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. 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 www.suddenoakdeath.org; in Great Britain at www.forestry.gov.uk/pramorum; and in Northern Ireland at www.dardni,gov.uk/forestservice.