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NEWS RELEASE No: 1553410 JULY 2012

Fourth genetic lineage of Phytophthora ramorum pathogen identified

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Issued jointly with the California Oak Mortality Task Force

British forestry scientists have identified a fourth genetically distinct lineage of the Phytophthora ramorum (P. ramorum) pathogen which is causing devastating tree diseases in the UK and the USA.

P. ramorum causes ramorum disease of larch trees in the United Kingdom (UK) and Ireland and sudden oak death disease of oak and tanoak trees in the USA, as well as diseases in more than 120 other plant species around the world.

It has been causing dramatic mortality in Japanese larch in the UK, including Northern Ireland and many western parts of Great Britain, especially southwest and north-west England, south Wales and southwest Scotland. The British Forestry Commission uses aerial surveys to detect outbreaks.

The increasing intensity of the P. ramorum outbreaks in the UK led researchers to analyze samples from the new findings further, which resulted in the discovery of a fourth genetically distinct lineage of the pathogen.

Announcing this at the Fifth Sudden Oak Death Symposium in California recently, Clive Brasier, an emeritus professor with the Forestry Commission's Forest Research agency, said he believed, based on genetic analysis, that the previously unknown European Type 2 (EU2) lineage had been recently introduced into south-west Scotland and Northern Ireland.

"We are still uncertain about the pathway by which the new lineage has arrived in the UK, and we are still trying to assess the extent of its distribution," Professor Brasier said. "We have tremendous biosecurity concerns, including this new form of P. ramorum. Over the past year alone in the UK we have discovered chestnut blight, Asian longhorn beetle, Chalara dieback of ash and several other invasive forest pests."

Three P. ramorum lineages were known to exist before the discovery: EU1 and North American types 1 and 2 (NA1 and NA2).

The EU1 lineage has primarily been found in European nurseries and forests, and has caused the current epidemic on larch throughout much of Great Britain. However, it has also been recovered from several US West Coast nurseries and waterways.

The NA1 lineage is most commonly found in California’s infected forests, but has also been recovered in some US nurseries, and the NA2 isolate, which is found in US nurseries, has also been detected in a couple of Washington waterways. Based on genetic analysis, these three lineages are considered separate, unique introductions, with each estimated to have arrived in its primary country during the 1990s.

Research on the four lineages indicates that the pathogen emanates from some unknown centre of origin. Each is a distant relative of the others, having diverged hundreds of thousands of years ago, and reuniting the lineages through trade and long-distance plant movement could have unknown and unintended consequences. The co-mingling of these lineages increases potential for sexual recombination, possibly creating more virulent strains.

 "This pathogen has been moved around the world and introduced into Europe and North America at least four times, and the likely vehicle for these introductions is nursery stock," Professor Brasier said. "The latest introduction into the UK is particularly frustrating, because a major effort to implement effective phytosanitary (plant health) measures has been under way for some time. Unfortunately, these pests and pathogens are often effective at evading the measures in place to protect trade and our forests from these new, unwanted arrivals.

"Really, the larger issue highlighted here is the need for a two-fold programme which includes: developing and implementing protocols that take unknown pathogens into consideration to help those in affected industries mitigate for them as much as possible; and increasing public awareness of invasive species issues. It is going to take a collective effort to successfully deal with this, but if done properly, we will save forests and prevent millions of pounds from being lost to often futile reactive responses."

In recognition of this need, plant health authorities and private-sector interests in the UK have jointly launched an integrated Tree Health and Plant Biosecurity Action Plan intended to ensure that the UK’s defences against exotic pests and pathogens are as robust as possible within the current regulatory framework. In addition, the European Union is conducting a review of its plant health regime to ensure it is fit for purpose in the 21st-century global trading environment.

Further information about the disease in the USA is available from; in Great Britain at; and in Northern Ireland at


  1. Ramorum disease is known as sudden oak death (SOD) in the USA because many of its host tree species there are North American native oaks. The pathogen has been found killing coast live oak, California black oak, Shreve’s oak, canyon live oak and tanoak in 14 coastal counties from Monterey to Humboldt as well as Curry County, Oregon. To date in California, more than a million trees have died and at least another million are infected. It also affects nearly 125 other plant species worldwide, including garden plants such as rhododendron.
  2. Despite its common name in North America, the UK's native pedunculate and sessile oak species (Quercus robur and Q. petraea) have been little affected by P. ramorum. However, it has proved particularly destructive of Japanese larch (Larix kaempferi) in the UK since it was first found to be the cause of significant mortality in this species in England in 2009. Larch trees also produce huge numbers of the infective spores of P. ramorum, which can be spead some distance from tall trees on the wind and in mists. To date more than 3 million larch trees have been compulsorily felled in the UK in the effort to bring the disease under control.


  • USA - Katie Palmieri, California Oak Mortality Task Force, +1 510 847-5482;;
  • UK - Charlton Clark, British Forestry Commission/Forest Research, +44 131 314 6500;