Dual origin of recent Dutch elm disease outbreaks in Europe

Abstract

Dutch elm disease was first recorded in 1918 in Western Europe and quickly spread to the US. The origin of the causal fungus, Ceratocystis ulmi, is unknown. A serious new epidemic began in the UK around 1970 and led to the discovery that there were two strains of the fungus: a highly pathogenic ‘aggressive strain’ responsible for the epidemic and thought to have been imported from North America^1,2, and a less pathogenic ‘non-aggressive strain’ believed to be endemic to the UK and adjacent parts of Europe since the 1920s (refs 1, 2). Recent disease outbreaks associated with the appearance of the aggressive strain in Holland³ (1972), France^4,5 and Germany^6,7 (1973) were assumed to be secondary foci of the UK epidemic. A similar outbreak in Italy⁷ (1973), however, seemed to be of longer standing (L. Mittemberger and O. Rackham, personal communications) and this, together with the discovery of the aggressive strain in Iran in 1975 (ref. 8), suggested that there was more than one source of the aggressive strain. I show here that there are two distinct races of the aggressive strain and that they have probably entered Europe independently.