During the last century, the fungi Ophiostoma ulmi and O. novo-ulmi, causal agents of DED, were introduced in Europe and North America due to globalization in the transport of plant material derived from elms. Their origin is unknown, but emerging evidence suggests that O. ulmi could be endemic to Japan.

After its introduction, the pathogen quickly expanded in various epidemic fronts. O. ulmi spread in the early twentieth century from the northwest to central and southern Europe, and eastward to the British Isles and North America. Around 1940, two subspecies of O. novo-ulmi were introduced into Europe and North America, and rapidly replaced O. ulmi as they are better adapted to temperate climates.

The disease was first recognized in Spain in 1932 by the forest pathologist Benito Martinez. The first O. ulmi pandemic caused around 10 to 40% elm losses, and natural selection of tolerant individuals. This fungus is known as the “non aggressive species”, since nowadays it is a relatively weak pathogen. O. novo-ulmi, the causal agent of the second pandemic that led to the virtual disappearance of adult elms in Spain, is known as the “aggressive species”.

Picture taken by scanning electron microscopy showing the hyphae of the DED pathogen (Ophiostoma novo-ulmi) colonizing an earlywood vessel. During xylem colonization the fungus causes cavitation of the water column, and results in a wilting syndrome. Photo: Luis García Esteban.

(Click on image to view larger)