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Phytophthora ramorum - Frequently Asked Questions

What is the situation?

Phytophthora ramorum (P. ramorum) is a fungus-like pathogen of trees and other plants which was first found in Britain infecting a viburnum plant in a garden centre in 2002. It has since been found in all four countries of the United Kingdom.

P. ramorum kills many of the trees it infects, and could have serious impacts on trees, woodland, the forest industries and the wider environment. The Forestry Commission, Defra, Fera and the Scottish and Welsh Governments, acting on scientific advice, have been destroying 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 shrub species such as rhododendron and viburnum, and heathland plants such as bilberry (Vaccinium myrtillus), 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 plants, especially Rhododendron ponticum. Rhododendron is common in many woodlands, and P. ramorum on Rhododendron produces spores that cause further infection. No trees have been found infected in Scotland.

However, in 2009 the pathogen was found infecting Japanese larch (Larix kaempferi) and a small number of other conifer tree species in Forestry Commission and privately owned forestry plantations in South West England and, in 2010, in Wales, Scotland, Northern Ireland and the Republic of Ireland. 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.

Large numbers of larch trees have died or are dying. Japanese larch foliage produces P. ramorum spores at about five times the level that Rhododendron does, and these can be dispersed over considerable distances, possibly tens of kilometres. Aerial surveys are used to identify areas of suspect dead trees. These are followed up by inspection and sampling on the ground. The pathogen thrives and spreads best in a moist, mild climate, and climate modelling suggests the southern and western seaboards of Britain are more likely to be affected than the drier eastern regions. This has been borne out by the outbreaks confirmed so far.

The locations of currently suspected and confirmed sites of P. ramorum on Japanese larch are available on our outbreak map.

A full list of hosts and findings in Europe is given at http://rapra.csl.gov.uk

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How worried is the Forestry Commission?

Britain’s trees, woods and forests make a huge and positive contribution to our landscape, economy, environment, culture, heritage and well-being, so we are always concerned by new threats to them. We were already very concerned when we found it infecting bilberry, which is an ecologically important ground-cover plant of woodland and heathland. It had already had a significant impact on the horticultural and amenity sectors, particularly heritage gardens, and the fact that it is now also infecting a commercially important species of conifer tree has added to our concerns. So, although we cannot yet predict what the impact wille be in the long term, we are treating it very seriously.

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What are the symptoms?

On trees, symptoms include lesions – sometimes known as bleeding cankers - which exude fluid from infected bark, visible as a black exudate which 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.

Infection by P. ramorum on Japanese larch can take two forms. Shoots and foliage can be affected, visible as wilted, withered shoot tips with blackened needles. The infected shoots shed their needles prematurely. Trees with branch dieback can have numerous resinous cankers on the branches and upper trunk.

On other plants, it infects 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.

Images of symptoms

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How many larch trees are infected?

We will never know the exact number because we are treating all Japanese larch on an outbreak site as infected, and requiring them to be felled. We are also still working to establish the extent of the disease and the number of sites affected. However, by October 2013 about 16,000 hectares of larch trees in the UK (i.e. including Northern ireland) had been felled or were under notice to be felled.

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What other species does it affect?

Globally, about 130 species of plants, including trees, have been identified as hosts. Those which have been found with potentially lethal bleeding lesions include southern red oak, red oak, sessile oak, Turkey oak, beech, southern beech, horse chestnut, sweet chestnut, and sycamore.

Other species found suffering leaf and shoot infections caused by P. ramorum include holm oak, ash, western hemlock, Douglas fir, birch species, eucalyptus species, Magnolia and Michelia.

The first tree found with P. ramorum infection in Britain was a non-native southern red oak in Sussex in 2003. Until 2009, the most-affected tree species in Britain had been beech. Some conifer species, including Sitka spruce (Picea sitchensis), the most-grown commercial conifer species in Britain, are thought to be potentially susceptible, and in April 2011 an infected small Sitka spruce standing under a heavily infected Rhododendron bush was found in the Republic of Ireland. A small number of other cases on Sitka spruce have been found since then.

Different starins of the P. ramorum pathogen have killed millions of native American oaks and tanoaks in California and Oregon, but Britain's two native species of oak, sessile and pedunculate oak, have been demonstrated in laboratory tests to be more resistant than their American cousins. Only a tiny number have been infected, and all of them were standing next to heavily infected Rhododendron bushes and were therefore exposed to heavy 'inoculum pressure'.

