Phytophthora ramorum (P. ramorum) is a pathogen of trees and other plants that was first found in Britain infecting a viburnum in a plant in a garden centre in 2002.
P. ramorum kills most trees that it infects, and could have serious impacts on trees, woodland, the forest industries and the wider environment. Since its discovery in Britain, the Forestry Commission and its partners in Defra, Fera and the Scottish and Welsh Assembly Governments, acting on scientific advice, have been taking action to destroy infected plants to limit its spread. They are also carrying out research to gain a full understanding of the pathogen and how it behaves, how it is spread, and how it might be controlled in future.
In Britain P. ramorum has mostly been found infecting ornamental shrub species such as rhododendron and viburnum, and heathland plants such as bilberry (Vaccinium myrtillus), which is known as blaeberry in Scotland and winberry in Wales. Until 2009 fewer than 100 trees had been found with the infection, and then usually only on trees standing very close to infected rhododendron, which produces spores that cause further infection, or other infected plants. Unlike the situation in America, very few of Britain’s native oaks have been infected, because they appear to be much more resistant to the pathogen than native American oaks. No trees have been found infected in Scotland.
However, in 2009 the pathogen was found infecting Japanese larch (Larix kaempferi) and other conifer tree species in forestry plantations in South West England and, in 2010, in South Wales. Large numbers of larch trees have died or are dying. It has also been confirmed that the foliage of infected Japanese larch produces spores at about five times the level that rhododendron does, and that these can be dispersed considerable distances, probably some tens of kilometres. This is the first time it has been recorded infecting a commercially important conifer species anywhere in the world, and represents a significant development in the pathogen’s behaviour.
Aerial surveys have been used to identify areas of dead trees, and these have been followed up by inspecting and sampling on the ground. More of this work is now under way to investigate other parts of Wales and the western sides of England and Scotland (which climate modelling shows to be at the highest risk), while we remain vigilant for signs of the disease in all parts of Britain.
A full list of hosts and findings in Europe is given at http://rapra.csl.gov.uk
Symptoms
In Europe P. ramorum has most frequently been found infecting the leaves and shoots of ornamental shrubs such as rhododendron, viburnum, pieris and camellia. Although it does not usually kill these plants, infected leaves of some of these ‘foliar hosts’ can generate many spores, and in sufficient numbers these spores can then infect the bark of certain tree species.
Typical symptoms on rhododendron include leaf-blackening, wilted shoots and die-back. On individual leaves, blackening of the leaf stalk usually extends into the leaf along the mid-vein, although blackening at the leaf tip can also occur. The progress of the disease can be so rapid that shoots wilt and the leaves hang down.
On trees, symptoms are lesions – sometimes known as bleeding cankers. Fluid is exuded from infected bark, visible as a black exudate that can dry to a crust on the trunk. The inner bark under this bleeding area is usually discoloured and dying. Trees die when the lesions become extensive on the main trunk.
Spread
P. ramorum can be spread in a number of ways. Evidence from California and Oregon indicates that it can be spread over several miles in rain, mists and air currents. We believe that this air dispersal is the most likely pathway in the case of the larch infections. We also know that Phytophthora pathogens can be spread on the footwear of people who have been walking in infected areas, and on dogs’ paws, bicycle wheels, tools and equipment etc, and this could be a possible pathway for the spread of P. ramorum to other sites in Britain. Movement of infected plants is also a key means of spreading it over long distances.
Treatment
The measures we are putting in place follow a risk-based approach. As well as removing and destroying any rhododendron on infected sites, other sporulating hosts (trees or plants on which P. ramorum can produce inoculum) are also being removed and processed into wood products or used as biofuel. Infective material can be burned on site, under controlled conditions.
We are also putting in place biosecurity measures to minimise the risk of the disease being spread from infected sites on footwear, vehicles, tools and equipment used in forestry operations. These include requirements to wash and disinfect the items, and to sweep up and destroy material removed by washing.
