Dutch elm disease

Since the early 20th century, elm populations worldwide have been strongly impacted by two successive pandemics of a vascular wilt known as Dutch elm disease (DED). These pandemics were caused by the closely related Ascomycete fungi Ophiostoma ulmi and Ophiostoma novo-ulmi. The biological and geographical origins of DED fungi, along with the mechanisms by which they kill their host are still unclear. On the other hand, vectoring of DED fungi by elm bark beetles clearly contributed to rapid spread of the pathogens throughout elm populations. Interspecific hybridization with the less aggressive O. ulmi, followed by intraspecific hybridization events, allowed the highly aggressive O. novo-ulmi to acquire traits that contribute to high fitness. Despite heavy losses of elms in several countries, integrated management of DED is possible. Moreover, detection of disease resistant individuals in several populations of susceptible elm species suggests that intraspecific crosses are a viable alternative to interspecific crosses with resistant species for DED resistance breeding.