Dutch Elm Disease

Dutch elm disease is caused by by two fungal strains, a non-aggressive strain (Ophiostoma ulmi, formerly called Ceratocystis ulmi)) and an aggressive strain (O. nova-ulmi). The fungi is usually spread by two species of elm bark beetles, but can also be spread by root grafts between trees in urban settings (Hanisch and others 1983 in SAMAB 1996e). Dutch elm disease was introduced to the United States through shipments of unpeeled veneer logs from Europe in the 1930s. By 1977, the disease had spread through most of the country, killing an estimated 46 million elms. (Schlarbaumand others1997)

Dutch elm disease has mostly affected urban populations of American elm (Ulmus americana), a widely planted shade tree. In forest stands, where elms are relatively isolated from one another and spread of the disease is sporadic, American elm is declining more slowly (SAMAB 1996e). Other native elm species, such as red elm (Ulmus rubra), can be infected with Dutch elm disease, but appear to have greater resistance (Schlarbaumand others1997).

Disease management and breeding programs

There are several control measures for preventing the spread of Dutch elm disease in forest stands. The most effective management option is to promptly remove stressed, dead and dying elms. However, this intensity of treatment often is not financially feasible. A more cost-effective method is the "trap tree" method, where infected elms are treated with a herbicide, causing the bark to dry rapidly and prevent bark beetles from completing their lifecycle, thus reducing their populations. However, this method creates hazardous trees (Haugen 1998).

Resistance has been bredinto cultivars using American elm strains that are tolerant of Dutch elm disease. Two new cultivars, "Valley Forge" and "New Harmony," were released by the U. S. National Arboretum in 1996 (U. S. National Arboretum 1996). Attempts to breed resistance into American elm using other Ulmus species generally fail due to a reproductive barrier between tetraploid American elm and other elm species, which are diploid (Santamour 1969, Schlarbaum and others 1997).

Although trees with good tolerance to Dutch elm disease have been found, very little is known about the mechanisms of tolerance. A current joint projectby the USDA Forest Service and the U. S. National Arboretum is studying the genetics of host resistance to improve tree selection in breeding programs and to illucidate quantitative inheritance of disease tolerance (Schlarbaum and others 1997).

For more information onDutch elm disease, see "How to identify and manage Dutch elm disease", on the Northeastern Area State and Private Forestrys website.