Dutch elm disease

Dutch elm disease is a severe fungal vascular infection caused by species of the genus Ophiostoma (specifically O. ulmi and O. novo-ulmi). This pathogen acts by clogging the xylem of the host tree, obstructing the transport of water and nutrients, which inevitably leads to wilting, crown dieback, and eventually the death of the tree.

The disease primarily targets trees in the genus Ulmus. The fungus colonizes the sapwood, causing the tree to produce tyloses and gums as a defensive response; ironically, these very defenses contribute to the vascular blockage, accelerating the decline of the tree's physiological functions.

Early symptoms include the flagging or wilting of leaves on individual branches during the early summer months. Over time, the foliage turns yellow and brown before dropping prematurely. A key diagnostic feature is the presence of brown or dark streaks within the outer annual rings of the wood, visible when a suspect branch is cut in cross-section.

Transmission of Ophiostoma is mainly mediated by elm bark beetles (Scolytus spp.). These insects feed on healthy trees, introducing fungal spores into the vascular system. Additionally, the fungus can spread underground through root grafts between adjacent trees, creating local hotspots of infection that can wipe out entire stands.

Management of the disease relies heavily on proactive sanitation. The removal of dead or dying elm wood is critical to limit beetle breeding grounds. For highly valuable specimen trees, preventive measures include systemic fungicide injections and the installation of root barriers to prevent transmission through root contact, as there is currently no known cure for a fully infected tree.