Xylella update: still no cure, control measures crucial

EFSA's Panel on Plant Health (PLH Panel) used computer models to simulate how Xylella (X. fastidiosa) spreads over short and long distances under different conditions. The modeling showed how important it is to take measures such as Controls established by the European Commission implement to prevent further spread and even eradicate outbreaks. It illustrated the relative effectiveness of buffer zones of different sizes to control an infected area.

The simulations also showed that it is important to control the insects that have been shown to transmit the pathogen in Europe - such as the meadow foam cicada Philaenus spumarius - as well as minimize the time lost between outbreak detection and implementation of control measures - e.g., removal of infected plants and establishment of demarcated areas.

Is there healing?

The evaluation confirms that there is still no known way to eliminate the bacterium from a diseased plant under field conditions. The effectiveness of chemical and biological control measures has been evaluated in recent experiments. The results indicate that such measures may temporarily reduce the severity of the disease in some situations; however, there is no evidence that they can X. fastidiosa could eliminate under field conditions and over a longer period of time.

Computer simulations showed that even though most of the EU territory has climate types under which the pathogen occurs elsewhere in the world, the areas most at risk are in southern Europe (see map). However, the modeling showed some deviations from this general rule, depending on the subtype of the pathogen. For example, the settlement potential of X. fastidiosa subsp. multiplex in northern Europe is greater than that of other subspecies.

The information on establishment risk will be useful for setting up monitoring and detection programs in different Member States. EFSA is currently developing guidelines with plant protection organizations across the EU for surveys on X. fastidiosa.

A hidden pest

The assessment includes an important section on variations in the asymptomatic period - the time between infection and expression of symptoms - in plants that are hosts for X. fastidiosa can function.

Literature searches and data analysis revealed significant differences in asymptomatic period for different combinations of host and subspecies. This information is useful for designing tailored surveillance programs and helps risk managers decide when it is safe to stop delineating an outbreak area.

Future research

Stephen Parnell, chair of the PLH panel's working group on. X. fastidiosa, stated, "This work was a complex scientific challenge with many uncertainties, but we were able to draw some important conclusions that will help risk managers, risk assessors, and researchers."

"Computer simulations are at the core of this scientific expertise. The models we developed are robust and, most importantly, flexible, so they can be adapted to study a variety of different scenarios and support emergency planning."

Dr. Parnell added, "Since the publication of our previous report in 2015, a wealth of new information has become available, much of it from EU-funded projects such as POnTE or XF-ACTORS origin. Thanks to these and other initiatives, we are gaining a better and better understanding of this dangerous plant pest, although many uncertainties remain, especially regarding subspecies and sequence types."

"It is imperative that we continue to invest in research so that we can not only contain outbreaks in the future, but also predict them."

Background:

Xylella fastidiosa is one of the most dangerous plant bacteria in the world. It causes a variety of diseases with enormous economic impact on agriculture, gardens and the environment.

The four most commonly reported subspecies are. fastidiosa, pauca, multiplex and sandyi. The bacterium lives in the xylem tissue of plants and usually spreads via vector insects that feed on the xylem sap.

X. fastidiosa was first detected in Europe in 2013, when an outbreak occurred in Apulia in southern Italy. Since then, the bacterium has also been reported in France (Corsica and Provence-Alpes-Côte d'Azur), Spain (Balearic Islands, Valencia, Madrid), central Italy (Tuscany) and Portugal (Porto). In all outbreak areas, EU emergency measures were taken to contain the pathogen.

• Update of the scientific opinion on the risks to plant health posed by Xylella fastidiosa in the EU territory
• Effectiveness of in planta control measures for Xylella fastidiosa
• Collection of data and information on biology and control of vectors of Xylella fastidiosa