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Phytophthora lateralis

Introduction

Phytophthora lateralis (P. lateralis) is an aggressive water mould, a fungus-like pathogen which affects, mostly, Lawson cypress trees (Chamaecyparis lawsoniana). It usually attacks and spreads through the roots of its host trees (see 'Susceptible species' below), girdling the root collar, resulting in rapid decline and death of the plant.

There are also genetic lineages which have been found to be capable of causing branch dieback, having been transmitted aerially in water droplets.

It has also been reported causing foliage infections on nursery plants of northern white cedar (Thuja occidentalis), and in the wider environment on Sawara cypress (Chamaecyparis pisifera) and western red cedar (Thuja plicata).

Thought to originate in Asia (a genetic lineage has been found in Taiwan), P. lateralis is the main cause of death of Lawson cypress trees in their native range in the West Coast region of North America. Outbreaks have also been recorded in France, the Netherlands and the Republic of Ireland.

The pathogen was first detected in the wider environment in the UK in 2010, and it has since been found infrequently in England and Wales, and more often in Scotland and Northern Ireland. Infection sites include forest stands, windbreaks, parkland and private gardens.

The most likely source of the outbreaks in the UK has been the importation of infected plants from neighbouring European countries – four confirmed outbreaks on mature trees in the UK have been on nursery sites or next to garden centres or plant sales areas. There have been reported findings of P. lateralis on plants in nurseries in the Netherlands, Belgium, France and the UK.

Susceptible species

Identified mostly on Lawson cypress,  P. lateralis has also been found in the UK on western red cedar, northern white cedar and Sawara cypress. However. its host range extends to other members of the Cupressaceae family as well as Pacific yew, juniper, periwinkle and petunia.  It has also been found in soil in nurseries associated with cyclamen, marigold and pomegranate.  Although reported on rhododendron, azalea, photinia and kiwifruit plants, these might have been misdiagnoses of similar phytophthoras.
 
In addition, Alaskan cedar (Cupressus nootkatensis) and Douglas fir (Pseudotsuga menziesii) have been found to be susceptible in experimental conditions.

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-coloured dead 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 those of other infections, such as those caused by Phytophthora cambivora or Armillaria spp, which are endemic to the UK.

Distribution

Confirmed findings at 5 January 2016

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.

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

Status

P. lateralis is not listed in the European Union’s Plant Health Directive.

How it spreads

Under EU phyto-sanitary (plant health) legislation, imports of Chamaecyparis are banned from non-EU countries. 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 when tree roots come 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 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 have occurred.

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 risk to the forest industries, with about 1000 hectares of red cedar 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

In December 2013, The Netherlands ceased statutory controls against P. lateralis. This action could increase the risk of the pathogen being imported into the UK on host plants and in soil. The existing pest risk analysis for P. lateralis in the UK was reviewed, and in November 2015 the decision was made to move from treating P. lateralis as a regulated organism (requiring statutory action in the form of eradication or containment measures) to a situation where statutory action would only be taken on findings in commercially traded plants, and not in the wider environment, unless there were exceptional circumstances. This could include vulnerable woodland environments and sensitive habitats, where isolated finds are detected in areas where the pathogen has not previously been detected, or if new scientific information becomes available.

The review of the PRA also recommended raising awareness with the trade of the need for surveillance, and providing guidance on dealing with and identifying the organism.  In the context of these decisions the following guidance provides some concise recommendations for reducing the risk of introducing or spreading P. lateralis.

Choosing stock

Be prepared to ask your supplier searching questions about their stock, and if the answers are unsatisfactory, choose another source.  For example, do they grow all their stock on site, or are plants imported from abroad, or from another wholesale nursery? If so, do they have separate quarantine areas, and how long do they observe imported plants for signs of ill health before they are sold on?  Do they only grow forestry trees, or do they trade in other potential hosts of P. lateralis?  What are their biosecurity procedures?

When new stock is received on site, check for signs of ill-health, and avoid planting unhealthy trees (and report them to an appropriate authority if necessary).  If planting larger, containerised trees, keep them in a quarantine area for at least six weeks if possible to check for development of potential symptoms before planting.

Site choice

Avoid planting susceptible tree species adjacent to or upstream of already established stands of the Cupressaceae family. Choose a suitable planting location away from areas prone to flooding or standing water. Consider different silvicultural (woodland management) techniques to increase forest resilience (e.g. planting in mixture, so that trees of individual species are widely spaced or in small intimate blocks). Keep records of planting locations, batches and sourcing. In urban situations, check surrounding areas to establish whether there are any individual trees, hedges or windbreaks of the Cupressaceae family which have suspicious symptoms or have suddenly died.

Operational Biosecurity

Always practise good biosecurity as a matter of routine to reduce the risk of accidentally transferring soil or plant material from site to site on footwear, machinery or equipment.  When undertaking work on or near plants of poor health, clean tools and equipment before moving to healthy plants. See the biosecurity section of our website for guidance.

Report symptoms

Familiarise yourself with the common symptoms of P. lateralis infection and use Tree Alert (below) to report suspected outbreaks.

Managing an infected site

If you are unfortunate enough to have an outbreak of P. lateralis, you might be contacted by one of the Plant Health Services if the outbreak is deemed to have exceptional circumstances requiring statutory action.

In most instances after P. lateralis is confirmed, it is likely that there will be no statutory action, and the landowner will be left to undertake the work voluntarily. In these cases, appropriate actions to take are to:

  • Kill remaining affected and nearby or adjacent host trees (usually by felling, but ring barking or chemical treatment can also be options).
  • Avoid moving infected plant material. The most effective method of disposal is to burn or chip affected material on site.
  • Avoid transferring soil or chipped material across the site or off it. Establishing ground cover (using non-host plant species) on any exposed areas of soil following management of infected trees would help reduce this risk of spread. Alternatively, consider using a membrane to cover exposed ground.
  • Make arboricultural workers, gardeners and contractors aware of the long-term biosecurity risk on site, and ensure they undertake appropriate biosecurity actions.
  • Consider awareness signage,  temporary fencing and restricting access to tracks and footpaths etc., particularly if there is a high level of public access on site, until the risk of accidental movement of soil from infected areas has subsided.
  • Remain vigilant for other unexpected dieback on the site, and report any with Tree Alert or to a local plant health officer as appropriate.

Reporting sightings

If, after consulting the above guidance, you think you 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 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 pose a high risk of spreading the disease, you should report it to us using our Tree Alert form.

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 (Apha for cases in England and Wales, or the Scottish Government).

 

Further information

P. lateralis factsheet

Last updated: 26th May 2016