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Professional Affiliations:

Illinois Arborist Association
Tree Care Industry Association
International Society of
Arborculture
Better Business Bureau

Special Bulletin

Ulmus: Dutch Elm Disease
Problem:

Other Important Bulletins:

Leaves wilt, curl, and turn yellow on one or more branches in the top of the tree; many leaves drop off. Trees may die slowly over a period of a year or longer. Or trees wilt and die within a few weeks, often in the spring soon after they have leafed out. Cross sections of infected branches may show a ring of brown dots in the wood, just underneath the bark. Small holes may be found in the bark of infected branches.

Analysis
Dutch elm disease is caused by a fungus (Ceratocystis ulmi) that invades and plugs the water-conducting vessels in the tree. The fungus enters the wood through feeding wounds made by elm bark beetles. In the spring, the adult beetles emerge from holes in the bark of elm trees where they have spent the winter. If the trees are infected with the Dutch elm fungus, the beetles have sticky spores of the fungus on and inside their bodies. The beetles disperse to healthy elm trees, where they feed in crotches of small twigs, usually high in the tree, and deposit the fungus spores in the wounds.

The fungus then spreads through the tree. The infected elm usually develops the disease that summer. The fungus produces a toxin that interferes with the water-conducting vessels in the wood, reducing the amount of water available to the leaves. The foliage on the infected branch wilts, turns yellow, and drops. Surrounding branches, and eventually the entire tree, become infected and the tree dies. When elms are closely planted (50 feet or less between trees), the fungus may spread through natural root grafts between trees. Trees infected through the roots generally wilt and die rapidly, often when growth begins in spring. These trees do not show browning in the wood when the branches are cut.

Solution
Curing a tree of Dutch elm disease is usually not possible. Disease development may be delayed on lightly infected trees (where less than 5 percent of the foliage and branches show evidence of the disease) that were initially infected by bark beetles rather than by root grafts. Removal of early infections by tree surgery may save a tree for a number of years. Remove yellowing branches at least 10 feet below the point where brown streaks are visible in the wood. Sterilize pruning tools with rubbing alcohol after each cut. Spray the wound with an insecticide containing methoxychlor. Systemic fungicides (fungicides that are carried throughout the tree), injected into the tree by a trained arborist, may increase the life span of a lightly infected tree. Contact an arborist when yellowing is first noticed. Prevention involves 3 different measures that should be carried out on a communitywide basis.

(1) A good sanitation program will slow the spread of the fungus. Since bark beetles carrying the fungus breed in dead or dying elm wood, all dead or dying trees, damaged limbs, and prunings should be removed and burned or buried. The bark of all stumps should be peeled to just below ground level.

(2) Controlling bark beetles with insecticides is practical only on valuable specimens, when used in conjunction with a sanitation program. Spray with an insecticide containing methoxychlor in early spring before leaves come out. Use high-pressure spray equipment, and cover the entire tree thoroughly.

(3) To prevent transmission of the fungus through the roots of closely planted trees, grafted roots should be severed either by mechanical trenching or by soil injection of a chemical. This is important if there is a diseased tree nearby. Contact a professional arborist.

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