This disease is not of Dutch origin, but because early work on the disease was done by Dutch pathologists in the 1920s, the disease has been called Dutch Elm Disease (DED). In all probability the disease is of Asiatic origin.
DED is caused by a fungus called Ophiostoma ulmi (formerly Ceratocystis ulmi) that was introduced to the U.S. in the early 1930s. The American elm, Ulmus americana, is extremely susceptible and the disease has killed hundreds of thousands of elms across the U.S. All native elms are susceptible, as are European elms, but the Asiatic elms, U. parvifolia (Lace bark elm) and U. pumila(Siberian elm) are highly resistant to the disease. The disease is still a threat today, but fortunately, several resistant American elm and hybrid elm selections are available or being developed.
There are two insect vectors responsible for transmitting DED: the native elm bark beetle (Hylurgopinus rufipes) and the European elm bark beetle (Scolytus multistriatus). The beetles carry the fungus from infected trees to healthy trees as they feed on twigs and upper branches. They lay their eggs in the bark and wood of unhealthy trees. Emerging larvae form galleries (tunnels) underneath the bark and the fungus grows through the galleries until it reaches the tree’s water conducting cells, or xylem. Chemicals produced by the tree during its attempt to fight the disease plug up the xylem, causing the tree to wilt. The beetles typically have two generations per year in the Midwest. DED can also be transmitted through root grafts. Root grafts between trees are especially prevalent in cramped urban and suburban parkways. The disease usually does not spread in this manner beneath roads because the road foundation prevents root grafts between trees on opposite sides. Driveways and sidewalks are usually not effective barriers to root growth.