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Are European larch (Larix decidua) and hybrid larch (Larix x eurolepis) at risk?

The first infected European larch was confirmed in March 2011 in an area of Cornwall, south-west England, with infected Japanese larch nearby. However, it is too soon to know how susceptible European larch is, whether it will be a sporulating host, and if it is, how heavily it will sporulate. So far no infections have been confirmed in hybrid larch, although in light of its genetic relationship and physiological similarities to European and Japanese larch, we would not be surpised if it is also susceptible to P. ramorum, and we continue to closely monitor all larch crops closely.

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What measures are you taking to control it?

The measures we are putting in place follow a risk-based approach. As well as removing and destroying any Rhododendron on infected and high-risk sites, other sporulating hosts (trees or plants, including larch, on which P. ramorum sporulates) are also being removed and either processed into wood products, used as biofuel, or 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 by forest workers or visitors. These include requirements to wash and disinfect the items, and to sweep up and destroy material removed by washing.

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What are you doing about the outbreaks on larch?

We are working closely with scientists in Forest Research and colleagues in Defra and the Food & Environment Research Agency (Fera) to confirm the extent of P. ramorum involvement in the dieback of 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. Timber from infected larch can still be used, so we are permitting logs from infected woodland 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. These precautions are set out in our ‘Moving wood - operational precautions” protocol.

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Why are you felling infected larch trees and not all infected trees of other species?

Because P. ramorum sporulates on larch, that is, it produces spores, or inoculum, which spread the disease. It does not sporulate on most of the other tree species it is known to infect. Most other trees found infected have been felled, primarily for safety reasons, although some specimen trees with light infection of the foliage have been pruned and kept under observation.

The only other tree species on which it has been observed sporulating in Britain are Holm oak, ash and sweet chestnut.

Where P. ramorum infects only the bark of trees, it does not sporulate on them, so there is no need to fell them for disease management reasons. However, if they are dying or declining they might become dangerous and need to be pruned or felled for public safety reasons.

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How is it being spread?

In a number of ways. Evidence from California and Oregon indicates that P. ramorum can be spread over several miles in mists, air currents, watercourses and rainsplash. We also know that Phytophthora pathogens can be spread on footwear, dogs’ paws, bicycle wheels, tools and equipment etc. Movement of infected plants is also a key means of spreading it over long distances.

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Do we understand the entry pathways for the disease into larch and other trees?

We have evidence that the spread of P. ramorum in woodlands has occurred initially through infection of rhododendron. Trees such as beech and turkey oak only became infected when in close proximity (within a few metres) to already infected rhododendron. Infected rhododendron have probably acted as a pathway for the pathogen into larch plantations, but the ability of P. ramorum to infect and sporulate on the needles of larch has meant that infection can then move from larch to larch and is not limited by the distribution of infected rhododendron.

Does Phytophthora ramorum enter trees via a wound?


P. ramorum does not need to infect via wounds. In the field the spores of P. ramorum are produced on certain plant species known as ‘sporulating hosts’. Rhododendron and larch are the most significant sporulating hosts. Infected larch needles generate huge numbers of P. ramorum spores just before they are shed in autumn, but no spores appear to be produced in or on infected bark. Infection through bark results from contact with hundreds, or maybe even thousands, of spores. This is only likely to occur when the spores are washed from infected needles in rainfall and collect in water pockets on twigs or thicker bark surfaces. Interestingly, Phytophthora spores are thought to act together to achieve infection: it is a group action, and the number of spores in the group has to be large enough to succeed.

Research by Forest Research, Fera and researchers in the USA shows that P. ramorum is adapted for very effective spread via moist winds and water, both locally (within ~100m) and over distances of 1-3 km (less frequent). Wounding is not needed for infection; indeed it might be counter-productive, particularly with larch. Animal and bird studies in the USA to confirm spread followed by infection have been inconclusive to date. Spread via animals moving Phytophthora-infested soil and/or leaf litter seems likely, because the spores would be protected in a moist matrix, and there is some anecdotal evidence to support this.

Are grey squirrels implicated in the spread of P. ramorum?

See this separate page dealing with questions about grey squirrels and other animals' possible involvement.

Is it always fatal to trees?

