Dutch Elm Disease (DED) - 2000
David L. Roberts, Ph.D.
Senior Academic Specialist
Michigan State University
Dutch Elm Disease (DED), caused by the fungus Ophiostoma ulmi (Buism.) Nannf., O. novoulmi Brasier (formerly Ceratocystis ulmi), is a serious threat to American Elms around the United States. It has essentially eliminated most of American Elms from the U.S. The disease was first introduced to the Americas in the 1930's from Europe, on elm logs of all things! Why we were importing logs at that time is beyond my comprehension.
Over the past couple of years DED has been quite prevalent in many areas of Michigan. In fact, the disease has taken a sudden upward turn in incidence and I think this increase is probably due to stressed trees, especially drought stress, and a potentially higher chance of insect transmission during stress conditions. This year, (summer of 2000) is shaping up to be the worst year for loss of elms to DED than any year I can remember. There is still a rather significant population of elms in Michigan and in some locations, for example the Grosse Point area, Ulmus Americana comprises the major component of the urban forest; these trees are quite valuable to the residents. While we have a significant elm population on the MSU campus, the incidence of DED has reached an alarming rate compared to previous periods, despite the valiant efforts of the grounds department.
The fungus grows through the vascular system of the tree and prevents water movement to branches and leaves. Hence, DED is actually a wilt disease. The fungus is spread from tree to tree by elm bark beetles and through root grafts. Control or management of the disease is by a multiple endeavor approach.
Here is a list of some techniques that will help minimize DED and perhaps help us to better ward-off the disease. Some of them can be quite costly, so implementation of these efforts will depend on a variety of issues with cost being a primary consideration and feasibility being a secondary one.
Minimize Stress I don't think there's any question that stressed trees are more likely to attract the insect vectors and hence the disease. Deep-root watering during dry periods may help. Moderate fertilization may also help but keep in mind that fertilization, especially abundant amounts, may also stress trees further.
Trenching In addition to transmission by elm bark beetles, the DED fungus is spread through root grafts between trees. Trenching is a viable technique to sever these roots and minimize spread through these root unions. Trenching may have to be accomplished or designed into tiers as primary and secondary efforts. Trenching is usually performed mid-way between trees but may be difficult in some dense stands.
Eliminate Weedy Elms In many areas such as wood lots and property borders, elms grow naturally to about 20-30 feet, contract DED and die. These weedy elms serve as a great source of elm bark beetles and the DED fungus. These populations of insects and fungal propagules create a great disease pressure for nearby grand old elms. Remember, any large, old elms are simply escapes and are not resistant to the disease.
Avoid Pruning & Wounds Although not said, there is no reason why the DED fungus cannot be spread by means other than elm bark beetles and root grafts. For example, sap beetles can spread oak wilt and could potentially spread DED as well. I believe pruning wounds created during the warm season greatly increase the chances of DED. We know that slime flux bacteria (the cause of the stain on most elm trunks) are contracted by elms in such a manner. Presumably, the fermentation products emitting from the wounds with slime flux bacteria could attract insect vectors.
Injection There are several micro and macro injection techniques and products on the market. These products can be used in a preventative and curative manner. Some offer three-year rates and one-year rates. Some researchers and arborists have related better results with higher inject rates, higher injection volumes and higher number of injection sites (by exposure of the root flares). I suspect that this is true because the main impediment to success of the chemical in controlling the fungus is limitations in distribution of the chemical through the tree. Some have found success with injection even though 10-30 percent of the tree may be infected. Obviously, the less infection, the
better the chances of success by injections.