Dutch elm disease (DED) is most easily detected during early summer when the leaves on an upper branch curl and turn gray-green or yellow and finally brown. This condition is known as flagging, but a flag alone is not absolute assurance that the tree has DED. Brown streaks in the wood beneath the bark of affected branches is further evidence. Only laboratory testing can confirm positively that the tree has DED. Symptoms will continue to spread through the tree. Very susceptible species may die in the year of infection, while species with some resistance may take two years to die.

The spores of Dutch elm disease (DED) fungus overwinter in the wood of the infected host. Two insect species are able to transmit DED; the native elm bark beetle (Hylurgopinus rufipes) and the European elm bark beetle (Scolytus multistriatus). These beetles lay their eggs in infected trees. When the adult beetles emerge, they carry the fungal spores with them when they travel to healthy trees to feed on twigs and upper branches. From the feeding sites, the spores travel to the tree’s water-conducting cells or xylem. Chemicals produced by the tree during its attempt to fight the disease contribute to the plugging of the xylem, causing the tree to wilt. The beetles typically have two generations per year in the Midwest.

DED can also be transmitted through root grafts. Root grafts between trees are especially prevalent in cramped urban and suburban parkways. Root grafting is most common between two elms of the same species, but can occur between elms of different species.

Cultural Management

Valuable trees should be inspected weekly, from early May through July, and monthly through September. Proper and early identification of the disease is essential for management. Dutch elm disease (DED) is difficult to diagnose in the field and a lab diagnosis is needed. Samples should be sent to a diagnostic laboratory as soon as DED is suspected. The University of Illinois Plant Clinic will confirm DED for a nominal charge. Some arborist companies have their own labs or have access to other labs and can assist with obtaining a proper diagnosis.

Sanitation is a key preventative measure. The bark beetles breed in dead or dying elm trees and piles of elm wood with the bark attached. Therefore, trees that completely wilt and die are suitable for beetle reproduction and should be removed. Destroy the infected wood and bark by chipping and composting (chips must attain temperatures of at least 120 degrees F), or at a minimum, remove the bark from cut logs and let the logs dry out. Cut logs from diseased trees should not be kept for firewood unless all of the bark has been removed and there is no evidence of bark beetles. Transporting diseased elm firewood may spread DED to otherwise disease-free areas. Covering and sealing cut logs and chips in clear plastic during the summer will allow the sun to heat up the wood and is another way to kill the beetles and fungus. Prolonged sunny weather and high temperatures are necessary, however, for this method of sanitation, called solarization, to be effective.

An infected tree may be saved by pruning out the diseased branch promptly after seeing the first flag. Pruning may be useful in dealing with new infections that occur in the upper crown of the tree. Pruning is most effective in the very early stages of disease development when no more than 5% of the tree is symptomatic. A trained, certified arborist should be hired to do this type of pruning.

If a tree shows many flags or completely wilts and dies, it should be removed quickly so that beetles and root grafts do not transmit the disease further. Root grafts should be severed before removal of a diseased tree whenever possible. Severing root grafts is an important tool for preventing spread when elms are growing close together (within 25 to 50 feet of each other). This technique requires an arborist who has been properly trained. Roots may be severed mechanically or chemically.

Resistant cultivars

Disease resistant cultivars of American elm and disease resistant hybrid elms are available. These cultivars and hybrids are resistant to the disease, but are not completely immune.

Chemical management

Systemic fungicide treatments that can be injected into infected trees are available.These need to be applied by licensed arborists. These products are most effective when used preventatively on valuable trees that are not yet infected or are in the very early stages of infection. If properly applied, American elms may be protected for 3 years.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).