Xylella fastidiosa (X. fastidiosa) is a bacterium which causes disease in a wide range of woody commercial plants such as grapevine, citrus, olive and several species of broadleaf trees widely grown in the UK, as well as many herbaceous plants.
Xylella fastidiosa affects its host plants by invading their water-conducting systems, moving both upstream and downstream. 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. Although X. fastidiosa is not known to be present in the UK, there is a heightened risk of its being accidentally introduced since it was discovered in Italy in 2013 and Corsica and mainland France in 2015. (See Distribution.)
There are four known sub-species (ssp.) of the bacterium:
• Xylella fastidiosa ssp. multiplex, which can probably infect the widest range of host plants, including Britain’s native pedunculate oak (Quercus robur) and wych elm (Ulmus glabra), as well as plane (Platanus occidentalis) and northern red oak (Q. rubra)
• Xylella fastidiosa ssp. fastidiosa, which is found in Central and North America and Taiwan, and can affect grape vines and citrus, coffee and almond plants
• Xylella fastidiosa spp. pauca, which affects coffee, citrus and olives
• Xylella fastidiosa ssp. sandyi, which affects oleander plants in the USA
In nature, the pathogen is exclusively transmitted by xylem-fluid feeding insects from the Cicadellidae and Ceropidae families. There are several species of insects in the UK which could vector (spread) X. fastidiosa, 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 bacterium for the feeding insects, which can then transmit the bacterium to other hosts. There can also be some transfer of the bacterium between neighbouring plants via root grafts.
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 olives trees over a large area in southern Italy., In 2015, X. fastidiosa was identified in Corsica, initially in Polygala myrtifolia, but now in other hosts. However, infection has not been identified in olives in Corsica. To date, there has been one interception of X. fastidiosa in the UK in an ornamental coffee plant, which was destroyed.
The Italian outbreak on olive trees is caused by a strain related to ssp. pauca, which has also been intercepted on ornamental coffee plants imported into France, Germany, The Netherlands and Italy. In Corsica, the outbreak was found on imported plants of Spanish broom and milkwort which had been planted out, and was caused by the sub-species multiplex. The plants involved had been planted as early as 2007, and had been supplied by Corsican nurseries using plants from Italy and mainland France.
Symptoms range from leaf scorch (browning) 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. 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).
The visible symptoms on plane, maple (Acer), oak and elm trees 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 X. fastidiosa, including:
• infection of horse chestnut trees (Aesculus hippocastanum) by the Guignardia aesculi fungus, which causes a brown leaf blotch with a yellow halo. The same tree species can also be affected by horse chestnut leaf miner caterpillars (Cameraria ohridella), although these cause browning between the veins of the leaves rather than around the margins
• wilting and browning of the foliage of elm trees suffering from Dutch elm disease
• anthracnose on plane trees caused by the fungus Apoignomonia veneta, which results in twig death and leaf blight. Powdery mildew (Erisiphe platani) can also cause yellowing and distortion of young plane leaves
The pest is subject to EU emergency measures. The control strategy primarily aims to keep this bacterium out of the UK if possible. Pending landings of host species such as plane, elm and oak plants must be pre-notified to the UK plant health authorities to enable inspection.
Other regulations are in place (Appendix 3 of the Pest Risk Analysis) which restrict movements of specified host plants from the infected region of Apulia in southern Italy, and from third countries outside the EU, to reduce the risk of entry.
However, if it did become established in the UK, control would focus on the targeted removal of host plants and management of the vector insects’ habitats.
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.
The UK Plant Health Risk Register (Fera) has details of the pathogen’s risk rating and other information.
Although X. fastidiosa is not known to be present in the UK, there is a heightened risk of its being accidentally introduced since its discovery in Italy, Corsica and mainland France. We therefore urge the public, especially tree and plant professionals, to remain vigilant for signs of it, and to report suspicious trees to us. Xylella fastidiosa is a quarantine organism, so there is an obligation to report any trees suspected of being infected by it. Please report suspected cases to us with our Tree Alert on-line disease reporting form.
If the affected trees are horse chestnut, plane or elm trees, first check in the Symptoms section above whether the symptoms might be caused by other pests and diseases.
The University of California Xylella website has an extensive host list and current research findings.