Some broadleaved trees can recover if affected branches are pruned early enough, but the evidence so far is that infected larch will die quite quickly.

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Is there any chemical treatment that could be used?

There are no chemical treatments currently available which are effective against P. ramorum. Some fungicides can suppress the symptoms, but none can kill the pathogen. In the long term one of our research options might be to see whether there is a biological or chemical treatment that can be deployed to inhibit the disease. However, the use of such treatments requires thorough research to ensure that they don’t themselves cause environmental harm, and they must be approved before use by the Chemicals Regulation Directorate.

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Is Phytophthora kernoviae involved?

There have been no findings of P. kernoviae in any of the newly identified hosts of P. ramorum (i.e. Japanese larch, western hemlock, Douglas fir or birch). Although both pathogens are members of the Phytophthora genus, they are only distantly related. It does not automatically follow that a host of one will be a host to the other.

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Is there anything the public can do to help prevent or minimise its spread, and are you restricting public access to infected areas?

Access to public forests is only being restricted while trees are being felled, which is normal practice for safety reasons.

We still welcome visitors to all of our forests, but the public can help us to prevent or minimise the spread of the disease. Visitors will be advised by local signage of the most appropriate action to take. Ways in which they might be asked to help include:

  • keeping to marked paths;
  • keeping dogs on leads;
  • not removing any plant material, such as cuttings, from the woodland;
  • removing soil and mud from boots and shoes before leaving the woodland; and
  • thoroughly washing boots, shoes and bicycle wheels before visiting other susceptible areas, such as woodlands, gardens or garden centres and nurseries.

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What does it mean for the forestry and timber industries?

At present, until we have greater clarity on the extent to which P. ramorum is involved and how many sites are affected, we are uncertain what it means for our forestry and timber industries.

However, we are working hard to minimise the impacts on jobs and businesses by allowing logs from infected trees to be harvested and used, subject to biosecurity precautions being taken, and helping owners to harvest and sell logs from trees in infected sites.

Details of the assistance and guidance available to woodland owners and the industry are available at the links on the left-hand side of this page.

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Do affected forest owners have to fell their trees early?

Unfortunately, yes, even if their infected trees have not yet reached economic maturity (the optimum age for felling a tree for timber production). Forest Research scientists advise that the disease can be spread from infected larch trees of any age. This means that felling infected larch trees, preferably in winter before the new needles develop in spring, is still the best means to control its spread. We are therefore requiring infected larches and other sporulating trees to be felled. However, we are continuously reviewing that advice as the situation develops and our understanding of the disease improves.

Details of the assistance available to owners who are required to fell immature larch trees are available at the links on the left-hand side of this page.

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How does it affect the timber? Is it still usable?

There is no evidence to suggest that P. ramorum's presence in a tree makes its timber unusable. There is no risk of further spread from sawn wood from infected trees.

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What precautions are you taking with timber and logs from infected areas?

We are requiring a number of precautions to be taken when handling and removing logs from infected forests, including:

  • cleaning off forestry machines before they leave infected sites;
  • stacking logs clear of the ground on bearers while awaiting removal from the forest;
  • sweeping off the lorry trailer and wheels after loading and before returning to the road;
  • taking the logs only to mills licensed to receive logs from infected forests;
  • sweeping off trucks on hard-standing at the mill before they return to the road;
  • sweeping up and destroying yard residue; and
  • rendering the infected bark and chipped wood products safe by burning them for energy or putting them into manufacturing processes which use heat and pressure, such as fuel pellet or chipboard manufacture.

We are also banning the use of bark and residue as garden mulches or similar.

Forestry Commission plant health inspectors visit the licensed wood users to ensure these precautions are being observed.

More-detailed biosecurity advice for those moving infected larch material is available in our ‘Moving wood - operational precautions” protocol.

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Will you be advising forest managers not to plant larch or other affected species?

It is too early to give any firm recommendations, and we will keep this situation under review. However, it is probably unwise to plant Japanese larch on a site that has already been infected, at least for several years following clearance, because the pathogen can remain viable in the soil for some time.

We generally do not dictate to forest managers what they may or may not plant. However, we do need to ensure that we achieve value for the taxpayers’ money when planting in public forests and when helping other forest managers with planting and restocking (replanting) grants. It is therefore possible that at some point we will have to assess whether the risk of larch trees not reaching economic maturity has become too high to justify spending public money on planting them. However, this will depend on the long-term progress of the disease.

