According to the experts, Xylella fastidiosa’s arrival in the UK is a question of when, not if. So with that in mind, what can we reasonably expect would happen next?

A WHIRLWIND of alien insect pests and microbial pathogens continues to sweep the UK. There have been unmitigated disasters such as sudden larch death and Chalara ash dieback, near misses like Asian longhorn beetle and ongoing sagas such as Ips typographus. However, there remains a largely unmentioned elephant in the room caused by a microscopic plant pathogenic bacterium which few want to talk about in detail. Some mention it in passing, the latest being Dr Matt Elliot in the March 2024 issue of Forestry Journal

Xylella fastidiosa is a plant pathogenic bacterium which exists as a series of sub-species, having a collective host range exceeding 400 different plant species. It is currently present in a number of European countries including Spain, Italy, Portugal and France. Among its most notable achievements are the near destruction of 21 million olive trees and the olive oil industry in Puglia, Italy, and very significant damage to almond production around Alicante, Valencia in Spain.

Xylella fastidiosa has all the hallmarks of a potential pandemic creator – a huge collective host range, lethal consequences and no effective means of control. 

One more attribute opens up all sorts of implications for farmers, growers and foresters. Xylella fastidiosa is carried by common sap-feeding insects. As such, a huge part of disease management comes down to the wholesale destruction of wild and cultivated plants tapped and fed on by these insects, covering an almost limitless range and number of plants including trees. 


The extent of vegetation destruction required means the amount of damage caused by plant health authorities will be as extensive (if not more so) as any damage done directly by the bacterial pathogen. No wonder nobody wants to talk about the disease, because the potential consequences for the UK’s primary plant industries are truly horrendous.

The fabric of the following article was prepared exactly six years ago, in March 2018, but was never published. At the time, the UK was still a member of the European Union and bound by the emergency measures drawn up for Xylella fastidiosa at the time. They still exist today, essentially unchanged. The UK is no longer in the EU, but would almost certainly have to set in motion measures of the same scope and severity as those established by the EU.


Forestry Journal: the combined group of Xylella subspecies (strains) has a wide and large host range. The pathogen first appeared in Portugal as Xylella fastidiosa subsp. multiplex on lavender.the combined group of Xylella subspecies (strains) has a wide and large host range. The pathogen first appeared in Portugal as Xylella fastidiosa subsp. multiplex on lavender. (Image: FJ)

According to UK government health authorities (APHA – Animal and Plant Health Authority) and non-government organisations (Royal Horticultural Society), Xylella fastidiosa will arrive in the UK, the only question being when. So if there was an outbreak of Xylella fastidiosa, what would happen next? 

The following snapshot of EU emergency measures on the statute book for Xylella fastidiosa in 2018 remains essentially unchanged. It provides an insight into the potential seriousness of the pathogen, the scope of measures required should it be confirmed in the UK and the extent of collateral damage expected for plant-related businesses and the environment.

Regulated plant species 
The EU considers two categories of regulated plant species:
• Host plants – plants for planting (other than seeds) belonging to genera and species recorded as susceptible to X. fastidiosa on EU territory.
• Specified plants – as above but recorded as infected anywhere in the world outside the EU.

Eradication measures
Confirmation of X. fastidiosa in any EU country requires immediate establishment of a demarcated area as follows:
• The infected zone covering all infected plants, plants with symptoms indicative of infection, and all other plants liable to infection due to close proximity or having come from a common source with the infected plants or plants grown from them.
• Beyond there is a buffer zone of 10 km or 5 km for outbreaks subject to containment measures or eradication measures, respectively. And 1 km for isolated outbreaks where no natural spread has occurred and eradication measures have been immediately taken.

Plant removal and testing
• Phytosanitary treatments to be applied against all potential vectors of X. fastidiosa within a 100 m radius of the infected plants.
• Removal of all host plants of the Xylella subspecies concerned within the 100 m radius and irrespective of health status.
• Sampling and testing for the pathogenic bacterium in all specified plants within the 100 m radius.

Confounding factors
• The number of plant genera and species proven susceptible to infection by X. fastidiosa was constantly rising, with common walnut (Juglans regia) and Morello cherry (Prunus cerasus) among those added to the list in 2018. 
• The Forestry Commission says there are four known subspecies of X. fastidiosa – multiplex, fastidiosa, pauca and sandyi. However, United States sources including NCBI (National Centre for Biotechnology Information) describe additional subspecies such as ‘morus’ to which Morus (mulberry) is susceptible.
• For reasons of climate Xylella fastidiosa subsp. multiplex is the only one that could realistically establish in the UK, but this subspecies has the widest host range which includes oaks, elms and plane trees. It is worth remembering that Xylella fastidiosa subsp. pauca which has been decimating olive groves in Italy was introduced into this southern European country on ornamental coffee plants imported from tropical Central America.
• Xylella fastidiosa is defined as a ‘xylem restricted’ plant pathogen vectored (carried and transmitted) by xylem sap feeders belonging to the insect families Cicadellidae (leafhoppers) and Cercopidae (froghoppers). This makes Philaenus spumarius (meadow spittle bug or meadow froghopper) the most ubiquitous and frequent potential vector insect for the UK. 
• However, research carried out in Italy into X. fastidiosa on olive has also identified X. fastidiosa in Euscelis lineolatus. This is a phloem-feeding leafhopper (Cicadellidae) thus indicating a potential vectoring role for phloem sap-feeding insects. Euscelis lineolatus is spread throughout England and Wales with a number of common wild grass species among known hosts.
• Phytosanitary treatment against vector insects will almost certainly mean spraying with ‘hard’ chemical insecticide which will open a whole new ‘can of worms’. Will the UK plant authorities recommend a contact insecticide (kills the insect on contact) or a systemic insecticide which enters the plant (via roots or leaves or both) to kill the insect when it sucks the sap? Which non-target insects and other arthropods will be affected?
• The number of legally approved insecticides is shrinking all of the time. Should a suitable approved insecticide product (e.g. a systemic insecticide transported in the xylem) not be available, would the UK plant health authorities sanction the use of a previously banned insecticide product, even if environmental damage was the reason for its withdrawal? 
• Given the potential for serious, long-term damage should X. fastidiosa gain a strong foothold in the UK will the plant health authorities ignore some generally adhered to safety measures, such as not spraying pesticides near water courses or felling trees and grubbing shrubs during the bird nesting season? 


Forestry Journal: Xylella fastidiosa subsp. pauca has virtually destroyed olive growing within the Puglia province of Italy, which traditionally produced 50 per cent of Italy’s olive oil.Xylella fastidiosa subsp. pauca has virtually destroyed olive growing within the Puglia province of Italy, which traditionally produced 50 per cent of Italy’s olive oil. (Image: FJ)

Losses for plant-based industries and businesses will not stop there. The EU says uncertainty surrounding host plants affected by X. fastidiosa in ‘European Union Territory’ means that emergency measures restricting the movement of plant material out of demarcated areas, and from infected zones into respective buffer zones, will be forced to include a long list of specified plants, which currently covers 200 species across 35 genera. Movement of specified plants is only permitted when they fulfil certain conditions (e.g. are grown under protected conditions, have been sampled and tested for X. fastidiosa prior to movement and plant traceability requirements are satisfied). 

The EU does not specify compass and time span of such restrictions, but reliable sources within the UK, including APHA and HTA (Horticultural Trades Association), have talked about 5–10 km for a 5–10-year period. The EU does not mention the words ‘Exclusion Zone’, but some of these same UK sources do, adding that such restrictions would prove ‘business busting’ in their severity.