Xylella fastidiosa and its implications for UK horticulture

Xylella fastidiosa is a bacterial pathogen that can cause severe damage to a wide range of plant hosts. The pathogen is not present in the UK but has been confirmed in parts of Southern Europe and is endemic in the Americas.

In 2013, the pathogen was reported in olive trees in Italy; in 2015, in France, in Polygala myrtifolia and in Spain, in both 2016 and 2017, in various plant subjects. Other odd outbreaks on imported plant material across Europe have also been recorded, which were subsequently quickly eradicated.

In the event of an outbreak of Xylella fastidiosa in the UK, there would be a significant impact on local horticultural businesses.

Emergency EU legislation demands that, in the case of an outbreak, all host plants within 100 m must be destroyed and the area treated to control the insects responsible for transmitting the disease. There will also be measures placed on businesses trading host plants within a 5 km radius for up to five years.

Plant health strategies imposed by the Animal and Plant Health Agency (APHA) to reduce the risk of the pathogen being imported from affected countries through infected host plants remains the most important way to prevent a disease outbreak in the UK.

AHDB is continuing to review the latest research conducted in countries where the disease is already present and are in contact with APHA to monitor developments.

Xylella fastidiosa is a bacterial pathogen which affects the water-conducting tissues of the plants it infects.

X.fastidiosais a diverse species, subdivided into at least six subspecies (ssp.), but the three main subspecies include X. fastidiosassp. fastidiosa, ssp. pauca and ssp. Multiplex – the latter being the main threat in the UK due to its climatic requirements.

Symptoms include leaf scorch, chlorosis, wilt, poor-quality fruit, dieback and eventually death, many of which can be confused with other disease or disorder symptoms.

The pathogen can infect a wide range of plants, with over 350 species from 75 families already identified as hosts.

Lists of important commercial plant species have been circulated by Defra/APHA.

Unlike many other bacterial diseases which are dispersed by water splash, X. fastidiosa is dispersed over short distances by a large range of sap-feeding insects.

A review of potential insect vectors in the UK identified 18 species in four families. In Europe, only the meadow spittle bug is confirmed to be a vector of the pathogen. It is very common and widespread in the UK and feeds on a large range of mainly herbaceous plant hosts, but it can move onto trees in late summer, in dry years.

The majority of the other potential UK insect vectors, including leafhoppers and froghoppers, are commonly recorded in grasslands and marshes and are therefore less likely to be found near commercial nurseries, thereby posing less of a risk as a pathogen vector.

Long-distance dispersal is via the movement of infected propagation and finished plant material, either within countries or internationally.

Anyone importing host plants from the EU needs to ensure that the plants are accompanied by a valid plant passport confirming they have been sourced from disease-free areas.

From 1 March 2018, there have been additional measures for high-risk hosts, including: annual inspections of production sites, plant sampling and testing prior to plant movement and the requirement to hold records for three years.

Imported susceptible host plants should ideally be held in a quarantine area well away from production areas and inspected for any disease symptoms. If potential disease symptoms are noted, your local APHA Plant Health Inspector must be contacted immediately. If confirmed, plant material will be destroyed and surveys undertaken to determine if the pathogen has been intercepted, to prevent any outbreak.

Pathogen detection and identification of X. fastidiosa is currently based on DNA analysis techniques. Research is being undertaken to develop sampling procedures for the early detection of the pathogen in plants which are free of disease symptoms and to evaluate new diagnostic methods for use in both the laboratory and field.

No single approach is likely to be sufficient to control the pathogen, and a range of management options are likely to be needed, including:

• Ensuring imported plant material is disease-free
• Adoption of appropriate cultural practices depending upon the crop, including procedures to isolate imported host plants
• Introduction of effective monitoring practices by staff members
• Efficient use of environmentally friendly vector-control measures (to be devised)

Plant protection products and other measures

Research has been carried out in Bari, Italy, to examine the control of the meadow spittle bug in olive plantations with a range of conventional insecticides, bioinsecticides and other substances. Neonicotinoid and pyrethroid insecticides were found to have the highest efficacy against juvenile stages and adult populations. An assassin bug was also assessed as a potential biocontrol for the vector by other researchers in Italy.

Resistance breeding

Resistance breeding could provide a long-term control measure for some of the more important crops grown either in larger numbers or over wider areas. Molecular and classical breeding techniques have been used to develop resistance in grapes in California while maintaining grape quality, and olive varieties are currently being screened for potential resistance traits.

Disease information

AHDB Review of bacterial pathogens of economic importance to UK crops – Xylella fastidiosa supplement

Defra Plant Pest Factsheet

EU Plant Health and Biosecurity

APHA: Xylella – what the future holds? – Presentation from AHDB Ornamentals Conference, February 2018. Please note the presentation was correct at the time of writing and hosts and areas affected may change with time.

Trade implications

Defra UK plant health guidance Xylella fastidiosa: Information about controls for importers and users of trees, shrubs and herbaceous plants

Defra consolidated EU Xylella fastidiosa host list: List of plants which require a plant passport

HTA Xylella fastidiosa: What you need to know

Research

European Xylella Conference 2017

XF actors – research project into Xylella