Advice about replanting on infected sites is available in our guidance note available from our Advice to owners and operators page.

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Will you be imposing further import controls?

We are looking into this, but the final decision will need to be taken at a European Union level and will be informed by the results of the ongoing research. There are already import controls regulating all conifer wood entering Great Britain which, at the moment, appear to be sufficient.

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What resources are you putting into tackling this threat? How are they being organised?

As well as our own staff time, expertise and contingency funds, we are using money from the UK Government’s £25 million fund for Phytophthora management to deal with this problem in England and Wales.

The use of this fund is being co-ordinated and managed by Fera (Food & Environment Research Agency). Research into the larch infection is being funded from this programme, as are clearance of infected Rhododendron and other sporulating hosts in high-risk woodlands, and support for surveillance and diagnosis.

We are also working closely with the Scottish Government on joint surveillance in woodland, heathland and other areas at risk, while horticultural inspectors carry out inspections at nurseries and garden centres. Contingency plans are in place to increase our response should the situation change.

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If you can’t bring this disease under control, what’s the worst-case scenario?

A lot will depend on how the pathogen evolves in Britain. If it does not evolve to infect any other tree species, or worsen its current effect, the worst case is that we could lose a high proportion of our larch trees. Larch of all three species (Japanese, European and hybrid larch) occupy about 134,00 hectares, or 5 per cent of the total woodland area of Britain. (About 47,000ha or 4.3 per cent in England; 23,000ha or 8 per cent in Wales, and 65,000ha or 5.1 per cent in Scotland.). Therefore the overall environmental, landscape and economic impact would not be great in the widest British context, although of course it would be significant in some regions. If, however, it begins to infect other widely used species in significant numbers, the impact could be greater.

At present the other most worrying aspect is the fact that a range of woodland and heathland plants is also susceptible to P. ramorum and its distant relative, P. kernoviae.

Bilberry has been found infected in the wild, but other species, including heather, have been shown to be susceptible in laboratory trials. These are ecologically important plants of woodland and heathland, and there would be serious consequences for those habitats if either pathogen were to cause significant damage to these species.

Bilberry is closely related to blueberry, and if the pathogen were to infect blueberry the economic consequences for the blueberry industry could also be serious.

These risks and uncertainties are among the reasons why we have moved quickly to increase surveillance, deal quickly with outbreaks, and research the disease as thoroughly as possible.

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Is there any good news?

In 2011 we found fewer infection sites and a smaller total area of infected trees than in 2010. In addition, most new sites were in close proximity to previously infected sites. However, we cannot afford to be complacent, because a number of factors can affect the disease's progression, including weather patterns, and some new sites were identified in previously unaffected regions such as North West and central England and South-West Scotland. Evidence is also emerging that sites with infected larch might be 'clustered', with areas of apparently healthy trees between these ‘clusters’. It is therefore possible that we might be able to contain the disease within these clusters and protect the other healthy areas if we can move quickly enough to remove the infectious trees and plants and prevent the disease from spreading from them.

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Who’s doing what about P. ramorum?

The Forestry Commission is the government department responsible for protecting England's and Scotland's woods and forests from pests and diseases. It is advised on the science of pests and diseases by Forest Research, its scientific research agency. It reports to Defra Ministers in England and to the Environment Minister in the Scottish Government.

The Forestry Commission and Forest Research work in close partnership with scientists and officials at Fera and SASA (Science & Advice for Scottish Agriculture). Fera has responsibility in England and Wales for research and expertise on the full range of environmental and food pests and diseases, and SASA is its Scottish counterpart.

Since 1 April 2013 forestry policy and public forest management in Wales has been the reponsibility of a new Werlsh Government body, Natural Resources Wales (NRW), which peforms the functions previously carried out by Forestry Commission Wales.

The government departments and agencies involved have formed a Phytophthora Project Board in England and Wales, along with partners from the private forestry sector and non-government organisations with an interest in woods and forests, and a Phytophthora Steering Group in Scotland. These groups work in partnership and oversee and advise on all aspects of disease research and management.

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Further information

As well as www.forestry.gov.uk/pramorum, there is scientific information on the Forest Research website.

Last updated: 29th September 2014