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Phytophthora lateralis

Introduction

Phytophthora lateralis (P. lateralis) is a pathogen which generally attacks and kills the roots of its host trees, although aerial infections of branches and foliage also occur.

Thought to originate in Asia, P. lateralis is the main cause of mortality in Lawson cypress (Chamaecyparis lawsoniana) in its native range in the West Coast region of North America, where the disease is thought to have been present since 1923. Nursery outbreaks were recorded in France in 1996, 1998 and the Netherlands in 2004, and have since been eradicated. Further outbreaks were discovered in France and the Netherlands in 2010, about the same time that the disease was first discovered in the UK.

There are two distinct 'lineages' of P. lateralis present in the UK: one which occurs in the Pacific North-West of the USA, and another which has to date been found only in Scotland.

Distribution

Confirmed findings at 28 April 2014

P. lateralis outbreak map
Large size map (PDF)

Outbreaks of infection have been confirmed in Lawson cypress trees in South-West England, Yorkshire, Scotland and Northern Ireland. The first case was identified in the UK in 2010 at Balloch Castle Country Park in West Dunbartonshire, Scotland. Where positive cases are identified, infected trees are destroyed and the area monitored. The disease is most prevalent on the western side of Central Scotland, where 21 separate Statutory Plant Health Notices (SPHNs) for the destruction of infected trees have been issued.

In Scotland, P. lateralis was also found in 2014 on a single western red cedar (Thuja plicata) at Bridge of Allan, Stirling, and in 2013 on Sawara cypress (Chamaecyparis pisifera) near Tarbert in Argyll.

How it spreads

Under EU phyto-sanitary (plant health) legislation, imports of Chamaecyparis are banned from countries outside of the EU. Despite this, P. lateralis entered the EU, possibly on other host species which are not banned, such as Pacific yew (Taxus brevifolia), or on soil associated with non-host plants originating in areas where the pathogen occurs.

The disease spreads by tree roots coming into contact with P. lateralis spores present in the soil or water. Planting infected plants, or using soil which is harbouring spores, are key pathways for introducing the disease on to new sites. Once present in an area, spores from infected trees can also be spread in water run-off, or transferred on footwear.

We do not know how P. lateralis got into the UK, but the most likely route is that it entered on imported, containerised plants. We have not yet been able to confirm the pathways by which the pathogen got to these specific sites, although the fact that different genetic lineages of the pathogen are present at geographically close sites (i.e. Balloch and Greenock) suggests that separate, independent introductions of the pathogen have occurred.

Symptoms

The foliage of infected Lawson cypress trees initially turns a pale green, then a reddish-brown as the tree dies. As the pathogen extends from the roots and root collar up the trunks of affected trees, tongues of darker killed phloem (observable by removing the outer bark) become visible, contrasting with paler healthy tissue. The entire trunk can become girdled, leading to the tree’s death. Disease symptoms caused by P. lateralis can be confused with other infections such as those caused by Phytophthora cambivora or Armillaria spp, which are endemic to the UK.

Susceptible species

Lawson cypress is the main species of tree likely to be infected by P. lateralis in Britain. However, it can be easily confused with other tree species, for example, the Leyland cypress (Cupressocyparis leylandii), which is not known to be susceptible to P. lateralis. It is therefore important first of all to be sure that the affected tree is indeed a Lawson cypress. The ends of Lawson cypress shoots droop downwards and its crushed foliage smells like parsley, whereas crushed foliage of Leyland cypress has a resinous smell.

Other susceptible tree species include Pacific yew (Taxus brevifolia), western red cedar (Thuja plicata), eastern white cedar (T. occidentalis) and several other cypress species.

Impact

P. lateralis is a virulent pathogen which usually kills the trees it infects, primarily Lawson cypress. Few trees recover from an attack.

Lawson cypress is found throughout the UK, and we believe P. lateralis is capable of surviving in all parts of the UK. We estimate there are fewer than 2200 hectares of Lawson cypress growing in woodland in the UK. 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 threat to the ornamental plant industry if it became established here.

P. lateralis also affects other cypress species and Pacific yew, a close relative of Britain's native common yew (Taxus baccata). Its recent confirmation in western red cedar in the UK means it could prove a more significant risk to the forest industries, with about 1000 hectares growing in public forests.

Treatment

Treatments such as soil drenches can be effective in some limited situations, such as nurseries. However, they are unlikely to be effective in the wider landscape, where their use would also raise a number of other environmental issues.

Action

Infected trees are felled and destroyed, either by burning, or chipping and deep burial - provided that no other Lawson cypress trees are in the vicinity. Where appropriate, disinfectant mats are placed at entry/exit points from public sites, and staff and visitors are asked to use them to kill any spores from the pathogen which might have been picked up on their footwear. Notices are also erected at public sites to inform visitors of the infection and to encourage them to observe sensible biosecurity measures, such as keeping to footpaths, keeping dogs on leads, and refraining from taking cuttings from plants in the woodland.

Research is continuing to gain as wide a knowledge and understanding of the disease as possible, and to try to trace the pathways by which it entered the UK. We are also surveying the areas surrounding outbreak sites to determine whether it is present at any other locations in those areas.

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.

Reporting sightings

If, after consulting the above guidance, you still think you might have identified a possible case, and it involves trees in a private garden, you should consult a reputable arborist. The Arboricultural Association can help you find a local one.

If the suspected case involves a number of trees on an estate or public site such as a park or churchyard, especially where high numbers of visitors posing a high risk of spreading the disease, you should report it to the Forestry Commission via Tree Alert.

Nursery owner or managers, or anyone involved in a business handling young trees and planting stock, should report suspected cases of infected trees directly to the relevant plant health authority (Fera for cases in England and Wales, or the Scottish Government).

Further information

P. lateralis factsheet.

Last updated: 12/17/2014