Dutch Elm Disease is Back

Twin Cities residents should be aware that Dutch elm disease has come back with a vengeance this summer. Actually, it had never really gone away.

According to Lori Mordaunt, Science Officer at Rainbow Treecare, the Twin Cities area is experiencing one of the worst DED outbreaks since it arrived here in the 1970s. "For approximately 50 years Dutch elm disease has devastated unprotected elm populations across the United States , spreading westward from its origins on the East Coast. However, with the development of responsive community programs and effective fungicidal injection treatments, the disease was later brought under reasonable control. But it had never really gone away."

Cities such as Minneapolis, Edina, Richfield, Minnetonka, St. Louis Park and Bloomington are still home to large inventories of American elm trees. In fact, the Twin Cities boasts one of the largest American elm populations in the country. Through rigorous sanitation programs area cities have been able to keep the disease in check, with recorded annual losses running somewhere between 2% to 4%. But losses for 2004 are predicted to go much higher given the impact of reduced city budgets and decreasing public awareness of the disease.

According to veteran ISA Certified Arborist, Al Olson, "During the past three weeks I've been seeing signs of DED consistently across the area. Throughout Bloomington , Minneapolis , and the adjacent suburbs I'd estimate that I've seen one diseased tree for about every two blocks. It's really something. If a home owner doesn't have DED now, I'd say that they've been quite lucky, or that they've taken a proactive approach to protecting their trees."

Reasons for the recent surge in Dutch elm disease infections are not totally clear at this point, but several possibilities seem likely. First, with recent city budget cutbacks, it has been difficult for some municipalities to remove infected trees as quickly as in past years. A diseased tree standing longer allows the responsible fungus (Ophiostoma ulmi) to spread to other trees. Root grafting is one method of transmission. The other vector is the elm bark beetle that carries the fungus from diseased to otherwise healthy trees. These beetle infections can be prevented through the proper injection of an EPA (Environmental Protection Agency) approved fungicide.

Another reason seems to be a general lack of public awareness. Homeowners have become complacent about DED, having never heard of it, or believing that it was resolved back in the 1980s. "A lot of people just don't care anymore", said Gary Johnson, a professor of Urban Forestry at the University of Minnesota.

"The heavy spring rains of this past season encouraged greater than average vascular tissue growth in the trees and correspondingly larger water-conducting vessels, where the fungus operates", according to Mordaunt. "In that kind of environment DED moves faster than experienced in drier years when the vessels are more constricted."

Dutch elm disease kills elms regardless of their health. It is considered the most costly shade tree disease ever and will remain active in a community as long as there are susceptible trees.

To properly manage Dutch elm disease, four steps are generally required. First; identify the disease by the "flagging", or wilting of leaves typically located at the ends of isolated branches in the canopy. Infected leaves turn dull green to yellow and then curl. The disease moves down the limb and then into the main stem. Second; sanitize the area. Remove infected branches from trees (if identified in time) or remove entire trees, immediately, in order to halt further spread of the disease throughout the neighborhood. Third; isolate infected trees from nearby healthy trees by soil trenching, which prevents spread of the disease via root grafting. Fourth; protect valuable remaining elms with the use of an approved Dutch elm disease fungicide. Contact your city forester or an ISA Certified Arborist who is familiar with modern tree injection technology.

Rainbow Treecare is the leading tree care company in the United States that protects elms.

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