Dutch Elm Disease on American Elm Trees

Ulmus Americana Before the Invasion

giant elm tree
ThereseMcK / Getty Images

Bruce Carley, in his article on saving American elm trees from Dutch elm disease, paints a pretty picture of Main Street USA in the first half of the 20th century. It was a street typically lined with these majestic giants, which unfurled their weeping branches Rapunzel-like over the heads of passersby, affording shade on hot summer afternoons. There was no other tree quite like them:

"The interweaving limbs of the stately trees that lined the streets ascended into a towering canopy with a graceful, arching beauty...spreading horizontally at heights often greatly exceeding 100 feet...." [Bruce Carley]

Dutch elm disease (Ceratocystis ulmi) changed all that. Dutch elm disease is a wilt fungus that grows in the sapwood of elms. The fungus was first encountered in 1921 in the Netherlands. Over the next few years, elms across central and southern Europe were found to be succumbing to the fungus.

History of Dutch Elm Disease

American elm trees (Ulmus americana) are the most susceptible of all to Dutch elm disease. American elm trees are also known as water elms, soft elms, white elms, or Florida elms. American elm trees are found throughout Eastern and Central North America. Their range extends as far south as northern Texas and Florida.

Cleveland, Ohio, witnessed the first case of Dutch elm disease in the U.S. in 1930. This silent killer arrived in a shipment of logs from France. Dutch elm disease spread East quickly; within two years, American elm trees in New Jersey were falling prey to the deadly fungus.

The Dutch elm disease had "killed 77 million trees by 1970," wrote Phil McCombs in a 2001 Washington Post story that begins with this picturesque description of how American elm trees once lined the streets of many a town:

"Once upon a time in America, great leafy high-arching cathedrals of elms lined the streets of villages and cities from the Atlantic to the Rockies, casting a deep cool shade upon life's turmoil."

Why Dutch Elm Disease Hits American Elm Trees So Hard

For all the tranquility such mass plantings bestowed, this monocultural practice was one of the culprits in the downfall of American elm trees. The deadly fungus, it turns out, can spread underground from the roots of one victim to the roots of another nearby. This is what happened when the roots of adjacent American elm trees "grafted" together, essentially linking the lives of what had been two distinct entities.

The demise of one thus became the demise of the other. The monoculture and its consequent root grafting meant that an infected sap could pass from one American elm tree to another in a chain reaction that would decimate a whole row along a street.

The planting of American elm trees en masse was not the sole culprit, though. The microscopic spores of the fungus are also transmitted from diseased victims to healthy specimens by two kinds of beetle that tunnel under the bark. One is a European bark beetle (Scolytus multistriatus), an import that preceded Dutch elm disease itself. The other beetle is a native bark beetle, Hylurgopinus rufipes. Photos of both of these carriers of Dutch elm disease can be found at the Utah State extension site, as well as additional information about Dutch elm disease.

What Plant Cloning Can Do to Help

Thanks to the plant cloning work of tree geneticist, Alden Townsend, the prognosis for Ulmus americana is now good. In the late 1990s, approximately 25 years of work with U. americana came to fruition when the announcement was made that Townsend had succeeded with two new strains: American elm tree clones resistant to Dutch elm disease became a reality.

Named U. americana "Valley Forge" and U. americana "New Harmony," Townsend's clones are now on the market. The plant cloning work of tree geneticists continues in hopes of developing new American elms that will be even more resistant to Dutch elm disease.

For now, if you are not able to purchase one of these plant clones of American elms, or if you are trying to save a long-established tree, follow these guidelines:

  • Prune dead or dying branches off American elms, from fall to late winter. This procedure, called limbing, is best handled by professionals.
  • Avoid pruning American elms from April through August. The elm bark beetle is attracted to freshly cut elm and is most active during this period.
  • Be on the lookout for the signs of Dutch elm disease. Leaves of infected American elms will wilt in the summer. They will first turn yellow, then curl, and finally become brown. The signs usually first appear in the crowns of American elms.
  • If signs appear, dispose of infected American elms properly. In rural areas, they may be burned. In urban areas, take them to a designated disposal site.

Note that American elms are considered one of the worst trees to plant for allergy sufferers. For those who are not allergy sufferers, American elms make for splendid specimen plants. American elms are cold hardy to zone 3.

Humans will not be the sole winners if the former ubiquity of the American elms is restored through plant cloning work. For Baltimore orioles, American elms had always been a favorite nesting tree. The male oriole is one of nature's most striking birds, with screaming orange markings punctuated by jet black plumage. Baltimore orioles prefer American elms for nesting because of the drooping habit of the trees' branches. Orioles' nests hanging from the ends of branches of American elms are nearly impossible for predators to access.

Bruce Carley tells us how the cities of Portland, Maine and New Haven, Connecticut were once home to so many American elms that each locale earned the title, "City of Elms," long before the words, "plant cloning" had ever been heard. But thanks to plant cloning, the prospects are now better that people will someday honor more hometowns with the epithet, "City of Elms." Plant clones may yet restore "Elm Street, USA."