Studies on Dutch elm disease
Research at the Forest Research Station, Alice Holt (UK) by Dr C.M. Brasier has shown that aggressive isolates of Ceratocystis ulmi can be classed into two types, differing in morphology and separated by reproductive barriers. These two types are nevertheless more similar to each other than they are to the non-aggressive strain, which was already widespread in Europe before the new outbreak and are also genetically isolated from it.
North American aggressive isolates all belong to one type, referred to as the NAN race. Isolates of the aggressive strain from UK, France, Netherlands, Austria and Switzerland are all of this race. The other type, known .as the EAN race, is absent from N. America and has been found in Italy, Denmark. Bulgaria, USSR and Iran. In the Federal Republic of Germany and Ireland, both races are found, but geographically separated.
Dr Brasier suggests therefore that the recent outbreaks of Dutch elm disease in Western Europe are of dual origin : through the importation of the NAN race of the aggressive strain from North America into the United Kingdom in the late 1960's, and then to Ireland, France, Holland and Germany in the early 1970's; and through migration of the EAN race northwards from Italy, Central Europe or farther east, to Germany, Denmark and Ireland. It seems probable that the EAN race of the aggressive strain was present in southern and eastern Europe before the recent outbreaks in western Europe.
Sources
Nature 281 : 78-80 (1979-09)