The Dutch elm disease is caused by three fungal species: a) Ophiostoma ulmi (Buisman) Nannf. sensu stricto, initially termed the non-aggressive subgroup of O. ulmi sensu lato (= Ceratocystis ulmi), a more weakly pathogenic fungus, responsible for the first pandemic of the disease in Europe and North America in the 1920s—1940s; b) Ophiostoma novo-ulmi Brasier was initially termed the aggressive subgroup of O. ulmi sensu lato. It is responsible for the current second pandemic, killing most mature elm trees across Europe, North America and central Asia. As O. novo-ulmi spreads it is replacing O. ulmi sens. str. It comprises two subspecies: O. novo-ulmi subsp. novo-ulmi and O. novo-ulmi subsp. americana; c) O. himal-ulmi Brasier & M.D. Mehrotra was recorded on Ulmus wallichiana in the Indian Himalayas and is able to infect U. procera (English elm) (Brasier 1991, Brasier & Mehrotra 1995, Brasier & Kirk 2001).
Elm bark beetles (Scolytus spp. and Hylurgopinus rufipes) carry spores of the pathogen in their bodies. When the insects feed on vascular tissues on branches of the tree, the fungus is inoculated and colonizes the xylem vessels causing its clogging and producing a vascular wilt. T