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Phytophthora ramorum (P. ramorum) is a fungus-like pathogen of plants that causes extensive damage and mortality to trees and other plants.
The first UK finding was made on viburnum 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 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 (Germany and The Netherlands) are now known to have had findings of the pathogen (as a then unknown Phytophthora) on shrubs dating back to 1993, but these are also likely to have been introduced.
P. ramorum has been found on nursery stock in a number of other European countries including Belgium, the Czech Republic, Netherlands, 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. Then in 2010 it was found on Japanese larches in Wales, Northern Ireland and the Republic of Ireland, and 2011 it was confirmed at locations in western Scotland.
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.
Q. rubra (northern red oak; Fagus sylvatica (beech), Castanea sativa (sweet chestnut), Q. cerris (Turkey oak), Q. ilex (holm oak); Aesculus hippocastaneum (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. P. ramorum also occasionally infects European larch (Larix decidua) and hybrid larch (Larix x eurolepis). It has only once been confirmed on Sitka spruce (Picea sitchensis), another commercially important conifer species widely grown in the British Isles. In 2009 P. ramorum was unexpectedly found infecting and killing large numbers of Japanese larch trees (Larix kaempferi) in south-west England. This 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.
On trees, symptoms include lesions – sometimes known as bleeding cankers - that exude fluid 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. 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.
Diagnosis uses a combination of: visual inspection by trained observers; field tests of symptomatic bark and needles with test kits known as lateral flow devices (LFDs), which indicate Phytophthora infection to genus level; and laboratory tests of bark and foliage samples to isolate the pathogen and confirm its species. Laboratory tests to confirm the presence of P. ramorum in larch bark and foliage present a number of technical challenges, and return a conclusive result in only about one-third 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.
A number of simple precautions such as cleaning footwear, tools, vehicles and clothing are strongly advised in outbreak areas to prevent further spread.
No cure has been found and there are no chemical treatments currently available that are effective against P. ramorum. There are some fungicides that can suppress the symptoms, but none that 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.
Given the seriousness of this pathogen, we are serving statutory Plant Health Notices on woodland owners requiring their infected trees to be felled. We are ensuring that private-sector interests are addressed and taken into consideration as we develop our strategy for dealing with this threat, through representation on our Outbreak Management Team from the Confederation of Forest Industries (ConFor) and the UK Forest Products Association (UKFPA), and through meetings with key sector representatives. The Forestry Commission has suffered extensive infection within our own Japanese larch woodlands, and we are undertaking felling on all of our infected sites. We have also defined three risk zones in Great Britain, based on the risk of pathogen spread in each from ‘high’ in Risk Zone 1 to ‘low’ in Risk Zone 3. A map showing the risk zones is available. The boundaries of the risk zones are kept under review as the situation develops. A system has been established for recording the rate of spread of the pathogen based on notifications and aerial survey, and regular updates are made to the outbreak map.
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Management programme report (2011)
P.ramorum in Larch – Update note (2012)
Symptoms on Larch
Map of risk zones
Guidance for Inspectors
Guidance for owners of felled sites (England)