May 13, 2004 at 2:56 p.m.
Dutch elm disease is caused by a fungus that is usually fatal to elms native to the upper Midwest. It attacks American elm, red and rock elms and occasionally non-native elms.
DED is thought to have originated in Asia and was first introduced to this country before 1930 on elm-veneer logs from Europe. It rapidly spread, reaching Minnesota in 1961. In North America the disease has killed millions of elm trees, cost billions of dollars in tree removal and resulted in the loss of priceless shade and habitat.
The fungus is a close relative to the fungus that causes oak wilt. It is a vascular wilt pathogen that can quickly kill even large trees once they become infected. The disease is usually first noticed as branches with wilting, limp yellow and brown leaves that often fall from the tree. Symptoms usually progress through the crown of the tree until the whole tree is dead.
The fungus is introduced into the tree by two ways. One is by elm bark beetles, both native and European, which feed on small branches in the spring and summer. The other way is by transmission into the root system of healthy trees from nearby diseased trees by means of root grafts.
Symptoms usually start in the spring and summer and can move through the tree and kill it in a single season. Occasionally, trees infected the previous year do not show symptoms until early in the following spring. Sometimes it takes two or more seasons for trees to die.
There is little resistance in American elm populations, though some other types of native elms may tolerate infection. Asiatic and European elm seem to show more resistance. The time of year, rainfall amounts, and tree vitality can also affect DED infection. Elms are most easily infected in spring when water-conducting elements are large. Trees are also more susceptible to infection when they are growing vigorously. DED incidence has increased in the last few years due to mild winters, drought stress and an increase in storage of bark-intact elm firewood. These factors cause an increase in bark beetle habitat and survival, which in turn increases the incidence of the disease.
The native and European elm bark beetles that spread DED are attracted to stressed and dying trees. They tunnel into these trees to lay eggs and complete their life cycle. They can also colonize dead elm wood with intact bark, pupate, and emerge as adult beetles. The adults feed on twig crotches and the inner bark of branches, introducing the fungus into wood vessels. Several generations of bark beetles are born each year in Minnesota.
Infections caused by root graft transmission is where the fungus travels from diseased elms to nearby healthy trees. This method progresses rapidly where the trees are very close together on a boulevard.
There are several ways used to control the disease, with sanitation the most effective. Removal of infected, stressed, and dying elms, as well as pruning of infected, dying and dead branches, is essential. The wood must be debarked and destroyed to prevent emergence of the bark beetles. Pruning to eradicate branch infections is most effective if done soon after a new infection has been identified and if less than 5 percent of the crown to the tree shows symptoms. Pruning is not effective in controlling infections that arise by way of root graft transmission.
There are a number of fungicides registered in the management of DED. Arbotect 20-5 and Alamo are reportedly the most effective and can be used on either a preventive or a therapeutic basis. Fungicides are injected at the root flare and may be effective for several years. Only large, high-value elms should be treated, due to the expense of the procedure and the risk posed to the tree from repeated injection wounds. Also, only trained arborists should inject fungicide, in combination with a good sanitation program.
There are some new American elm cultivars that are resistant to DED. They include the Princeton, Valleyforge, and New Harmony. Asiatic elm species tend to have good resistance to DED but do not exhibit the vase-like form of the American elm. Two of the Asiatic elms that are resistant are the accolade and cathedral.
When planting trees it’s a good idea to mix cultivated varieties and species to ensure that future disease or insect pests will not decimate the local tree population. Most effective DED management programs use a combination of management practices.
You can leave your gardening questions on the Master Gardener voice mail at 651-237-3080 or if you have internet access you might find your answers at the following sites.
www.extension.umn.edu/county/chisago Check out the “Hot Topics” box in the middle of the page for current Chisago County Master Gardener news and events.
You can also click on “Ask a Master Gardener” next to the cute little flower on the right hand side of the page. Here you can search 1,000s of answers from Master Gardeners around the state.
If you don’t find your answer, you can submit a question online or search for University publications, Bell Museum of Natural History. For information about snakes, skunks, raccoons or other wildlife around your yard, call the wildlife information line at (612) 624-1374 or www.bellmuseum.org.