Intercontinental Spread and Continuing Evolution of the Dutch Elm Disease Pathogens

The two enormously destructive pandemics of Dutch elm disease (DED) that have spread across Europe and North America this century have been caused by two very different fungal pathogens. First to appear, in the early 1900s, was Ophiostoma ulmi. Second to appear, around the 1940s, was the much more aggressive O. novo-ulmi which exists as distinct Eurasian (EAN) and North American (NAN) forms, equivalent to subspecies. Current knowledge of the origins and spread of these DED fungi across the Northern Hemisphere is presented. O. novo-ulmi has been rapidly replacing O. ulmi across most of Europe and North America. It may well have picked up “useful” genes from O. ulmi in the process. In addition, where the EAN and NAN forms of O. novo-ulmi are now geographically overlapping, they are freely hybridizing. As a result, a new form of O. novo-ulmi may emerge in the future. In 1993 yet another aggressive DED pathogen was discovered in the Himalaya; this was named O. himal-ulmi. Rather than causing epidemic disease, O. himal-ulmi appears to be in natural balance with Himalayan elms and elm bark beetles.

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