Purdue News

June 12, 1998 Dutch elm disease threatens remaining elm trees WEST LAFAYETTE, Ind. -- It's not autumn yet, but if the leaves on your elm tree have been wilting, it's time to take action -- it could be a sign of Dutch elm disease. Infected trees usually should be cut down to prevent the spread of the disease to other elms. "Dutch elm disease is one of the most destructive diseases of elm trees in the United States," said Paul Pecknold, professor of botany and plant pathology at Purdue University. "It can affect all members of the elm family, but species vary in susceptibility. For example, American elm is highly susceptible, but Siberian elm is considered to be tolerant, and Chinese elm is highly resistant." Iowa, Michigan, and Missouri all have reported an increase in the number of reported cases of Dutch elm disease over the past four years. According to Pecknold, Dutch elm disease is the second-best-known plant disease, after potato late blight, which led to the Irish potato famine. "Dutch elm disease changed the face of the American landscape by killing so many of our elm trees," Pecknold said. "It appeared in the early 1900s, and now there are only pockets of elm trees or individual survivors left." According to Pecknold, the disease first appears in mid-summer as yellow foliage on one or more branches. The affected foliage soon becomes wilted and brown, which is called "flagging," because the individual leaves resemble a flag hanging limply from a flagpole. "Often the disease can kill a tree in just one growing season," he said. Dutch elm disease is caused by a fungus, but it is spread by two species of beetles. These insects breed under the bark of dead elms, picking up the spores of the fungus. After the beetles emerge as adults, they feed on twigs of healthy elm trees. The fungus spores are left behind in the wounds on the twigs. The fungus begins to spread by growing in the tissue that moves water through the tree, the xylem. When the xylem layer becomes blocked, the tree no longer can carry water to its tissues, and it begins to die. Because dying or dead trees are preferred breeding sites for the beetles, the disease cycle is very efficient in maintaining a supply of contaminated insects. (The disease can also spread from infected elms to adjacent elms when the roots of two trees make contact and grow together below ground.) To limit the damage caused by the disease, Pecknold suggested eliminating bark beetle breeding sites by removing dead and dying elm trees. "Sanitation is critical in controlling this disease," he said "All of the diseased wood must be removed from the area. Do not use dead elm wood as firewood, because the disease can live in firewood, too." To prevent the disease from spreading from infected trees to adjacent elms through root grafts, the roots between the trees should be severed by trenching to a depth of three to four feet. Trenching should be done before removing infected trees, Pecknold said. Some companies offer treatments for the trees, but Pecknold recommended that they only be used in specific circumstances for selected highly valuable trees located in high disease areas. "Fungicide injection treatments have met with some success, but repeated injections can do more harm than good," he said. "These fungicides are generally effective for two to three years, however repeated injections are discouraged because of the mechanical damage that results from drilling into the buttress roots of the tree to inject the fungicide. This can be a case where the cure is worse than the disease." Dutch elm disease-resistant varieties are available, but Pecknold does not recommend them for planting in Indiana because of their susceptibility to other disease and insect pests. "We have not yet come up with a good disease- and insect-resistant elm for the homeowner," he said. "We hope that they will be coming with future breeding programs." For more information, refer to Purdue Extension publication BP-50, Dutch Elm Disease, available from your county Cooperative Extension Office. The publication is also available on the Web at https://www.agcom.purdue.edu/AgCom/Pubs/BP/BP-50.html Source: Paul Pecknold, (765) 494-5821; e-mail, pecknold@btny.purdue.edu Writer: Steve Tally, (765) 494-9809; e-mail, tally@purdue.edu; Web, https://www.agcom.prudue.edu/AgCom/homepages/tally/ Purdue News Service: (765) 494-2096; e-mail, purduenews@purdue.edu To the Purdue News and Photos Page