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Phytophthora ramorum

Phytophthora ramorum (P. ramorum) is a a fungus-like pathogen called a water mould. It causes extensive damage and death to a wide range of trees and other plants.

The generic name for the disease which it causes is Ramorum disease. The disease is known in the USA as 'sudden oak death' because different genetic forms of the P. ramorum organism from those present in the UK have caused significant damage to North American native oak and tanoak species. However, the genetic forms of the P. ramorum organism found in the United Kingdom have had little effect on Britain's two native oak species: pedunculate or 'English' oak and sessile oak (Quercus robur and Q. petraea respectively).

Larch trees, which are widely grown in the UK for the timber market, are particularly susceptible, and large numbers have been affected. P. ramorum infection on larch trees is sometimes also referred to in the UK as 'Larch tree disease', 'Japanese larch disease' (although European and hybrid larch are also hosts) and 'sudden larch death'. 


Our situation report summarises the situation across the UK at 31 May 2018.

In addition, the UK outbreak map shows where ramorum disease has been confirmed or presumed in larch trees across the United Kingdom. The coloured dots represent the April-to-March years in which infection was confirmed or presumed.

More-detailed outbreak maps and country-specific information and guidance are available for Scotland, Wales and Northern Ireland from:

Outbreak stage

P ramorum sporangia on japanese larch needle.jpg

The first UK finding was made on a viburnum plant in February 2002 at a garden centre in Sussex. The first record of P. ramorum on a mature tree in the UK was on a 100-year-old  southern red oak (Quercus falcata) in November 2003, and it was found on Japanese larch (Larix kaempferi) in South-West England in 2009.

It has since been found on larch, including European and hybrid larch (Larix decidua and L. x eurolepis), in many parts of the UK, especially in the wetter, western regions. Other woodland species it is affecting in the UK include beech, sweet chestnut and bilberry.

Rapid action to destroy affected trees and other plants to prevent them from spreading the disease further afield, as well as biosecurity measures, has reduced the rate of spread. This action has kept tree losses, especially of larch trees, below what they could have been.


It is impossible to know when P. ramorum first entered Britain or from where. However, contrary to some views, research has shown that it did not arrive here from the USA, where different genetic lineages of the pathogen occur. The evidence suggests that it is native to some parts of Asia, and recent expeditions to Vietnam have detected P. ramorum there.

Other European countries, including Germany and The Netherlands, are now known to have had the pathogen (as a then unknown Phytophthora) on shrubs as long ago as 1993, but these are also likely to have been introduced.

There are four distinct genetic lineages of Phytophthora ramorum in North America and Europe, each of which is  largely clonal. They are referred to as North American lineages 1 and 2 (NA1 and NA2) and European lineages 1 and 2 (EU1 and EU2).

Initially only the EU1 lineage was known in the UK and wider Europe, but in 2011 a fourth evolutionary lineage, the EU2, was discovered in south-west Scotland and Northern Ireland. EU2 is genetically and behaviourally distinct from the much more widespread EU1, and the presence of two lineages suggests at least two separate introductions of P. ramorum into the UK, with the arrival of the EU2 probably the more recent event.


P. ramorum has been found in a number of European countries, including Belgium, the Czech Republic, Denmark, France, Germany, Ireland, Italy, Norway, Spain (including the Balearics), Slovenia, Sweden and The Netherlands. Among these countries, only the UK, Ireland and The Netherlands have reported it on trees.


Few trees in the UK were affected until 2009, when P. ramorum was found infecting and killing large numbers of Japanese larch trees in South-West England.

It is likely that in many cases it had spread to larch trees from Rhododendron ponticum, an invasive, non-native species present in many British woodlands. Rhododendron is highly susceptible to P. ramorum infection, and infected rhododendron produce large numbers of the spores which spread infection. Ramorum disease has since been found on larch species in all four countries of the United Kingdom, and the Republic of Ireland.

Some plant species which can be infected with P. ramorum do not go on to produce the spores which spread the disease. These are known as terminal hosts, and those which do produce spores are called sporulating hosts.

The evidence indicates that P. ramorum spores can be spread over several miles in mists, air currents, watercourses and rainsplash. It can also be spread on footwear, dogs' paws, tools, equipment, and bicycle and other vehicle wheels. Movement of infected plants is also a key means of spreading it over long distances.

Movement over short distances by wild animals such as deer and wild boar might also be possible, although there is little to indicate that squirrels might transfer P. ramorum from tree to tree.

Great Britain has been divided into three risk zones based on the risk of the pathogen spreading in each one. These zones help to inform the disease management strategy. For more information see the 'Official action' section below.

