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. 

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.

   Dutch Elm Disease (DED) is quite common and is often cited as the primary cause of elm death in Michigan. Another disease which we do not discuss with any regularity is elm yellows(EY). Both diseases have some similarities: lethal to native elm species, transmission by insects, and the causal organisms are systemic within the host plant. There are differences: DED is caused by a fungus and EY is caused by a Phytoplasma (more closely related to bacteria); the transmission of EY is typically by leaf hoppers while that of DED is typically by elm bark beetles; and both diseases are not controlled by the same techniques, for example; EY will not be managed by fungicide injections. If your fungicide injections for DED management have not been effective in preventing elm death, you might want to consider EY.

   EY tends to occur in localized areas, while DED is currently widespread throughout Michigan.
Differentiating between the two diseases should not be too difficult. The flagging, death of elms over a 1 to 3 year period and vascular streaking associated with DED is common knowledge among most professionals. For EY, as the name “yellows” implies, infected trees tend to become off color (yellow) often beginning in sections or limbs. Symptoms tend to occur in mid to late summer with yellowing, drooping leaves and premature defoliation. The symptoms may spread to other parts of the tree over 2 or 3 seasons. Depending on elm species and local situations, elm death usually occurs within 1-3 years. In the case of rapid death from EY, shriveled brown leaves may adhere to trees for several weeks, mimicking DED. While vascular discoloration occurs with EY, it is more characteristic of a general browning (often in the trunk and not in the upper branches) than the streaking associated with DED. A distinguishing characteristic associated with EY for several elm species is the production of a winter green smell (from methyl salicylate) when the bark is peeled from the stem in symptomatic, not dead, elms. You can enhance the smell by enclosing some discolored inner bark in a jar for several minutes; opening the jar to your nose should provide you with the pleasant oil of winter green smell. 

   Management of DED has been discussed extensively elsewhere. The management of EY is achieved primarily through sanitation measures of isolating and removing diseased trees. Because of the unique characteristics of the Phytoplasma incitant associated with EY, storage of wood for firewood or lumber is not an issue in the case of EY.  Control of vectors of EY is not feasible.