XVI International Botanical Congess

Two highly destructive pandemics of Dutch elm disease have occured across the northern hemisphere this century. Each has been caused by a different, introduced fungal pathogen. First to appear, in the early 1900s, was Ophiostoma ulmi, this was followed in the 1940s by the even more aggressive O.novo-ulmi, which exists as distinct Eurasian (EAN) and North American (NAN) forms, equivalent to subspecies. Evidence for the origins and history of spread of these pathogens will be presented. As O. novo-ulmi has rapidly replaced O. ulmi, rare and transient hybrids have occured between them, in consequence, O. novo-ulmi appears to have acquired 'useful' (anti-viral) genes from O. ulmi. Where the EAN and NAN 'subspecies' of O. novo-ulmi geographically overlap, they are now freely hybridizing. As a result, it is likely that yet another form of O.novo-ulmi will emerge in the near future.