Not present in UK
Notifiable – see ‘Report a sighting’ below
Scientific name of causal agent - Xylella fastidiosa, including sub-species
Xylella is a plant disease which can affect several species of broadleaf trees widely grown in the UK, such as oak, elm and plane, as well as a wide range of other commercially grown plants. Among them are grape, citrus, coffee, olive, almond and blueberry species, and many herbaceous plant species.
It is caused by a bacterium called Xylella fastidiosa (X. fastidiosa), which has four known sub-species. It is a quarantine organism, which means suspected sightings must be reported immediately to the plant health authorities.
Xylella is the generic name for a range of diseases which the bacterium causes on different plants: see 'Additional resources' below for more information about these.
X. fastidiosa has been found in the wider environment (that is, outside trade premises) in France, Spain, Italy, the Americas and Taiwan. Although an infected imported plant has been intercepted in the United Kingdom, it is not known to be present in the wider environment here.
Xylella has the potential to cause damage - and in some cases, significant damage - to a wide range of plants grown in the UK if it became established here. This would have implications for the biodiversity and ecology of woodland and other habitats. Many of the plant species it can affect are grown commercially, so there could be economic implications for our plant-based industries.
Xylella’s threat arises from the virulence of certain sub-species on certain plant species, and the fact that its four sub-species between them can affect a very wide range of plants.
There has been a heightened risk of its being accidentally introduced to the UK since its discovery in mainland Italy in 2013 and, since then, in France and Spain, including Corsica and the Balearic Islands. It has caused the compulsory destruction of tens of thousands of commercial olive trees in southern Italy in an attempt to stop it spreading further afield.
X. fastidiosa affects its host plants by invading their water-conducting systems, moving both up and down the plant. In so doing, it restricts or blocks the movement of water and nutrients through the plant, with serious consequences, including death, for some host plants.
The different sub-species of the X. fastidiosa bacterium can infect different plant species. Listed against each sub-species below are some of the main host plants.
- Xylella fastidiosa sub-species multiplex can infect probably the widest range of host plants, including Britain’s native pedunculate oak (Quercus robur) and wych elm (Ulmus glabra), plane (Platanus occidentalis) and northern red oak (Q. rubra).
- X. fastidiosa ssp. fastidiosa, which is found in Central and North America and Taiwan, can affect grape vines and citrus, coffee and almond plants.
- X. fastidiosa spp. pauca can affect coffee, citrus and olive plants.
- X. fastidiosa ssp. Sandyi can affect oleander plants in the USA.
See 'Additional resources' below for more extensive lists of host plants.
Symptoms and identification
Symptoms range from leaf scorch, or browning (below) to dieback and death. Symptoms vary depending on the host plant species and its degree of susceptibility, but include marginal leaf scorch, wilting of foliage and withering of branches.
Picture: Sandra Jensen, Cornell University, Bugwood.org.
Severe infections in some of the most damaging combinations of host plant and Xylella sub-species can result in dieback, stunting and eventual death, e.g. with olive trees or grape vines (on which it is known as Pierce’s disease).
On plane (Platanus species), maple (Acer spp.), oak (Quercus spp.) and elm (Ulmus spp.) trees the symptoms include leaf scorch, sometimes also with dieback of twigs and branches. The characteristic leaf symptoms which are visible in summer include browning at the leaf margins (but not along the main veins), and there is often a yellow edge to the browned areas.
However, a number of other disorders can produce symptoms similar to those caused by Xylella, including:
- wilting and browning of the foliage of elm trees suffering from Dutch elm disease, caused by the fungus Ophiostoma novo-ulmi;
- anthracnose on plane trees, caused by the fungus Apiognomonia veneta, which results in twig death and leaf blight;
- powdery mildew on plane trees, caused by the mildew Erysiphe platani, which can also cause yellowing and distortion of young leaves;
- leaf blotch of horse chestnut trees (Aesculus hippocastanum), caused by the Guignardia aesculi fungus. The brown leaf blotch typically has a yellow halo; and
- browning of horse chestnut leaves caused by feeding by horse chestnut leaf miner caterpillars (Cameraria ohridella), although these cause browning between the veins of the leaves rather than around the margins.
