The American elm, Ulmus americana, is a large, vase-shaped tree that was a minor timber species used for furniture, cooperage, and construction of boats. Many cities and towns across America once planted American elms along streets to provide shade (Karnosky, 1979). The species was ideal for urban settings because it withstands soil compaction and drought. In 1930, however, urban American elms began to die from a disease caused by an introduced fungus, Ophiostoma (=Ceratocystis) ulmi. The fungus was introduced on shipments of unpeeled raw veneer logs from Europe and rapidly spread from three infestation centers (Stipes & Campana, 1981). The fungus is vectored by the native elm bark beetle (Hylurgopinus rufipes) and the exotic smaller European bark beetle (Scolytus multistriatus). When trees are infected with the fungus, they react by producing tyloses and gums that will eventually block the vascular system and cause demise. Dutch elm disease can be passed from tree to tree by root grafts, which are common in urban settings. American elm is the least resistant of North American elm species to Dutch elm disease. Other native elms, for example, red or slippery elm (Ulmus rubra), have more resistance.
The disease has spread to most of the contiguous 48 states with the exception of a few southwestern states. The demise of urban trees, however, has given a misleading impression that forest trees are also dead. In United States forests, the disease is still radiating south and westward. American elms over 75 cm (30 inches) DBH are still common in Tennessee. A new pathogen, O. novo-ulmi, which first appeared in North America but is of unknown origin, has caused a second, more damaging epidemic in both North American and Europe (Brasier & Buck 2001).
Insecticides and fungicides have been used to protect urban trees with varying success. Breeding resistance into American elms from other elm species has been complicated by the tetraploidy of the American elm, in contrast with the diploid nature of all other elm species. Nevertheless, resistant hybrid triploids have been produced (Sherald et al., 1994). Another approach that has met with good success is crossing among surviving individuals, followed by clonal propagation of resistant progenies (Smalley & Guries, 1993; Townsend, 2000). This work has been led by Alden M. Townsend of the U. S. National Arboretum and the Elm Research Institute, a private, nonprofit organization, and a number of resistant elm clones have been released.
Brasier, C. M. and K. W. Buck. 2001. Rapid evolutionary changes in a globally invading fungal pathogen (Dutch elm disease). Biological Invasions 3: 223-233, 2001.
Karnosky, D. F. 1979. Dutch elm disease: a review of the history, environmental implications, control, and research needs. Environmental Conservation 6(4): 311-322.
Sherald, J. L., F. S. Santamour, R. K. Hajela, N. Hajela, and M. B. Sticklen. 1994. A dutch elm disease resistant triploid elm. Can. J. For. Res. 24: 647-653.
Smalley, E. B., and R. P. Guries. 1993. Breeding elms for resistance to Dutch elm disease. Ann. Rev. Phytopath. 31: 325-352.
Stipes, R. J. and R. J. Campana (eds.). 1981. Compendium of elm diseases. Am. Phytopath. Soc., St. Paul, Minn. 96 pp.
Townsend, A. M. 2000. USDA genetic research on elms. In The Elms: Breeding, Conservation, and Disease Management, C. L. Dunn, ed. Kluwer Academic Publ., Nowell, Massachusetts. pp. 271-278.