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Dutch Elm Management

Cultural strategies
Today, some communities maintain active programs to manage Dutch elm disease because they have found that it is cheaper to manage the disease than to remove the large dead trees that it leaves behind. Some communities focus on cultural practices for disease management, including the avoidance of monocultures of elm trees, the removal of all dying or recently dead branches, trees, and cut wood (sanitation), and the breakage of root grafts between adjacent elms. To be successful, diligent inspection of all elm trees in an area several times each growing season is required. Wood must be burned, chipped or buried so that it cannot provide a home for beetle vectors (Figure 16).

Figure 16

Organized community sanitation programs can delay the loss of elms. It has been estimated that the time when half of the elm trees in an area have been lost can be delayed by between 7 and 30 years. If privately owned trees are included in a program of inspection and mandatory removal, the longer end of this range is more likely. At best, this is a delaying tactic in the battle against Dutch elm disease.

Chemical strategies
In the past, insecticides were sprayed on elm trees in attempts to kill the beetle vectors of Dutch elm disease (Figure 17). This management strategy was expensive, not very effective, and came under attack from people concerned about the impact of insecticide use on wildlife and people.


Figure 17	Figure 18	Figure 19

More recently, fungicides have been injected into trees infected by or at risk of infection by the Dutch elm disease pathogens (Figure 18). These systemic chemicals are most effective if they are used to prevent new infections or to prevent the movement of the fungi into parts of a tree that are not yet colonized. Several different fungicides have been used, but all are relatively expensive, and none is completely effective. For these reasons, chemical management of Dutch elm disease is commonly used only to protect elm trees of high value, such as those along the Mall in Washington, D.C. (Figure 19) or large trees in the yards of well-maintained properties.

Breeding for resistance
The long-term solution to Dutch elm disease lies in the development of disease-resistant cultivars of elms. Several Asian elm species have moderate to high resistance, and breeding programs in both Europe and the U.S. have introduced resistance from these species into native elm species (Figure 20). Other programs have focused on identifying and cloning individual American elm specimens that have moderate resistance to Dutch elm disease. The American elm breeders also would like to maintain the elegant vase shape of the American elm - the quality that made it a highly desirable shade tree. As a result of decades of efforts by elm breeders, several hybrid and clonal elms are now available that have very good resistance to Dutch elm disease (Figure 21).


Figure 20	Figure 21