Phytophthora lateralis is a pathogen that generally attacks and kills the roots of its host trees, although aerial infections of branches and foliage also occur. It spreads by its roots coming into contact with the spores in the soil or water. Planting infected plants, or using soil that is harbouring spores, are key pathways for introducing the disease into new sites. It can also be spread in water run-off. It kills the roots and can extend up the trunk of affected trees and girdle them, leading to the tree’s death.
There are two distinct 'lineages' of P. lateralis present in the UK, one that occurs in the Pacific north west of the USA and another that has to date only been found in Scotland.
Outbreaks of inection have been confirmed in Lawson cypress trees (Chamaecyparis lawsoniana) in South-West-England, Yorkshire, Scotland and Northern Ireland.
P. lateralis was found for the first time in Great Britain in 2010 at Balloch Castle Country Park in West Dunbartonshire, Scotland, although there have been recent outbreaks in other European countries, including France and The Netherlands. Other outbreak sites in Great Britain include Greenock Cemetery in Inverclyde, Yorkshire Water land at Blubberhouses, near Otley in North Yorkshire, and one in Kintyre, Scotland.
One site has tested positive for infection near Aberdare in Wales. The infected trees were felled by Merthyr Council. Additional trees on the site were sampled in JUly 2012 and there were no further positives.
A further positive was reported in February 2013 in a site in east Sussex. The trees were around 15 years old and have since been felled. The source of the infection is not known although the site is close to a busy road. Further inspection and monitoring is planned.
P. lateralis usually kills most Lawson cypress trees that it infects, and is the main cause of mortality in Lawson cypress in its native range in south-western Oregon and north-western California.
The foliage of infected Lawson cypress trees initially turns a light olive-grey; later on it withers and turns a reddish-brown colour, then the tree dies. As the pathogen extends from the roots and root collar up the trunks of affected trees, tongues of killed inner bark become visible by their darker colour, and the entire trunk can be girdled.
Lawson cypress trees are the only species of tree likely to be infected by P. lateralis in Britain, but they are easily confused with other tree species of similar appearance. It is therefore important first of all to be sure that the affected tree is indeed a Lawson cypress, and this guide to identifying Lawson cypress trees and P. lateralis symptoms, and this factsheet , can help.
It mostly occurs in western North America, where Lawson cypress is a native species, including the states of California, Oregon, Washington and British Columbia. However, it is thought to have originated in Asia, and was recently found associated with yellow cedar (Chamaecyparis obtusa var. formosana) in Taiwan. Outbreaks were also recorded in France in 2009 and the Netherlands in 2010.
On site, the dead and dying trees will be felled and destroyed, either by burning, or chipping and deep burial provided no other Lawson cypress are in the vicinity. Where appropriate, disinfectant mats are placed at exit points from public sites, and staff and visitors are asked to use them to kill any spores from the pathogen that they might pick up on their footwear. Notices are also erected at the public sites to inform visitors of the infection and to encourage them to observe sensible biosecurity measures such as keeping to the footpaths, keeping dogs on leads, and not taking cuttings from plants in the park.
Overall, we are continuing to research the disease to gain as wide a knowledge and understanding of it as possible, and to try to trace the pathway by which it entered Britain. We are also surveying the surrounding areas to see whether it is present at any other location in the area.
About 27 native common yew trees (Taxus baccata) in Balloch Castle Country Park are also dying, but we believe the cause is another Phytophthora, P. cinnamomi, which is endemic in Great Britain. However, under conditions of natural infection, P. lateralis can affect other species, especially some other Chamaecyparis species and Pacific yew (Taxus brevifolia), a close relative of Britain's native common yew (Taxus baccata).
If, after consulting the above guidance, you are still sure it is a possible case, and it involves trees in a private garden, you should consult a reputable arborist. The Arboricultural Association, at www.trees.org.uk, can help you find a local one.
If it involves a number of trees in a public site such as a park or churchyard, with high numbers of visitors posing a higher risk of spreading the disease, contact the Forestry Commission at:
• for places north of the Humber-Mersey line:
Tree Health Diagnostic & Advisory Service, Forest Research, Roslin, Midlothian EH25 9SY; e-mail: firstname.lastname@example.org; tel: 0131 445 2176.
• for places south of the Humber-Mersey line:
Tree Health Diagnostic & Advisory Service, Forest Research, Alice Holt Lodge, Wrecclesham, Farnham, Surrey GU10 4LH; e-mail: email@example.com; tel: 01420 22255.
Is it harmful to humans or animals?
There is no evidence that it can harm humans or animals.
How serious is it? What damage could it do in Britain?
A lot depends on whether it becomes established here and how it behaves here. P. lateralis is a virulent pathogen that usually kills the trees that it infects, primarily Lawson cypress. Few trees recover from an attack. Lawson cypress is found throughout Britain, and we believe P. lateralis is capable of surviving in all parts of Britain.
It also affects Pacific yew, a native of North America and a relative of our native common yew. Although Pacific yew (T. brevifolia) is much less susceptible than Lawson cypress, and usually only succumbs when it is close to infected Lawson cypress trees producing a lot of spores, we don’t yet know whether our native yew is susceptible.
We estimate there are fewer than 2200ha in woodland in Great Britain overall. However, Lawson cypress and its many colourful cultivated varieties are popular for amenity planting in parks and gardens, and are among the most important conifers in the UK ornamental plant trade. This means it could represent a serious threat to the ornamental plant industry if it became established here.
Phytophthora ramorum is also present at Balloch and in South-West England - could these two hybridise, and if so what would be the consequences?
Yes, there is a possibility of this happening. P. lateralis and P. ramorum belong to the same sub-group of the large Phytophthora group of organisms, which means they might be able to exchange genetic material. This could involve anything ranging from genetic modification of P. lateralis or P. ramorum to formation of a new species of Phytophthora. It is impossible to predict what the consequences of hybridisation would be. Hybrids can prove to be little different from their 'parent' species; they might not be fit enough to survive for any length of time; or they might be more virulent and infect species of trees and plants that the 'parent' species are not currently known to affect.
How did it get to these places?
We have not yet been able to confirm the pathway by which the pathogen got to these sites, although provisional DNA sequencing indicates that the Greenock samples are distinctly different from the Balloch samples, suggesting (until further confirmation) that we might be dealing with independent introduction of this pathogen to Greenock, rather than a spread from Balloch.
How did it get into Britain?
Again, we do not know for sure, but the most likely route is that it entered Britain in the soil of the root balls of plants or semi-mature trees imported for planting, mostly in gardens.
Could it have spread out of these sites by now? And how?
Yes, that is possible. It could have spread on the footwear of people who visit or work in the infected sites, or in watercourses flowing through or by them. We encourage owners of Lawson cypress trees to check them for symptoms and tell us or a reputable arborsit if they think their trees might be infected.
Is there any treatment available for infected trees?
There are treatments, such as soil drenches, that can be effective in some limited situations, such as nurseries, but they are unlikely to be effective in the wider landscape, where their use would also raise a number of other environmental issues.
What is the pathogen’s official status?
P. lateralis is on the Alert List of the European & Mediterranean Plant Protection Organization (EPPO). It is not currently listed in the European Union’s Plant Health Directive.
Has a pest risk analysis been done for the UK?
More information about P. lateralis is available from the Forest Research website.
More information about Balloch Castle Country Park is available from the parks area of the West Dunbartonshire Council website.
Last updated: 17 May 2012