Dutch elm disease is a fatal fungal disorder spread by the elm bark beetle. The fungus can be carried from a diseased tree to a healthy one by the beetle, by the roots of an infected tree grafting together underground with the roots of a healthy tree, or by contaminated pruning tools. There is no cure for the disease once a tree is infected.
American elms (Ulmus americana) are the most susceptible to the fungus; Siberian and Chinese elms are more resistant but can still contract the disease. Much research is currently underway to breed disease-resistant elms that still exhibit the desirable, open vase-shape of the American elm. One such tree is the new Accolade™ (Ulmus ‘Morton’) elm introduced by Chicagoland Grows®, Inc.
Description & Symptoms
The fungus causes the vascular system of the tree to become clogged, preventing the flow of water and nutrients from the soil to the upper canopy of the tree. This manifests itself in the characteristic "flagging" of solitary upper branches, where leaves suddenly wilt, turn green, yellow, or brown, curl, and drop off the tree. Often these symptoms are first noticed in June when one or two dead branches appear high in the tree.
Infected branches will also exhibit brown streaking in the sapwood immediately under the bark. Tissues must be cultured in a laboratory for positive identification of Dutch elm disease. Because there are other wilt diseases that can produce similar symptoms, homeowners should have their trees tested immediately if they observe branch dieback. Diseased trees must be quickly removed to prevent spread to neighboring elms.
The disease can spread quickly throughout the infected tree and to neighboring trees. If symptoms appear in late summer or early fall, homeowners might confuse the yellow or brown leaves with premature fall coloring. Dutch elm disease can kill smaller trees in one season; large mature trees might succumb in several years.
Treatment & Solutions
There are no chemicals available to homeowners to cure this disease. Commercial fungicides are available for Dutch elm disease; however, only licensed applicators are able to purchase and apply these fungicides. These products are injected into the infected tree and the need for retreatment ranges from one to three years. Fungicides injected into infected trees are less effective. Fungicides injected into trees with infected interlocking roots will also not be effective.
Losing mature trees that are over a hundred years old can be an emotional experience for homeowners, and some will go to any length to prolong the years they can enjoy their trees. When choosing a large shade tree for parks, lawns, or parkways, homeowners are encouraged to plant disease- and insect-resistant varieties and to avoid planting only one type of tree in their neighborhoods.
For those who own American elms, keeping the tree healthy and reducing stress is the best prevention. Make sure elms receive one inch of water per week, especially during periods of drought. Consider deep-root fertilization every few years. Avoid growing grass right up to the trunk of the tree. Gently peel back turf to create as large an area as possible (optimally out to the drip line of the tree) where shredded mulch or wood chips can be applied two inches thick. Watch your tree diligently for the first signs of problems: bare branches in the upper canopy, yellowing or dropping leaves in spring or summer, late or stunted growth. Also, keep a close watch on the health of neighboring elms, since the disease spreads quickly.
For more information, recommendations, or a list of certified arborists, please contact Plant Information Service at (847) 835-0972 or via e-mail at firstname.lastname@example.org.