Dutch Elm Disease

Initially, infected trees have scattered dead branches.

Symptoms of Dutch elm disease typically begin to show up during May and June on elms. All native elms are susceptible to this disease; however, the American elm is highly susceptible. Many European and Asian elms are less susceptible. The first symptoms typically include leaf yellowing, followed by wilting and branch by branch dieback. Trees can die fairly rapidly, within a matter of weeks, or the dieback can be limited to single branches, and disease progression can occur over several months or years. Because several other diseases can often be mistaken for Dutch elm disease, it is important to look for the characteristic brown streaking in the sapwood. Positive diagnosis of this disease requires a laboratory test.

Dutch elm disease is caused by two closely related fungi, Ophiostoma ulmi and O. novo-ulmi, with the latter being the more aggressive fungi and is largely believed to have replaced the other species. Trees can become infected either through the feeding activities of contaminated bark beetles or through root grafts with an infected tree. The primary means of infection into a new area is through two species of bark beetles. Bark beetles are attracted to stressed, dying or recently dead trees in which to lay eggs. If the tree is infected with DED, the larvae and later the adults become coated with the sticky-coated spores growing under the bark. As the adults leave the brood tree, they fly to healthy trees in which to feed, where they inoculate the healthy trees. Within a population of elms, the fungus can also move, via the vascular system, to healthy trees that have roots grafted with the infected tree.

Unfortunately, there is no cure for this disease; however, there are certain steps you can take to reduce the rate of the spread of this disease in your landscape. In an urban environment, sanitation is very important to reduce the amount of trees that are lost due to Dutch elm disease. Diseased trees should be removed promptly to reduce breeding sites for the beetles. Wood should be debarked and or destroyed within 2 to 3 weeks of detection during the growing season. During the dormant season, removal should be completed before April, when the overwintering beetles emerge.

Elms within 25 to 50 feet of the infected elm have a good chance of having grafted roots and becoming infected. The root grafts should be destroyed before the infected elm is removed. This can be accomplished with chemicals or by mechanical severing and will require the services of a professional. The sanitation approach is difficult in rural or naturalized areas where many elms still exist along roadways, fence rows and in woodlands.

Brown streaks in sapwood

Because the beetles are attracted to stressed or weakened trees for breeding, maintaining plant health is important. Provide adequate amounts of water and fertilizer. Mild infections of Dutch elm disease may respond to pruning of diseased branches. Systemic fungicides injected into the tree at 1 to 3 year intervals have proven beneficial in providing protection prior to infection. An arborist should be consulted to discuss costs and potential benefits.

Some elm species have a high level of resistance to Dutch elm disease, so consider replacing diseased trees with a resistant elm. The true Chinese elm, Ulmus parviflora, is one possible choice as are the “American liberty” and “urban” elms. A related species, Japanese zelkova, Zelkova serrata, has a vase shape that is similar to American elm.

For pictures and more detailed management information about this disease see the following:
agebb.missouri.edu/pdc/trees/elm.htm
www.na.fs.fed.us/spfo/pubs/howtos/ht_ded/ht_ded.htm

Laura Kabrick, Extension Assistant, Plant Diagnostic Clinic, UMC (573)882-3019