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Phytophthora ramorum


 The outbreak map shows where ramorum disease has been confirmed in larch across the whole United Kingdom.

Outbreak stage

P ramorum sporangia on japanese larch needle.jpgPhytophthora ramorum (P. ramorum) is a fungus-like pathogen which causes extensive damage and mortality to a wide range of trees and other plants.

The generic name for the disease it causes is Ramorum disease. The disease is known in the USA as 'sudden oak death' because different strains of the pathogen cause disease and mortality among North American native oak and tanoak species. However, the strains of P. ramorum found in Britain have had little effect on British native oak species. It is sometimes referred to in Britain as 'Larch tree disease' and 'Japanese larch disease' because larch trees are particularly susceptible, and large numbers have been affected.

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 Quercus falcata (southern red oak) in November 2003.


It is impossible to ascertain when P. ramorum first entered Britain or where it came from. However, contrary to some views, research has shown that it did not arrive here from the USA, where different forms of the pathogen occur. The evidence suggests that P. ramorum is native to another part of the world, possibly Asia. 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.


P. ramorum has been found in a number of other European countries including Belgium, the Czech Republic, Denmark, France, Ireland, Italy, Spain (including the Balearics), Slovenia and Sweden.


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 has since been found on Japanese and other larch species in all four countries of the United Kingdom, and the Republic of Ireland.

Method of spread

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.

Susceptible trees

These include larch (Larix) species, Quercus rubra (northern red oak), Q. cerris (Turkey oak), Q. ilex (holm oak), Fagus sylvatica (beech), Castanea sativa (sweet chestnut) and Aesculus hippocastanum (horse chestnut). In Britain we have found Douglas fir to be a host but, unlike the situation in the USA where the impact has been minor, a number of younger Douglas fir trees of about 5 – 10 years of age have been killed in south-west England. It has 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 (Larix kaempferi) in south-west 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 sweet chestnut trees (Castanea sativa), but until 2015 the only specimens found infected were individuals exposed to heavy inoculum pressure by their close proximity to heavily infected plants of other species, usually larch trees or rhododendron shrubs. However, 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 have been found to generate P. ramorum spores, although in much smaller quantities than infected Japanese larch foliage generates. 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 our tree health surveillance programme has increased its focus on sweet chestnut health, and we have produced a symptoms guide to help owners and managers recognise when their sweet chestnut might have Ramorum disease. Anyone who suspects they have found the disease on sweet chestnut trees is asked to report it to us immediately using Tree Alert (below).


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 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 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.

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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). 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 symptomatic tissue. Laboratory tests are required to either 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 present a number of technical challenges, and return a conclusive result in about 80% of all symptomatic material sent in for testing, so we cannot rely solely on laboratory tests. If, however, laboratory analysis confirms the presence of another causal agent, the site will not be classed as infected, but will be kept under surveillance.


Simple precautions such as cleaning footwear, tools, vehicles and clothing 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. So the objective of any control approach must 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. In the case of infected larch, this means affected trees should be felled or otherwise killed as quickly as possible after detection of the disease and before the next spring or autumn period of sporulation begins on the needles. 

Movement and processing licences

Any movement of Phytophthora-affected wood from a forest site, or any subsequent movement of the affected material from a mill or processing site, requires a Movement Licence. 

Phytophthora-affected wood may only be moved to a facility that holds a valid Processing Licence.

Bark industry

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


Given the seriousness of this pathogen, we serve Statutory Plant Health Notices (SPHNs) on woodland owners requiring their infected trees to be felled. We ensure that private-sector interests are addressed and taken into consideration as we refine the strategy for dealing with this threat. This is achieved through representation on our P. ramorum Processor Licensing Working Group, the Confederation of Forest Industries (ConFor) and the UK Forest Products Association (UKFPA), 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 felling on all of our infected sites.

Three risk zones in Great Britain have been defined, based on the risk of spreading the pathogen in each, from ‘high’ in Risk Zone 1 to ‘low’ in Risk Zone 3. The boundaries of the risk zones are kept under review as the situation develops. A system records the rate of spread of the pathogen based on notifications and aerial surveys, and regular updates are made to the outbreak map.

Reporting suspected cases

Tree Alert iconPlease chack the symptoms guide before making a report.

England: email us  or call:  0300 067 4321

Scotland or Wales - please use Tree Alert.

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Last updated: 18th July 2017