All species of elms native to North America are attacked by the fungus that causes Dutch elm disease, as well as other genera in the elm family. In Colorado it is most damaging to the American elm. Siberian elm is tolerant of the pathogen and is an important potential breeding site for insect vectors of the pathogen.
Diagnosis and Damage
Trees affected with Dutch elm disease do not survive and this disease has all but wiped out native stands of American elm as well as urban and farmstead plantings.
Biology and Disease Cycle
This fungus is spread from declining or dead elms to healthy elms via insect vectors or root grafts. The lesser European elm bark beetle, Scolytus multistriatus has also been found to transport the fungus from tree to tree. The beetles breed in declining or dead trees or logs infected with the fungus. Sticky fungal spores adhere to the insect’s body and are carried to healthy trees where adult beetles go to feed on twig crotches. The fungus is consequently introduced into the host tree and invades and grows in the water-conducting vessels, causing wilting and death of the tree. Once in a host tree, the fungus can travel to nearby (35 to 50 feet) elm trees via root grafts.
Dead elm wood or declining elm trees are prime breeding grounds for the beetle vectors, increasing the likelihood of disease spread. Susceptible elms of the same species planted within close proximity to each other favor the spread of the pathogen through root grafts. There is no cure for Dutch elm disease and the primary emphasis in management is prevention. Systematic inspection of every elm within a community to detect the early symptoms of infection, isolation by disruption of root grafts between infected and healthy trees, and prompt removal and disposal of all dead and dying elm material with intact bark are key to a good management program. Early infections can be removed from elm trees by pruning. A minimum of 8 to 10 feet of streak-free wood (no vascular discoloration) must be removed. Individual high value trees may be protected by injection of chemical fungicides at labeled intervals.