The American elm (Ulmus americana), with its large size and outstanding arching umbrella habit, was long considered the ideal street tree. Hardy and tolerant of urban conditions, it was planted by millions across North America and the “tree-lined street” many of us have in our imagination are of those endless rows of American elms. Unfortunately, its role as America’s favorite street tree came to a crashing halt with the arrival of Dutch elm disease (Ophiostoma spp.), a fungal disease carried from tree to tree by elm bark beetles. Once established in a tree, the fungus can then spread to neighboring trees through their roots when they touch.
Dutch elm disease probably originated in Asia, but went largely unnoticed there, as Asiatic elm species have good developed genetic resistance to it over thousands of years. It was first noticed in Europe in 1910, but was only identified in 1921 in the Netherlands (hence the name Dutch elm disease). It first arrived in North America in 1928 when elm wood from the Netherlands containing bark beetles was shipped to New York, then transported to Ohio, starting two separate outbreaks. Since then, it has spread widely and newer, more virulent strains of DED (Dutch elm disease) have only increased the problem.
The disease now occurs throughout Europe and much of the United States and Canada, although its spread westward has been partly checked by the Rocky Mountains. It’s presently absent from Alberta and British Columbia while elms in Quebec City and Winnipeg are “holding their own”, helped by their colder climates (the beetles that carry the disease don’t overwinter in the north, but can fly there from the South during the summer). More than 77 million elms have died of DED in North America so far. Today, in many areas, any mature elms that are left only survive because their owners are willing to pay for expensive preventative treatments that have to be repeated every 2 or 3 years.
Hybridization and Selection
Fortunately many researchers have been working on solutions since the 1940s and have come up with some interesting solutions.
For example, healthy, mature American elms have been found in areas otherwise devastated by the disease. Cuttings from these survivors have been tested and many were found to be resistant to DED, even when inoculated with the spores of the disease. They aren’t entirely immune to the DED: they may catch it if they are raised under stressful growing conditions, such as repeated droughts. Even so, some of these varieties that have been successfully used in landscapes for over 40 years and are still thriving.
Another possibility is using naturally disease-resistant Asiatic species as substitutes, notably the Japanese elm (U. davidiana japonica), which is remarkably similar in appearance to the American elm, although of a smaller size.
Finally, there is considerable hybridizing going on, notably between desirable North American and European species and disease-resistant Asiatic ones, and several very interesting hybrid elms, all with good DED resistance, are available in many nurseries.
Pick Own-root Elms
If you intend to buy a DED-resistant American elm (U. americana), make sure to get one growing on its own roots. To save production time, some growers have been grafting resistant varieties onto elms grown from seed, but this can allow the disease to enter the tree through its roots. It’s therefore best to choose trees identified as non-grafted or growing on their own roots.
Elms Resistant to Dutch Elm Disease
What follows is a list of varieties of elm that offer both the majesty of the American elm and a good resistance to Dutch elm disease. Most nurseries will carry at least one of these varieties, so pick your favorite and help bring back the majestic American elm to its status as a beloved street tree.
- U. x Accolade™ ‘Morton’ zone 4
- U. americana ‘Brandon’ zone 3
- U. americana ‘Delaware’ zone 3
- U. americana ‘Independence’ zone 3
- U. americana ‘Jefferson’ zone 4
- U. americana ‘New Harmony’ zone 4
- U. americana Prairie Expedition™ ‘Lewis and Clark’ zone 3
- U. americana ‘Princeton’ zone 4
- U. americana ‘St. Croix’ zone 4
- U. americana ‘Valley Forge’ zone 4
- U. americana ‘Washington’ zone 3
- U. x ‘Arno’*, zone 6
- U. x ‘Cathedral’, zone 4
- U. x Commendation™ ‘Morton Stalwart’, zone 4
- U. x Danada Charm™ ‘Morton Red Tip’, zone 4
- U. davidiana japonica ‘Jacan’ zone 3
- U. davidiana japonica ‘Discovery’ zone 3
- U. davidiana japonica ‘Freedom’ zone 3
- U. davidiana japonica (formerly U. wilsoniana) ‘Prospector’ zone 4b
- U. x ‘Fiorente’*, zone 6
- U. x ‘Frontier’ zone 5
- U. x ‘Homestead’ zone 4
- U. x ‘New Horizon’ zone 4
- U. x ‘Patriot’ zone 4b
- U. x ‘Pioneer’ zone 4b
- U. x ‘Plinio’*, zone 6
- U. x ‘Rebona’ zone 4
- U. x ‘Regal’ zone 5
- U. x Resista® ‘Sapporo Autumn Gold’ zone 4
- U. x ‘San Zanobi’*, zone 6
- U. x Triumph™ ‘Morton Glossy’ zone 4
- U. x Vanguard™ ‘Morton Plainsman’, zone 4
*Varieties developed for growing in warmer climates (zones 6 to 9) than the usual American elm.