Continuing Education and
Communication Services
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
7/2/01
Contacts:
Paula Flynn, Extension Plant Pathology, (515) 294-5168,
pflynn@iastate.edu
Elaine Edwards, Continuing Education and Communication
Services, (515) 294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning July 6
Dutch Elm Disease -- It's Still Out There
By Paula Flynn
Extension Plant Pathology
Iowa State University Extension
The 1930s were the beginning of bad times for our nation's elms. That's when the Dutch elm disease fungus found its way to North American and began killing millions of native elm trees. The disease has been found in all of Iowa's counties, and it's estimated that 95 percent of our urban American elms have fallen victim to the disease.
This famous disease was devastating because our native population of elms did not have any natural resistance to defend themselves against the newly introduced fungus. The fungus is native to Asia and was introduced into Europe shortly before World War I. It traveled to North America from Europe sometime before 1930 in crating material made from elm logs.
Even though the disappointment of losing the elms that created graceful canopies over miles of neighborhood streets has faded, the fungus is still with us. This is the time of year we are reminded that the disease is still out there. Wilted, bright yellow leaves draw attention to elm trees as they begin to die.
A few branches in the topmost part of the tree usually show the yellowing and eventual browning of leaves first. This is referred to as flagging. Other branches begin to die, sometimes quite quickly, in just a few weeks, but in other cases dying stretches over a period of several months.
The fungus that causes Dutch elm disease, Ophiostoma novo-ulmi, finds its way into elm trees by a couple of different avenues. Through their breeding and feeding, elm bark beetles can move the fungus from diseased trees to healthy trees over distances of up to several miles.
Another way the fungus can travel from a sick tree to a healthy tree is through the root system. The roots of elms located within 50 feet of each other tend to fuse together. Trees that are infected this way usually die quickly.
Once inside a tree, the Dutch elm disease fungus does its damage by growing in the water-conducting vessels. The tree responds by producing structures and substances to block the fungus. This causes even more blockage of the water-conducting vessels. Symptoms of wilt result. When the bark is removed from these wilting branches, narrow brown streaks in the outer sapwood usually are easy to see.
Keep in mind that other factors can cause elm leaves to turn brown and fall, such as elm leaf beetle feeding or injury caused by storm damage. Although the symptoms of leaf wilting and the presence of brown streaking are good indicators of Dutch elm disease, the most conclusive diagnostic method is to isolate the fungus from sick-looking branches. In Iowa, this test can be performed in the Plant Disease Clinic at Iowa State University. Samples can be submitted through your local county Extension office or mailed directly to the Plant Disease Clinic at 351 Bessey Hall, Iowa State University, Ames, IA 50011. The best sample is one that contains three to four branch segments 1/4 to 1 inch in diameter and 6 to12 inches in length. The branches should be collected from areas that are showing wilt symptoms, but are not yet dead. Wrap the sample in dry newspaper, place in a plastic bag and mail in a box. There is a $10 laboratory fee.
A common question that arises is whether we have come up with anything new to manage Dutch elm disease since the 1930s. The most important control measure has stayed the same. Diseased trees should be removed promptly. Diseased limbs with the bark attached should not be used as firewood because it serves as a breeding site for beetles. Current evidence shows that wood chips from infected trees does not pose a risk and can be used as landscape mulch.
Severing the root grafts between adjacent diseased and healthy elms is another control strategy to block the spread of the disease. This can be done with trenching equipment or with chemicals. Fungicides are another option for Dutch elm disease management. Fungicides are injected into the tree through the root flares, where the trunk connects to the roots. These injections are best used to prevent the fungus from invading trees. They are much less successful on trees that have started to show the disease, and are not successful at all on trees showing many dead branches. Fungicide injections are expensive, and may have to be repeated every one to three years.
Finally, there is a glimmer of hope for those who want to include elms in their landscape. Researchers have been working on selecting and developing elms that are tolerant or resistant to the Dutch elm disease fungus. Photographs and descriptions of some of these new elm releases can be found in the Iowa State University bulletin SUL 4, "Dutch Elm Disease." Management practices for control of Dutch elm disease also are detailed. This publication can be obtained through your local county extension office.
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