Life cycle and symptoms of dutch elm disease

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Last Updated: 
January 23, 2004

CORVALLIS – Dutch elm disease, a fungal disease of elms, is spread by several bark beetles, most importantly, the smaller European elm bark beetle (Scolytus mutistriatus), Oregon State University researchers say.

The beetles breed in diseased trees but feed on healthy ones.

The fungus produces spores in the tunnels or "galleries" that beetles carve beneath the bark in dead or dying wood. Spores on or in the beetle's body infect healthy trees. Beetles overwinter as larvae under the bark of unhealthy or injured trees or cut logs.

Each spring, larvae complete their growth, pupate and change into adult beetles. Adults fly a short distance and feed on the bark of small branches of living elms from mid-May to early October. After feeding in the canopy of healthy trees, adult beetles 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.

The fungal spores that actually cause Dutch elm disease grow and secrete toxins into trees vascular systems, clogging the water conducting cells called xylem. This kills the living cells of trees.

Trees are most susceptible in spring, during shoot elongation. Entire trees may die in a few weeks or take a year depending on how rapidly the fungus goes down to the roots.

Early symptoms of Dutch elm disease include wilting leaves and sparse foliage – first on single limbs, according to OSU Extension plant pathologist Jay Pscheidt. Later the entire tree may be affected. Yellow leaves and premature leaf loss follow.

Symptoms may show up only a few days after infection has occurred. If any main limbs are infected via lower branches in spring, progress may be rapid, and trees can wilt and die in a short time. Or, if an infection is high and late in the year, disease progress may be slow. Symptoms may be confused with other vascular wilt diseases or summer heat stress.

Infected sapwood shows brown streaking, especially in its current-season growth. A cross-section often shows either a broken or continuous brown ring in outer growth rings. Cambium layers may also be discolored.

When elms grow within 40 feet of each other, their roots often cross and form natural grafts. 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.

If you think you have a diseased elm, Pscheidt recommends contacting a professional arborist for diagnosis, treatment and removal.

More detailed information about treatment and controls are on OSU Extension's "An on-line guide to plant disease control" website at: http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=435.

Author: Carol Savonen
Source: Jay Pscheidt