We are working closely with the scientists in our Forest Research agency as well as colleagues in Defra and the Food & Environment Research Agency (Fera) to confirm the extent of P. ramorum involvement in the dieback of the Japanese larch. We are also working to find out whether any other agents could have a significant role. We are requiring the infected trees to be felled to remove the living tissue on which the pathogen lives and sporulates. Japanese larch timber can still be used, so after felling the infected trees we are permitting logs to be taken under licence to authorised processing facilities with appropriate biosecurity precautions in place to prevent accidental spread. These precautions must be in place from the forest gate to the disposal of the sawmill residues. We have also provided advice to woodland owners and managers.
Susceptible trees
Except where disease levels are intense on foliar hosts such as Rhododendron ponticum, P. ramorum is unlikely to infect European species of oak such as common or pendunculate oak (Quercus robur) or sessile oak (Q. petraea). Laboratory tests on their relative susceptibility indicates that these species are more resistant than their American cousins. However, some conifer species such as Sitka spruce (Picea sitchensis) might be susceptible.
The first P. ramorum-infected tree outside the USA was confirmed in November 2003 on a mature southern red oak (Quercus falcata) in Sussex. Since then, the Dutch have confirmed P. ramorum infections on several beech (Fagus sylvatica) and red oak (Quercus rubra) trees. Until early December 2008, most infected trees with bleeding lesions in Britain were found in Cornwall, with beech the most frequently affected species. Other tree species found with potentially lethal infections caused by P. ramorum have been rare, but include southern beech (Nothofagus obliqua), horse chestnut (Aesculus hippocastanum), sweet chestnut (Castanea sativa), sessile oak (Quercus petraea), Turkey oak (Q. cerris) and sycamore (Acer psuedoplatanus). In addition, a few other tree species have been found suffering from leaf and shoot infections caused by P. ramorum. These include holm oak (Quercus ilex), Turkey oak, ash (Fraxinus excelsior), sweet chestnut, Magnolia, Michelia and Eucalyptus species.
Pest Risk Analysis
Pest risk analyses (PRA) for P. ramorum and P. kernoviae are available on the Defra website. A European PRA is now also available for P. ramorum. To see the full report (PDF-6147k) or see a Summary (PDF-1633k). A full list of hosts and findings in Europe and the USA is given at http://rapra.csl.gov.uk
Other names
P. ramorum has caused the deaths of millions of native American oaks and tanoaks in California and Oregon, giving rise to the its American nickname of ‘sudden oak death’. However, the nickname is a misnomer in Britain because laboratory tests have shown that our two native species, sessile and pedunculate oak, are much more resistant to the pathogen than their American cousins.
Who does what?
Defra’s Food & Environment Research Agency (Fera) has statutory responsibility for delivering policy and inspectorate functions in relation to Plant Health, including legislative responsibility for P. ramorum relating to plants and plant produce in England. Fera also co-ordinates UK plant health policy. The research arm of Fera provides scientific and technical advice and support to PHD/PHSI, and carries out all diagnostic services.
The Forestry Commission’s Plant Health Service (PHS) has overall policy and legislative responsibility for all aspects of P. ramorum relating to forest trees and wood.
Forest Research (an agency of the Forestry Commission) is responsible for research, scientific advice and diagnosis of P. ramorum on trees. Its Technical Support Unit carries out woodland surveys.
The Scottish Government Rural Directorate - Agricultural Commodities Branch is responsible for policy and legislative matters for P. ramorum relating to non-forestry plants material in Scotland, including liaison with Scottish stakeholders.
The Scottish Government Rural Payments & Inspections Directorate (SG-RPID) is responsible for enforcing policy through inspections, surveys, eradication campaigns and liaison with Scottish local authorities.
SASA (Science & Advice for Scottish Agriculture) provides scientific and technical support to the Scottish Government, along with diagnostic services.
The Welsh Assembly Government’s Plant Health and Biotechnology Unit has policy and legislative responsibility for plant health and P. kernoviae relating to non-forestry plant material in Wales. Under an agreement with Defra, all enforcement and inspection activities are carried out on its behalf by Defra’s PHSI.
