By Natalie Euler
Natural Resource Specialist
The American elm was formerly considered to be the ideal street tree because it was graceful, long-lived, fast growing, and tolerant of compacted soils and air pollution. The Dutch elm disease (DED) fungus was first introduced to the U.S. on diseased elm logs from Europe prior to 1930 and began devastating the elm population. It has now spread throughout North America and has destroyed over half the elm trees in the northern United States. The disease has been reported in all states except the desert Southwest.
DED symptoms are the result of a fungus infecting the water conducting system of the tree. Once the fungus is established within a tree, it spreads rapidly through the water-conducting vessels. The tree forms gums clogging up these vessels causing the tree to wilt and eventually die. Infected elm trees show wilting, curling and yellowing of leaves on one or more branches and usually the sapwood has brown streaks.
DED is spread by two kinds of bark beetles that attack elm trees: the smaller European elm bark beetle and the native elm bark beetle. They are elm pests because they carry the DED fungus as they move from infected breeding sites to feed on healthy elm trees. The numbers of the European species are reduced by cold winters while the Native species is more common and important in the spread of DED. Most emerging beetles feed on healthy elms within 1,000-1,500 feet of where they hatched. However, beetles may rise to altitudes of several hundred feet and are carried by air currents for many miles.
There is no way to eliminate DED once it begins in an area. Control programs try to manage the disease so that losses are spread out over a long period, minimizing the impact of the disease. Some control can be accomplished with community-wide sanitation programs designed to reduce numbers of elm bark beetles and prevention of the spread of the disease through natural root grafts from infected trees to adjacent healthy trees.
It is possible, to treat trees with insecticide to reduce beetle populations. These treatments are feasible both for communities and for individual homeowners, although individual action is of limited value. Some cities have implemented the spraying of tree bases with Dursban insecticide as part of their regular DED control program. It is especially useful for treatment of areas near river corridors where bark beetle populations are high and disease-carrying beetles move in the fall from infected native stands to residential areas.
The value of sanitation programs in controlling DED are often underestimated. Controlling the movement of firewood can help reduce bark beetle populations. The spread of DED through natural root grafts accounts for the majority of new cases each year. Transmission of the disease through the roots can be prevented by severing or killing roots between the trees. This can be done without harm to the healthy trees either by mechanical trenching or through the use of chemical barriers, which have been found to be quite effective in some situations.
Several DED-tolerant varieties of American elms and hybrid elms have been developed and may be available from tree nurseries. American elms include: American Liberty, Brandon, Delaware, Independence, New Harmony, Princeton, Valley Forge, and Washington. Hybrid Elms include: Accolade, Cathedral, Discovery, Frontier, Homestead, New Horizon, Patriot, Pioneer, Regal and Triumph.
Siberian elms are not immune to DED, but they generally show less severe symptoms and are not quickly killed. They do become infected and can act as a source of DED infection for neighboring American elms.
You can help prevent DED by keeping your American elms in good, healthy condition by pruning, fertilization and deep watering during periods of drought. It is important to control other pests that may weaken your trees and to support community interest in DED control programs.
My sources for this news release were the North Dakota State University Extension and U.S. Forest Service. If you would like more information about Dutch elm disease call Natalie Euler at the Conservation Office at 605-244-5222, Extension 4 or by email [email protected]