What is Killing Our Trees?
Dutch Elm Disease Changed Landscapes
This is the second in a six-article series called "What Is Killing Our Trees." The series will explore some of the lesser-known diseases that affect Michigan trees and potentially impact wildlife.
Perhaps no tree affliction has had as much impact on urban and rural landscapes as Dutch elm disease, a wilt fungus that grows in the sapwood of large elms and is spread by elm bark beetles. The American elm, which has been devastated by the disease since the 1930s, was once the signature tree of countless urban streets and rural roadsides in the eastern half of North America. Stately elms, with their deliquescent (vase-shaped) branching pattern, also towered over bottomlands, forming a graceful, arching canopy that enhanced the scenery along rivers and smaller drainageways. That all came to an end quickly.
Dutch elm disease was first documented in the U.S. in Cleveland, Ohio in 1930. It was apparently brought to our country accidentally in high quality logs shipped from France for Ohio's then-thriving furniture industry. Some argue that less serious forms of the disease may have already been found in America, but the Ohio outbreak had impacts on elms that far surpassed any previously seen. By 1970, Dutch elm disease had killed an estimated 77 million trees, and the Eastern U.S. landscape was irreversibly changed.
The disease originated in Asia and a strain that caused relatively minor impacts showed up in Europe around 1910. It killed only a small percentage of Europe's elms. On trees that survived, it often simply defoliated a few branches. A pioneering Dutch plant disease scientist, Marie Beatrice Schwarz, studied the fungus in detail in 1921 and thereafter it was called Dutch elm disease (even though Holland had nothing to do with its origin or spread). The disease moved slowly through northern Europe and reached Britain in 1927. Then, it almost died out in Europe by World War II.
Around 1967, while U.S. elms were being killed by the millions by the imported Dutch elm disease, North America accidentally sent a much more deadly strain of the disease to Europe. That strain arrived in England in a shipment of rock elm logs from North America. It promptly spread like wildfire and wiped out 25 million European elms in Britain alone. By the 1990s, there were only a relative handful of mature elms left in most areas of Europe. Among the casualties were majestic trees that had been featured for centuries in countless paintings of countrysides.
In Michigan, the devastation was also dramatic. In the 1950s, Dutch elm disease wiped out the trees that gave the Macomb County village of New Haven the nick-name "Elm City." Tremendous numbers of big elms died in the Detroit Metropolitan area by the 1960s, and it is now very hard to find native, big elm trees in any Michigan town.
Dutch elm disease appears to be here to stay. But young elms continue to be important parts of many natural tree stands. Elms often are not affected much until they reach seed-producing age and size and can support the spreading agent, elm bark beetles. Dutch elm disease eventually kills the elms before they reach maturity, but elm seedlings are common in many regions of Michigan and often very abundant on wetter soils in Southern Michigan.
Most tree experts agree that treatment of diseased elms is costly and usually prolongs the tree's life by less than 10 years. By the time an elm shows the tell-tale symptoms of wilting and/or yellowing of leaves on some of its branches, the disease is already causing the tree to plug its own internal tissues with a gum-like substance that prevents transport of water and nutrients. It's too late to save the tree, but long-used fungicides sold under the brand names Arbotect and Alamo can sometimes slow the inevitable death of the elm.
Prevention of Dutch elm disease in susceptible stands of elms has been accomplished in a few notable cases. In East Sussex, England, about 15,000 elms—including some over 400 years old—still stand. The area is isolated, between the English Channel and the South Downs, and local authorities monitor the stands intensively, removing any branches or trees that show any symptoms of Dutch elm disease. Alberta and British Columbia are the only Canadian provinces essentially free of Dutch elm disease—they hope to stay that way by intensive monitoring, control of elm log transport, and discouraging pruning of elms during the growing season (which attracts elm bark beetles).
There are lots of elms (e.g., Asian species) that are resistant to Dutch elm disease. But they don't have the long-favored growth form of the American elm. Horticulturists have been making progress in developing cultivars of American elm with some resistance and that have a growth form similar to the true American elm. These include the ‘Princeton,' ‘Independence,' ‘Valley Forge,' and ‘Jefferson,' cultivars among others. Demonstration planting projects are underway or planned in many parts of the Midwest and Eastern U.S. In 2005, the National Elm Trial—a 10-year effort to monitor plantings of 19 promising cultivars across the U.S.—began to yield data on the strengths and drawbacks of some of the new elms.
Dr. Patrick J. Rusz
|The Biology of Dutch Elm Disease|
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