What is it?
Xylella fastidiosa is a bacterium that lives in the plant’s ‘xylem’ (water channels). It causes disease by restricting or blocking the water and nourishment from travelling throughout the plant - thus causing the plant to rapidly wilt and die from lack of water. The bacteria are carried and spread by insects which feed in the plant sap.
It was first seen in the 1890s, in the grapevines of California. One hundred years later the bacteria were rapidly and widely spread by the aptly named insect, the Glassy-Winged Sharpshooter. As the insect feeds on an infected plant, X. fastidiosa attaches itself to the insect's mouth. The sharpshooter, an active and voracious feeder, then transmits the disease to additional plants while feeding and laying its eggs. Unfortunately plants can hold the fastidiosa bacteria, even if they are not affected by any of the diseases caused by X. fastidiosa. They act as 'reservoirs' for other insects to pick up the bacteria and carry them to other plants.
Where is it?
By the end of the 20th century, the bacterium had surfaced in Brazil, causing citrus variegated chlorosis disease (CVC).
In October 2013 it was found for the first time in Europe, in the olive trees of Apulia in southern Italy. Within 18 months it had spread throughout Southern Italy, affecting up to 1m trees. Almond and oleander plants in the region also tested positive for the pathogen, and probably acted as 'reservoirs'. Because the bacteria flourished in olive trees and in warm conditions it was thought unlikely to spread into Northern Europe.
But by July 2015, Xylella fastidiosa had reached Corsica. Three months later, a new subspecies affected the popular ornamental, Polygala myrtifolia, in mainland France, near Nice. The spread was relentless. In August 2016, the bacterium was detected in Germany in an oleander plant. By January 2017 it appeared in Majorca and Ibiza, and in June 2017, it was detected Spain. Polygala and Oleander are among many species widely propagated in warmer parts of Europe for sale worldwide. Now the whole of Europe is on high alert to try and contain the bacteria, impacting the plant trade.
How big a problem is it?
There is no doubt the spread and devastation of Xylella is a potential disaster. Its ability to infect a wide range of plants, and to ‘hide’ in symptomless plants makes the spread of the disease particularly difficult to control. It could be termed the 'foot and mouth' of the plant world.
There are numerous subspecies of X. fastidiosa affecting many different hosts. In Europe, 359 different species of plant have been shown to be susceptible, including grape, peach, citrus, olive, oak, sycamore, euphorbia, hebe, lavender and rosemary. Of the four known sub-species of the bacterium, it is Xylella fastidiosa ssp. multiplex, which could probably infect Britain’s native oak and wych elm, as well as plane and northern red oak.
There are several species of insects in the UK which could spread X. fastidiosa, including the common froghopper or cuckoo-spit insect (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.
There can also be some transfer of the bacterium between neighbouring plants via root grafts.
Can it be treated/prevented?
In Italy, various formulations of copper and zinc as spray or root drench were used on olive trees, as well as frequent application of natural manures and feeds. Despite reports of vigorous new growth in diseased trees, research confirmed the continued presence of X. fastidiosa. This indicates that there is no way yet known to eliminate the pathogen from the plant tissues. (The positive response of the treated olive trees is most probably due to the effect of concerted good husbandry, which as any organic grower knows, improves the vigour of the plants and their resilience to stress caused by bacterial infections.)
Can we prevent it arriving in the UK?
The disease, which could be deadly to our oak trees, would be difficult to eradicate if it was to arrive. Certain EU emergency control measures have already been put in place. The aim is to keep the bacterium out of the UK if possible. Therefore:
- All plant importers have to show evidence that their plants are sourced from areas that are free from Xylella.
- Proposed imports of host species such as plane, elm and oak plants must be pre-notified to the UK plant health authorities to enable inspection This will allow a sequence of spot checks at the UK borders.
- Other regulations are in place that restrict movements of specified host plants from the infected region of Apulia in southern Italy, and from 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. An outbreak (as opposed to an isolated incidence) would mean eradication of all possible hosts within 100m of the outbreak and very tight restrictions on commercial plant producers or garden centres within 10km of the outbreak for 10 years.
What plants are at highest risk of Xylella?
DEFRA has currently identified the following high risk host plants:
- Acer rubrum L.
- Catharanthus roseus (L.) G.Don
- Citrus sinensis (Linnaeus) Osbeck
- Coffea L.
- Gramineae Adans., Nom. Cons.
- Medicago sativa L.
- Morus rubra L.
- Nerium oleander L.
- Platanus occidentalis L.
- Prunus L.
- Prunus persica Batsch
- Quercus rubra L.
- Ulmus americana L.
- Vaccinium L.
- Vinca minor L.
- Vitis L.
- Woody plants
- Liliaceae (family)
Diagnosis of Xylella is not easy, as many other problems can cause similar symptoms.
Here is some helpful advice from the Forestry Commission on how to spot Xylella in trees:
*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.
If you think you have spotted Xylella, you must report it immediately.
For garden plants, contact the UK Gmt. Animal and Plant Health Agency APHA, or the Scottish Government’s Horticulture Unit. For trees, go to the Forestry Commission Tree Alert on-line disease reporting form. But if the affected trees are horse chestnut, plane or elm trees, first check in the Symptoms below for other possible pests and diseases.
A number of other disorders can produce symptoms similar to those caused by X. fastidiosa, including:
- infection of horse chestnut trees 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, 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.*