PROTECTING EUROPE'S FORESTS: HOW TO KEEP OUT BOTH KNOWN AND UNKNOWN PATHOGENS*

During the last century, the tree populations of Europe suffered from a number of serious epidemics resulting from the introduction of pathogens from abroad. Currently the principal approach to minimising the risk of introducing further exotic pathogens is through Pest Risk Analysis (PRA) in which measures are devised to prevent potentially dangerous pathogens from using their likely pathways of arrival. A relevant example is provided by Ceratocystis fagacearum (Bretz) J.Hunt, the cause of oak wilt in eastern North America, which was subject to critical evaluation in the 1970s and 1980s. Beneficial consequences can flow from such an exercise but the longevity and large size of trees may mean that crucial information can be obtained only from a research programme that takes many years to complete. Moreover, there are other difficulties with the PRA approach when it comes to forestry. Firstly, most forest pathogens are classified using morphological criteria which are inadequate for the recognition of differences critical to the ability to cause disease. This means that the occurrence of a named organism in Europe does no