The following documents contain further guidance to recognising the symptoms of the disease.
The European & Mediterranean Plant Protection Organization (EPPO) website has photographs of symptoms on a variety of plant species.
Report a sighting
Although Xylella is not currently known to be present in the UK, we must remain vigilant to the possibility of accidental introductions. This has become especially important since its discovery in some of our near neighbours and trading partners in continental Europe. We particularly encourage tree and plant professionals to be vigilant and report suspected sightings.
- Report sightings in Great Britain to us using Tree Alert.
- Report sightings in Northern Ireland to the Irish forestry authorities using TreeCheck, the all-Ireland tree disease reporting tool.
Please note that Tree Alert and TreeCheck both require photographs to be uploaded. These should be clear, well-lit, close-up pictures of symptoms.
If the affected trees are horse chestnut, plane or elm trees, first check in the ‘Identification and Symptoms’ section above whether the symptoms might be caused by other diseases or pests.
In nature, the bacteria are exclusively transmitted by insects from the Cicadellidae and Ceropidae families, which feed on plants' xylem fluid. There are several species of insects in the UK which could vector (spread) X. fastidiosa bacteria, including the common froghopper (Philaenus spumarius). Although such insects usually only fly short distances of up to 100 metres, they can be carried much longer distances by the wind.
Long-distance spread can occur by the movement of infected plants for planting. These plants can act as a source of the bacteria for the feeding insects, which can then transmit it to other hosts.
There can also be some transfer of bacteria between neighbouring plants via root grafts.
Control and official action
Xylella is subject to European Union emergency measures. The control strategy primarily aims to keep the bacterium out of the UK and other currently unaffected Member States if possible. Pending landings in the UK of Xylella host species, such as plane, elm and oak plants, must be pre-notified to the plant health authorities to enable inspection.
Other regulations are in place which restrict movements of specified host plants from the infected region of Apulia in southern Italy, and from third (non-EU) countries, to reduce the risk of entry. Additional EU protection against the risk of Xylella spreading includes movement restrictions on high-risk plants, and rapid responses to suspected findings of the disease.
Plant importers and other plant professionals who need information about the import controls applying to plant species susceptible to Xylella infection should contact:
- in England and Wales - the Animal and Plant Health Agency (Apha), tel: 01904 405138;
- in Scotland - the Scottish Government Horticulture & Marketing Unit; tel: 0131 244 8923; and
- in Northern Ireland - the DAERA Plant Health Inspection Branch, tel: 0300 200 7847.
If Xylella were to spread to the UK, control would focus on the targeted removal of host plants and management of the vector insects’ habitats, in accordance with the government's contingency plan.
This Guidance for the nursery and plant importing industries includes details of the control measures which would be taken if the disease were found in the UK.
We are playing a key role in a research partnership called BRIGIT - A consortium for enhancing UK surveillance and response to Xylella fastidiosa. The aim of the BRIGIT project is to enhance diagnostics, identify factors that could lead to spread, and formulate response strategies to mitigate the devastating effects of this disease.
Origins and background
Until 2013, X. fastidiosa was only known in the Americas and Taiwan. However, that year it was found to be associated with the rapid decline of olive trees over a large area in southern Italy. Xylella was then identified in Corsica in 2015, initially on myrtle-leaf milkwort (Polygala myrtifolia), but since then on other host plants.
To date, there has been one interception of X. fastidiosa in the UK, on an ornamental coffee plant, which was destroyed.
The Italian outbreak on olive trees is caused by a strain related to the pauca sub-species, which has also been intercepted on ornamental coffee plants imported into France, Germany, The Netherlands and Italy.
The Corsican outbreak was found on imported plants of Spanish broom (Spartium junceum) and milkwort which had been planted out, and was caused by the multiplex sub-species. The plants involved had been planted as early as 2007, and had been supplied by Corsican nurseries using plants from Italy and mainland France.
An outbreak in Germany was declared eradicated in 2018.