method because the magnitude of infection is so widespread that removing all the affected trees is impossible. Therefore the only solution is to keep the pest isolated and controlled via regular surveys and removal of infected plants in some selected parts of the infected zone with the aim to prevent further spread toward the north. All these measures are reflected in the decision 2015/789 established by the European Commission.

Vector: flying insect feeding on the plant's xylem

Outbreaks and spread of Xylella fastidiosa

Xylella was detected in Europe for the first time in October 2013 in the region of Apulia, Italy, affecting mainly olive trees. In July 2015 its presence was confirmed in Corsica, and a few months later in the South East of France (PACA), where it affects mostly ornamental plants. An isolated case was also reported in Germany in 2015, where four different potted ornamental plants were found infected in a small nursery in Saxony. In December 2016, the bacterium appeared in the Balearic Islands, Spain, where hundreds of plants have been infected in only a few months, including almond and olive trees, and very recently grapevine.

It is important to notice that all these outbreaks correspond to different subspecies of the bacterium, indicating that it may have a different source of infection which, at the present time, needs to be still confirmed. It is therefore critical to apply strict control measures in the different outbreak areas, to prevent its further spreading.

Lecce, Apulia (Italy). Place of the first outbreak. Devastated area of olive trees

There is still hope in research

Since there are no solutions, scientifically validated, to treat diseased plants in open field, the only - and dramatic - solution available so far is the felling and removal of infected trees. Fortunately, research in well-suited solutions is being carried out in order to find alternatives to the control of the bacterium. Experts from various scientific disciplines and countries are working together to discover new control methods for Xylella and its insect vectors. These researchers are part of two European funded projects: POnTE (Pest Organisms Threating Europe) and XF-ACTORS (Xylella Fastidiosa Active Containment Through a multidisciplinary-Oriented Research Strategy). Preliminary results are encouraging given that two olive tree cultivars, showing some levels of tolerance traits to the disease, have been identified. However, further studies are ongoing, although it seems so far the only way to ensure co-existence with the bacterium in the South of Italy.

A serious threat for the Spanish olive oil 

After the outbreaks in the Balearic Islands, there is a big concern that the bacterium may reach the rest of the Spanish territory. If this happens the first olive oil producing country in the world could face severe economic losses. Just as alarming is the fact that citrus and grapevine, both susceptible to be infected by Xylella, are also key crops for the Spanish economy.

The experience in Italy has confirmed the importance of information campaigns at an early stage to ensure immediate detection and application of strict eradication measures before it is too late.

YPARD-Spain initiative

From YPARD-Spain we want to shed light on this issue and make everybody participate in our initiative. Thus, an information campaign has been developed to spread the word about this disease, concentrating on the agricultural sector, but also making this information available to the general public. We would like to be as inclusive as possible to as well receive inputs from stakeholders. We kindly invite our French and Italian YPARD partners to take similar actions in their territories and to share experiences and ideas with us. 

Picture credits: Gloria Comadira/YPARD Spain