Cause: Ophiostoma ulmi (formerly Ceratocystis ulmi) and O. novo-ulmi, fungi with many subgroups. The first of two North American epidemics began in the 1920s with the introduction of a relatively nonaggressive subgroup which reduced the native elm population. A second, more destructive epidemic began in the 1940s with the introduction of a more aggressive subgroup, which was found in Oregon.
Diseased trees were first found in the Pacific Northwest in Boise, ID in 1968. The next reports were from Ontario, OR in 1973 and La Grande and Union, OR in 1974. A single tree was found in Portland, OR in 1976 and was removed promptly. No other trees were identified until a new outbreak in Eugene and Portland, OR in 1986. Since then, additional Oregon locations include Hillsboro in 1987, Salem in 1988, Corvallis in 1995, Heppner in 1999, Pendleton in 2000, and Medford in 2006. The disease continued its spread with reports from Moscow and Weiser, ID and Spokane, WA in 1990 and Tacoma and Bellevue, WA in 1994. The disease has continued to spread into the greater Seattle area. As of 2008 it has not been found in British Columbia.
The disease is spread by several bark beetles, the most important of which is the smaller European elm bark beetle (Scolytus mutistriatus). These bark beetles breed in diseased trees but feed on healthy ones.
The fungus produces spores (Graphium type) in beetle galleries beneath the bark of dead or dying wood. Spores on or in the beetle's body can be carried to and infect healthy trees during feeding. Beetles overwinter as larvae under the bark of unhealthy or injured trees or cut logs. In spring, larvae complete their growth, pupate, and change into adult beetles. The adult flies a short distance and feeds on the bark of small branches of living elms (beetles are active from mid-May to early October). After feeding in the canopy of healthy trees, adults seek breeding sites under the bark of dead or weakened trees or logs. They bore through bark and into the cambium area to lay eggs.
Spores in the feeding site grow into the tree's vascular system. The mycelium continues to invade the vascular system, forming new spores (Sporothrix type) and toxins that clog the xylem and kill living cells. The result is a wilting and dying of the tree distal to the areas the fungus invaded. Trees are most susceptible in spring, during shoot elongation. The entire tree may die in a few weeks or take a year depending on how rapidly the fungus goes down to the roots.
When elms grow within 40 ft of each other, their roots often cross and form natural grafts. The fungus in the vascular system of one tree can invade the adjoining tree through the graft.
Many elm species are highly susceptible to the disease including American, Belgian, English, red, rock, September, European white, and winged elms. Intermediately susceptible are cedar, European field, and wych elms. Chinese, Japanese, and Siberian elms have resistance. Oregon contiues to quarantine all Ulmus, Zelkova, and Planera spp. which must be certified free of Dutch elm Disease (and elm yellows) before entering the state.