Susceptible trees

Larch (Larix) species are the most commonly affected trees, but beech (Fagus sylvatica), southern beech (Nothofagus species) and some non-native oak species, including red oak (Quercus rubra), Turkey oak (Q. cerris) and Holm oak (Q. ilex) are also occasional hosts.

European sweet chestnut (Castanea sativa) is also a host, and increasing numbers have been found affected in southern England since 2015. (See next section). Horse chestnut (Aesulus hippocastanum) can also be infected.

Other conifer species such as Douglas fir (Pseudotsuga menziesii), true firs (Abies grandis and A. procera), and western hemlock (Tsuga heterophylla), can all be infected when growing near infected larch. In the USA, young Douglas fir and grand fir have been found to be occasional hosts, but the impacts have been minor.

It has also been confirmed on a small number of Sitka spruce (Picea sitchensis), another commercially important conifer species widely grown in the British Isles.

The discovery of the disease in Japanese larch trees in England in 2009 was the first time it had been found causing lethal infection (in the form of stem cankers) on a commercially important conifer species anywhere in the world.

P. ramorum on sweet chestnut

It has been known for some years that P. ramorum can infect European sweet chestnut trees. However, until 2015 the only specimens found infected were individuals exposed to heavy inoculum pressure (large doses of spores) because they were standing close to heavily infected plants of other species, usually larch trees or rhododendron shrubs.

Then in 2015 infected sweet chestnut trees were found at a small number of sites in South-West England, mostly in Devon and Cornwall, where there were no other infected plants nearby. Forest Research scientists are investigating this development to understand whether the trees could have been infected in the first instance by long-distance P. ramorum spread via moist air streams, with the disease then starting to ‘cycle’ among clusters of sweet chestnut trees, that is, spreading from chestnut to chestnut. This could occur, because infected sweet chestnut leaves do generate P. ramorum spores. The number of trees affected is relatively small, but the research will enable us to provide the best possible management advice for growers and owners, and minimise the potential for disease spread.

Meanwhile we have increased our surveillance of sweet chestnut trees, and we have produced a symptoms guide (see 'Symptoms' section below) to help owners and managers recognise when their sweet chestnut might have Ramorum disease. Anyone who suspects they have found it on sweet chestnut trees is asked to report it to us immediately. (See 'Reporting suspected cases' below).


Symptoms on trees 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 larch can take two forms. Phytophthora RamorumShoots 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 also 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 some 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.

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Diagnosis uses a combination of visual inspection by trained observers, and field tests of symptomatic bark and needles with test kits known as lateral flow devices (LFDs, pictured).

Lateral flow device

These are commercially available, pocket-sized kits which can give an indication in 5-10 minutes of whether a Phytophthora organism is present in the tissue. Laboratory tests are required to isolate the pathogen or detect its DNA to confirm the exact species of Phytophthora.

Laboratory tests to diagnose P. ramorum in larch bark and foliage return a conclusive result in only about 80% of the symptomatic material sent for testing, so we cannot rely solely on laboratory tests. When testing fails to produce a conclusive result, P. ramorum infection is presumed where all the other indications point to it.

If, however, laboratory analysis confirms that another organism is causing the symptoms, the site will not be classed as infected by P. ramorum, but will be kept under surveillance.

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Biosecurity precautions

Simple biosecurity precautions , such as cleaning footwear, tools, vehicles, machinery  and clothing between site visits, are strongly advised in outbreak areas to prevent further spread.

Treatment and management

No cure has been found and there are no effective chemical treatments available. There are fungicides which can suppress the symptoms, but none will kill the pathogen.

The objective of any control approach must therefore be to prevent or minimise any further spread of P. ramorum and the damage it causes. The best available scientific advice is to remove and kill the living plant tissue on which the organism depends for reproduction. This means that infected, sporulating trees, such as larch, should be felled or otherwise killed as quickly as possible after detection of the disease. If possible, this should be done before the next spring or autumn period of sporulation begins on the needles. 

We have published advice and guidance to help woodland owners and managers, and other parts of the forest industries, to do what they can to prevent or minimise spread of the disease. This also includes guidance on which species to use and which to avoid, and other factors, when replanting trees on affected sites.

Movement and processing licences

To minimise the risk of spreading the disease by commercial forestry and timber operations, any movement in Great Britain of Phytophthora-affected wood from a forest site, or any subsequent movement of the affected material from a mill or processing site, requires a Forestry Commission movement licence. 

In addition, Phytophthora-affected wood may only be moved to a facility which holds a processing licence.

Licences are issued to haulage and processing operators who can demonstrate that they have put in place and are practising the required biosecurity measures.

Movement and processing licences for Wales are issued by the Forestry Commission on behalf of Natural Resources Wales.

Licences for Northern Ireland are issued by the

Bark industry

We commissioned a study to improve understanding of the scale and nature of the softwood bark industry in Great Britain. The report considered how the increase in fellings of larch infected by P. ramorum was affecting the market, and how those impacts might develop over the following years.

