Major outbreak of plant pest - Xylella fastidiosa

Impact 5
upper risk error bar
upper likelihood error bar
risk indicator lower likelihood error bar
lower impact error bar
ID 57a
Risk theme Human, animal and plant health
Impact & Likelihood
Impact key
5 Catastrophic
4 Significant
3 Moderate
2 Limited
1 Minor
Likelihood key
5 >25%
4 5-25%
3 1-5%
2 0.2-1%
1 <0.2%


Xylella fastidiosa is a bacterium that is responsible for causing a number of named plant diseases. It was first detected in Europe in 2013 and is now established in France, Italy, Portugal and Spain. The bacterium has been reported from a very wide range of hosts, and the number of plant species that have been shown to be infected is constantly increasing. Legislation has been updated to prevent the introduction of X. fastidiosa on certain hosts, such as olive and rosemary, which are considered to be highly susceptible to the bacterium. The Animal and Plant Health Agency and Forestry Commission also carry out annual surveillance for X. fastidiosa.


The reasonable worst-case scenario is based on an outbreak of Xylella in an area containing 3 to 5 plant nurseries, with evidence of possible spread on plants and plant products to multiple premises across the UK. While it is difficult to quantify the costs associated with Xylella impacting plant nurseries, there could be moderate economic costs relating to lost working hours, lost stock and restrictions on trade (estimated to be over £7.5 million for 5 nurseries over 5 years). The cost of government intervention, including surveillance, is estimated to be £5 million over 5 years. In the short term, measures to remove plant species may impact air quality (through the burning of material) and water quality (through the use of herbicides and insecticides).

Key assumptions

It is assumed that if multiple outbreaks or outbreaks of other pests occur simultaneously, these would drain resources and likely mean that increased levels of impacts are seen.


A high-impact variation involves the bacterium being found in an area with large plant nurseries reliant on growing host plants of Xylella. The nurseries would be significantly impacted due to the scale of eradication needed. The risk could be compounded if a coordinated approach to mass testing between Northern Ireland and Great Britain is hampered, during the period where future arrangements for laboratory accreditation schemes and mutual recognition are being developed, following the UK’s exit from the EU.

Response capability requirements

Surveillance to monitor the spread of the bacterium, capabilities to diagnose the pest and procedures to report suspected cases would be required. In addition, the removal and disposal of infected and suspect plants, and the application of pesticides by registered spray operators to control vectors of the bacterium, would be necessary.


Depending on the situation, businesses such as garden centres and nurseries may be able to recover after a short period, whereas others may need longer or may never recover.