- Susceptible species
- Outbreak stage/distribution
- Pest risk analysis
Elm yellows is a disease caused by a group of organisms called phytoplasmas, which are specialised forms of bacteria. When the phytoplasma infect elms, they cause a range of symptoms, depending on the host plant's level of susceptibility. In very susceptible elms the phloem (inner bark) of the tree is attacked (hence the other name of the diseases: elm phloem necrosis), effectively girdling and stopping the flow of water and nutrients.
In 2013 plant health authorities investigated a case of ‘elm yellows’ (EY) phytoplasma disease on elm plants imported into England. See 'Outbreak stage' below.
Elm yellows has been found in a range of plant species in North America, Europe and Asia, and in elm trees in Italy, France, the eastern USA, and southern Ontario in Canada.
Within the wider EY group, a particular form infects elm (Ulmus), and so is referred to as Candidatus Phytoplasma ulmi. It can affect several species of elm trees, including English elm (Ulmus procera). Our native elm species, Wych elm (U. glabra), has not been confirmed as a host, and is considered to be resistant to EY in America.
Infection can be very destructive to some elms, particularly North American species, where the disease is also known as elm phloem necrosis. However, it is thought that European elm species tend to be much less affected than the highly susceptible American elm (U. americana). This has been the experience elsewhere in the European Union (EU), including Italy, France and Germany.
Although Britain has lost tens of millions of elm trees to Dutch elm disease (DED), many millions remain. These are mostly young specimens of English elm which have regenerated from the surviving roots of trees which have died, and others which have grown from the seeds of other, less-susceptible elm species, some of which can produce a lot of seed at a young age. Sadly, many of these also eventually succumb to DED.
The fact that elm yellows phytoplasma can affect elm species which are resistant to DED and which are important for conservation purposes, means it is important to prevent these organisms from becoming established in Britain if at all possible. This will also help to prevent possible spread to other wild and cultivated hosts. Taking swift, robust action at the first opportunity is the right approach.
Elm yellows disease can be spread by insects such as leafhoppers, and by the movement of infected plants. Spread from infected trees by natural means is unlikely during the winter, when trees have no leaves and insect vectors are not active.
The only known treatment is to destroy infected trees before the disease can spread to healthy trees.
• epinasty (downward bending of leaves or other parts resulting from excessive growth of the upper side)
• yellowing, dwarfing and premature casting of leaves
• formation of ‘witches’ brooms’ at the tips of twigs and branches
• precocious (early) opening of buds.
Symptoms can easily be confused for symptoms of DED, which commonly begin to appear on young and old elm trees from late summer. Symptoms on newly planted material are unlikely to indicate DED, but could be due to another natural cause. Trees affected by DED will die back and die rapidly, whereas EY could be expected to cause symptoms which do not result in the death of the tree.
Other diseases can cause similar symptoms on elm, so laboratory testing is needed to confirm infection by elm yellows phytoplasma.
Images courtesy Eric Collin, IRSTEA
Several hundred plants are known to be associated with the infected stock found in 2013, and which had been imported from Italy in 2012.
Diseased plants were identified at two nurseries in England. Some plants were distributed in small numbers from these nurseries to individuals and organisations, and others were planted out at Forestry Commission sites and by conservation groups.
No evidence was found to suggest that the disease had spread into the wider natural environment, and we might not even have an effective vector, or agent of spread. Surveillance was undertaken in the summer and early autumn, when any phytoplasma present could have been be detected by testing.
The infected imported plants and others propagated from them which remained in nurseries were destroyed under Statutory Plant Health Notices. We also traced elm plants from the same sources. Susceptible plants close to infected trees were monitored for signs of spread into the wider environment.
Elm was added to the list of tree species covered by a statutory requirement to notify imports from other parts of the EU. This helps to gather intelligence about the trade, raise awareness, and facilitate targeted monitoring. APHA's Plant Health & Seeds Inspectorate (PHSI) visited the nurseries involved in trading elms, to raise awareness and encourage early reporting of suspected symptoms.
Targeted surveillance by the PHSI and the Forestry Commission continued to assess whether the pathogen is present elsewhere, and to assess the potential role of insect vectors which could spread the disease. This helped to determine whether additional measures were warranted and justified. Because elm is an unregulated species, not subject to routine surveillance, it is too early to say whether such measures would be beneficial.
The European Food Safety Agency (EFSA) has published a “pest categorisation” of EY, which is the starting point for an EU review of the regulatory status of this organism. This reflects a UK request for such a review following the UK finding. The review will provide an opportunity to consider possible strengthened measures against the phytoplasma, such as Protected Zone designation, which could be considered if EY is not found to be established during surveillance.
The steps that would be taken in the event of an outbreak of the disease in Great Britain are set out in our Elm Yellows Contingency Plan.
The disease was first recorded in the United States. The elm plants involved in the first UK case were specimens especially bred in continental Europe for resistance to DED, and imported for use in projects to protect rare butterflies which depend on elm trees. EY has been reported in parts of continental Europe, so it is possible that they were exposed to the causative phytoplasma organism. The material had been used for further propagation since importation in 2012.
Because these resistant elms might have considerable conservation value for some rare butterfly species, the arrival and spread of EY in the UK could undermine conservation work to use these trees in this context, and also provide a pathway for EY to spread to native and naturalised elms in the UK.
There are general EU requirements aimed at preventing the introduction and spread of the pathogen in the EU. There are also specific requirements for elm planting material imported from outside the EU to prevent infected trees being imported. The trees involved in the UK finding were imported from within the EU, where outbreaks have been recorded.
The movement of elm plants within the EU is not regulated under plant health law. However, because the causal organism is regulated, with an obligation to prevent spread, the UK Government took action to require prior notification of imports of elm material from 6 May 2014.
The status of this organism needs to be reviewed at EU level, and we have provided details of these findings to the European Commission. An EFSA “pest categorisation” will provide the basis for initiating an EU review.
There are already well established elm breeding programmes in France, the Netherlands, Italy and the United States. Disease-free material from these programmes can be imported and propagated in the UK. In light of this development, we encourage the trade to carefully check the sourcing of plants they receive, to enhance their knowledge of the disease, and to report any suspected symptoms to the plant health authorities at an early stage.
A Pest Risk Analysis has been published on the Fera website.
Suspected cases in nurseries should be reported to:
- England and Wales - APHA
- Scotland - Scottish Government Horticultural & Marketing Unit.
Tree-care professionals are asked to look out for and report suspected cases in the wider environment using our Tree Alert form