Nature Notes
Dutch Elm Disease Didn't Wipe out the Island's American Elms
By Patricia Martin

One of the nice things about living on an island is that not every disease that comes down the pike gets brought over to infect our plants or animals. To some degree, we're protected by the water that surrounds us. This doesn't mean that organisms or diseases can't get here, but it's certainly made more difficult by the lake. It is something of a natural barrier. It is this idea that made the Mackinac Bridge the obvious checkpoint in trying to stop the Emerald ash borer from getting into the Upper Peninsula. This doesn't mean that people cannot bring in contaminated plants or animals, nor that they cannot come across the water by some other vector.

I was thinking about this in relation to the lovely American elms (Ulmus americana) that are still growing here in all their majestic height. There are very few places in North America where large, old American elms can be seen. Mackinac is one of the places where the Dutch elm disease didn't wipe them out. Dutch elm disease is caused by an ascomycetous fungus (Ceratocystis ulmi). The spores of the fungus are carried by beetles, and enter healthy tissue via wounds caused by feeding of the beetles on young shoots. It is fatal to most American elm trees.

Dutch elm disease did not reach Mackinac Island. Large old American elm trees still can be found here, including these two in Marquette Park.

It is believed that Dutch elm disease arrived in North America in the 1930s by infected wood that was imported from China. It spread from Ohio, where it was first reported. In 1950, the first case of Dutch elm disease was reported in the Detroit area in Michigan, and it quickly spread through the southern part of the state. In total, it is estimated that the disease killed more than 100 million elm trees in North America. Elm trees can be sprayed to prevent the spread of disease by insects, but it's costly and sometimes unsuccessful. By the way, in case you were wondering, the fungus is called Dutch elm because the disease was finally identified by Dutch researchers, although the disease comes from Asia.

The affects of the disease were particularly devastating because Elm trees had been widely planted in cities across America, as they're very tolerant of city life, that is, they grow fast, they're shallow rooted, do well along city streets, are long-lived, and are valued for shade and graceful habit. The government spent a fortune trying to stop the disease, unsuccessfully, and blocks upon blocks of American elms were removed from city streets. By the 1960s, the American elm, a staple of the nursery trade, had virtually vanished from nurseries. This is reminiscent of what is now happening with ash trees in Michigan. It is ironic that many ashes and Norway maples were planted to replace the American elms along city streets in the eastern United States and Canada.

The leaves on an American elm are oblong-obovate to oval and have coarse, double teeth.

For those of you who are not familiar with the American elm, it's a large, handsome tree, standing 75 to 100 feet tall, with a three- to six-foot diameter. Open-growing trees commonly divide near the ground into several ascending limbs, forming a vase-shaped crown. The leaves are oblongobovate to oval, and are usually four to six inches long with coarse, double teeth. The base of the leaf is not symmetrical, and it's dark green and rough above, and paler below. The fruit of this elm is a smooth oval samara (winged fruit) about one-half inch long. The bark is thick, ash gray with typical diamond-shaped pattern formed furrows. In cross section, the outer bark alternates light and dark layers. In the wild, they grow in swamp forest, such as river floodplains, bogs, and even cedar swamps. American elms are often found with silver maple, and are also found in rich upland hardwoods. On Mackinac, you can see lovely specimens in Marquette Park along the Fort Hill, on the school playground, and in the Mission area (there are several lovely elm trees near Lafayette Cottage, which is soon to be torn down).

Recently I read an article in USA Today about how the American elms may be making a comeback. In the 1990s, a researcher at the Department of Agriculture's National Arboretum research station in Beltsville, Maryland, identified several types of elm trees that are genetically resistant to Dutch elm disease. Nursery men began growing these varieties of American elms in the mid-1990s, and this year Home Depot decided to take on 12,000 elm trees to sell in stores, and it looks like more will be available in the coming years. The average cost is between $110 and $175 per tree. There is some concern that people may not flock back to the tree because of the tree's history, and skepticism about how disease-resistant they really are. It's good to know, however, that these trees are available for planting. Perhaps they can replace some to the Norway maples that are falling down or are being removed.

In the last month, I was talking to an arborist who works with cities and individuals who are doing street planting, and his advice to communities is simple: "Do not plant only one species of tree along the streets." In other words, do not create an artificial monoculture in which one disease can wipe out all the trees in a community. Mix up the plantings so that what has happened with elms, and now ashes, will not happen again.

Trish Martin is a yeararound resident of Mackinac Island, has earned a master's degree in botany from Central Michigan University, and owns Bogan Lane Inn.