Only elms (Ulmus
species) and closely related plants (Planera
) are susceptible to the Dutch elm disease fungus. American elm (U. americana
) is very susceptible. Lacebark or Chinese elm (U. parvifolia
) and the Siberian elm (U. pumila
) are the most resistant species, but natural infections of these species and their hybrids sometimes do occur.
The first symptom observed in American elm is yellow foliage on one or more branches, from late spring to midsummer. The affected foliage soon becomes wilted and brown; this symptom is called "flagging." A spring infection will cause the tree to wilt and die in the same growing season, but fall infection may allow the tree to survive the winter and die the following spring. As with other vascular diseases, brown discoloration of the sapwood may be visible on recently wilted branches.
The causal fungus is spread by the feeding activities of two species of bark beetle, the native elm bark beetle (Hylurgopinus rufipes
) and the lesser European bark beetle (Scolytus multistriatus
). These insects breed under the bark of dead elms and accumulate spores of the fungus as emerging adults. The insects feed on twigs of healthy elm trees and deposit spores of the pathogen in feeding wounds. The fungus colonizes the water-conducting tissue (xylem) of the twigs and moves throughout the tree. Affected trees die as the xylem becomes blocked. Because dying or dead trees are preferred breeding sites for the insect vectors, the disease cycle is efficient in maintaining a supply of contaminated insects. The disease also spreads from an infected elm to adjacent elms through root grafts.
Management of DED depends mainly on eliminating bark beetle breeding sites. Prompt removal of dead and dying elm trees and proper sanitation (chipping or debarking) of elm wood are initial steps of effective management. Root grafts between infected trees and adjacent elms should be severed by trenching with a back hoe or a vibratory plow to a depth of three to four feet. Trenching should be done prior to removal of infected trees. JULIE (800-892-0123) should be called before this is done to avoid cutting telephone, electric, cable, gas, or any other lines. If a new, upper-crown DED infection is detected early enough, the DED fungus can be eradicated from the tree by immediately pruning out the diseased limb or limbs. However, the fungus often moves too quickly for this method to be effective. Research has shown that certain fungicide injection treatments are effective in preventing DED, but this is an expensive procedure and should be regarded as a temporary measure for highly valuable trees. If an infected tree is standing on nearby property, fungicide injection may provide added protection against above-ground infection for the valuable specimen until the nearby threat can be removed. Without community-wide sanitation, Dutch elm disease is almost impossible to control.