The bacterium Xylella fastidiosa is native to the Americas where it causes disease in many important crops including citrus, coffee and grapevine. Until recently Xylella was absent from Europe but in 2013 the bacterium was identified as the cause of death of olive trees in southern Italy. There are now major outbreaks on ornamental plants in southern France (including Corsica), the Balearic Islands (Ibiza, Mallorca and Menorca) and southern Spain and most recently in Portugal, in Lisbon and the Algarve. 

Xylella infects a wide range of plants including many popular species grown in gardens, such as cherry, hebe, lavender and rosemary. The bacterium is transmitted between plants via insects which feed on plant

Sap is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

The bacterium causes a variety of symptoms which can include leaf scorch, wilt, dieback and plant death. Click this link to see image galleries of symptoms in Europe.

Symptoms of Xylella can be identical to symptoms caused by drought or other stresses such as frost damage, disease or establishment problems. Confirmation of Xylella infection requires laboratory testing. When a plant displays symptoms which are suggestive of Xylella there is therefore a need to prioritise and assess the risk posed by the plant showing these symptoms.

UK-grown plants or seeds pose a very low risk. Plants established in your garden for more than 5 years which have been previously healthy are also low risk. If you are importing plants from outside the UK then this carries a higher risk for plant diseases including Xylella.

High risk plants

Defra have identified nine Xylella high risk plant hosts in the UK. For more information on the identification of these hosts and the symptoms of Xylella infection to look out for please click on the names below:

• Polygala myrtifolia (polygala)
• Olea europaea (olive)
• Salvia rosmarinus (rosemary)
• Lavandula spp. (lavender)
• Prunus spp. (plums, cherries, almonds etc.)
• Nerium oleander (oleander)
• Coffea (coffee)
• Shrubby Veronica spp. (also known as hebe)
• Spartium junceum (Spanish broom)

Other important hosts include:

• Acer spp. (maple)
• Fraxinus spp. (ash)
• Platanus spp. (plane)
• Quercus spp. (oak)
• Ulmus spp. (elm)
• Vaccinium spp. (blueberries)
• Vitis spp. (grapevine)

A full and current list of Xylella host plants in Europe is available here.

1. Is the plant displaying symptoms which are typical for Xylella infection?
Check the image galleries of symptoms against your plants where you have concerns.

2. Where has the plant come from?

If the plant is UK sourced and grown (or grown from seed in the UK) then it poses a low risk for Xylella. Xylella has not been detected in the UK.

If the plant has been sourced from a region near a Xylella outbreak then the plant poses a higher risk. Outbreak areas in Europe currently include Italy (Apulia, Tuscany & Lazio);  Southern France (Provence-Alpes-Côte d’Azur, Monaco, Occitanie and Corsica); Portugal (Algarve, Lisbon and Porto) Spain (Valencia, Madrid and Balearics).

3. How was the plant imported?

Professional routes for import require a combination of inspection, notification and correct documentation which reduces the risk of infected plants entering the UK. However, if the plant was imported into the UK by an individual passenger in their luggage then the risk of undetected infection is likely to be higher. When you buy plants at a nursery or online it is best to use a known reputable source.

4. How long ago was the plant imported?

Most infected plants will show symptoms within a few years (if they are symptomatic hosts). For example, an olive tree imported 3 years ago would have most likely already displayed symptoms if it was infected with Xylella.

5. Is the plant a known Xylella host plant in Europe?

Has this plant been recorded in Europe as susceptible to Xylella infection?

A current list of Xylella host plants in Europe is available here.

Please do not send samples of suspected Xylella to the RHS

If you suspect that Xylella fastidiosa could be present in your garden you should not attempt to control the disease yourself. Collect together all available details including the host plant name, symptoms, origin, and import history and report your suspicions to the relevant plant health authority whose details can be found on the UK Plant Health Information Portal.

If you are an RHS member and have a plant health concern, please contact us via the Gardening Advice Service.

What happens if Xylella arrives in the UK?

If Xylella is confirmed in the UK, the UK government will implement measures based on retained EU law for control of Xylella.

If infection is detected at an early stage and is not thought to have spread, the infection will be classed as an ‘interception’. In the case of an ‘interception’ the infected plants will be destroyed, host plants in close proximity will also be destroyed, and further surveys of plants and potential vectors within a 200m radius for a minimum of two years will be undertaken.

If the infection is thought to have spread beyond the initial infection point then it will be classed as an ‘outbreak’ and more severe containment procedures will be followed. Control measures following diagnosis of an ‘outbreak’ include: destruction of host plants within 100m, a buffer zone of 2.5km radius with restricted movement and planting of ‘specified’ plants for a minumum of four years after the last finding of the pathogen, and control of the insects which spread the disease.

Actions taken in the event of an outbreak would have a significant impact on local landscapes and businesses, but the severity of the damage caused by Xylella if it established in the UK is not possible to predict.

Three subspecies of Xylella have been detected in Europe. Xylella fastidiosa subspecies multiplex, which has been found in a number of sites in Spain and France, is thought to pose the highest risk to the UK. This subspecies has higher climatic tolerance in cooler temperate regions and has the widest host range of the subspecies.

What is the RHS doing?

Prevention is better than a cure and the RHS working alongside the UK government and horticultural industry to prevent the introduction of Xylella into the UK.

The RHS is advising its staff and home gardeners through numerous media outputs; we are continuing to work closely with the UK plant health service; we are ensuring exhibitors at RHS shows are educated about plant health threats; and high risk plants are held in reception areas on entry to RHS gardens and inspected for disease.

All RHS Plant Centres have signed up to the industry (Horticultural Trade Association) best practice guidelines on sourcing plant material to reduce the risk of Xylella arriving in the UK. The RHS is also funding and co-supervising a PhD with Imperial College London to understand how to communicate the risks posed by Xylella with the public and policy-makers.