What genus of fungi causes Dutch elm disease?

The genus Ophiostoma causes Dutch elm disease.

Dutch elm disease is a vascular disease of elm trees caused by several related ascomycete fungi, including Ophiostoma ulmi and the more virulent Ophiostoma novo-ulmi. Ophiostoma himal-ulmi is another recognized species associated with the disease in the western Himalaya. The disease is therefore linked to a genus rather than to one single fungal species.

Elm bark beetles spread the fungus when they move between trees. The pathogen enters an elm through beetle feeding galleries and colonizes the tree’s water-conducting xylem. In response, the tree produces gums and tyloses that block the fungus, but these defenses also obstruct water movement. Leaves wilt, branches die back, and the tree may eventually perish from water stress.

The disease was identified in the Netherlands from 1921 onward, which explains its name; it is not a fungus native only to the Netherlands. A historical taxonomic complication is that older literature used the name Ceratocystis ulmi for the pathogen. Modern classifications place the principal Dutch elm disease fungi in Ophiostoma, making the stored genus the accepted answer.