Dutch elm disease in the UK
In the United Kingdom elm trees have for several years been severely affected by a new aggressive strain of Ceratocystis ulmi. As a consequence, a large number of elms died bringing about a surplus of elm timber.
There seem to be two chief alternatives for the origin of the aggressive strain : either that the aggressive strain arose by genetic change within an endemic non-aggressive gene pool, or that it was introduced to Britain 1).
There is now evidence that the aggressive strain has appeared locally in some continental European countries. Nevertheless, it is disturbing to learn that other countries on the continent of Europe are importing elm from Great Britain since it is almost certain that this elm will carry the aggressive strain and its beetle vector. Elm log importations are thus likely to greatly increase the possibilities of spread of the new strain and this risk should be considered very thoroughly.
1) BRASIER, C.M. & J.N. GIBBS, 1973. Origin of the Dutch Elm Disease Epidemic in Britain. Nature 242 (5400) : 607-609.
Sources
Forestry Commission, Farnham (1974-01-24)