Official action

Given the seriousness of this pathogen, we and Natural Resources Wales serve Statutory Plant Health Notices (SPHNs) on the owners of affected woodland in Great Britain requiring their infected and nearby trees to be felled.

We ensure that private-sector interests are addressed and taken into consideration as we refine the strategy. This is achieved through representation of the Confederation of Forest Industries (ConFor) and the UK Forest Products Association (UKFPA) on our P. ramorum Processor Licensing Working Group, and through meetings with key sector representatives.

We and Natural Resources Wales have had extensive infection in the public larch forests which we manage, and we have undertaken sanitation felling on all of our infected sites.

Official action is in part informed by three risk zones into which Great Britain has been divided, based on the risk of spreading the pathogen in each, from ‘high’ in Risk Zone 1 to ‘low’ in Zone 3. The zone boundaries are kept under review. We record the rate of spread of the pathogen based on notifications and aerial surveys, and periodic updates are made to the outbreak map.

Different regulations governing felling and replanting trees apply according to the risk zone and country in which the site is located:

Scotland also has a special South-West Scotland Management Zone in which statutory controls now applied relate only to the movement of infected timber and bark. Maps and full information about the disease in Scotland are available on the Forestry Commission Scotland website:

We are grateful to private-sector woodland owners for their public-spirited co-operation with the strategy. Without this co-operation it is likely that the disease would have affected a much larger area of trees, woods and forests.

Future prospects

A lot will depend on how the P. ramorum pathogen adapts to its hosts in Britain. If new genotypes emerge which increase its impact on current host species, the worst-case scenario is that we could lose a high proportion of our larch and sweet chestnut trees, particularly those growing in the wetter regions of west and south-west Britain. There might also be the potential for the pathogen to infect other species.

Before the discovery of the disease on larch in 2009, forests of larch occupied about 154,000 hectares, or 5 per cent of Great Britain's total woodland area. Therefore the overall environmental, landscape and economic impact of significant larch losses would not be great in a Britain-wide context, although it would be, and has been, significant in some regions. Sweet chestnut occupies 29,000 hectares of woodland, and the loss of veteran trees of this species would be important in terms of heritage value, and relatively more so than larch forests, most of which which were planted after 1945.

If, however, ramorum disease begins to affect other widely used species in significant numbers, the impact could be greater.

Also of concern is the fact that several other woodland and heathland species are susceptible to P. ramorum and its distant relative, P. kernoviae. Bilberry (known as blaeberry in Scotland and winberry in Wales) has been found infected in the wild, but others have been shown to be susceptible in laboratory trials. Among them is heather, although wild species of heather had only low susceptibility - ornamental cultivars of heather were more susceptible. These are ecologically important plants, 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 be serious.

These risks and uncertainties are among the reasons why we carry out almost continuous surveillance for ramorum disease, and deal quickly with outbreaks. As a result of this intensive surveillance and rapid action, the great majority of our larch forests remain healthy*, and the rates of new larch infection in England and Scotland have been trending downwards in recent years. There will undoubtedly be reversals of this trend in some years, predominantly when the weather is particularly conducive to spread and infection at critical times. For example, favourable weather for the pathogen is believed to have been responsible for the resurgence of the disease in Wales in 2017.

However, provided all parties continue to be vigilant and respond promptly to new findings, there is cause for confidence that we will achieve our strategic goal of maintaining the disease at a level which can be managed effectively in the drier, eastern parts of Britain in the course of usual forest management.

* The National Forest Inventory recorded 126,000ha of larch woodland at 31 March 2016, although this would have included felled areas awaiting replanting, which is unlikely to be with larch, and areas due to be felled for ramorum disease control.

Reporting suspected cases

Tree Alert iconPlease check the symptoms guides above before making a report.

In England, please email us  or call 0300 067 4321.

Images and publications

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Further information

This document sets out the procedures and the various factors which have to be taken into account when woodland in England must be felled and replanted as a consequence of P. ramorum infection.

Although written for the internal guidance of Forestry Commission plant health inspectors, we have published this guidance here to help woodland managers whose trees are or might be infected by P. ramorum (or other pests and diseases) to understand what to expect from our staff and procedures.

We commissioned a study to improve understanding of the scale and nature of the softwood bark industry in Great Britain. The report considered how the increase in fellings of larch infected by P. ramorum was affecting the market, and how those impacts might develop over the following years.

Different genetic forms of the P. ramorum organism from those present in the UK have caused significant damage to North American native species of oak and tanoak trees. States along the western/Pacific seaboard have been particularly affected. The disease it causes on oak and tanoak trees in the USA is called sudden oak death (SOD).



Last updated: 21st June 2018