You know that big, wide, tall, strong tree you see kids playing on in old movies? Well, that’s an American elm.
Elm trees are popular, and that's why you can find so many Elm Streets in cities and towns. You used to have to look very hard to find any elm trees. Over the last half-century, nearly all our American elm trees were "cut down"... by an organism so small you need a microscope to see it.
American elms are prized for city streets and parks and yards. They provide lots of shade. They can thrive in poor soil. They don't seem to mind the smoggy air in a big city. Elms can also cope with bitter-cold winters and boiling-hot summers.
But in the early 1940s, the nightmare began. A disease called Dutch elm disease began spreading rapidly from one elm tree to another. The disease is caused by a fungus. A fungus can't walk or fly, but bugs known as Elm Bark beetles can. And they did, traveling throughout the land, carrying the fungus like an invisible hitchhiker.
The beetle was accidentally brought to our country on logs of elm trees imported from France for making furniture. Over the years, the bugs and their hitchhikers hopped off railroad cars on the way to furniture makers in Ohio.
Once infected with the fungus, leaves on elm trees turn yellow, then a dead brown. The fungus gradually creeps through the branches into the trunk. In a year, some branches die. Soon after that--often by the second year--the whole tree is dead.
"By the 1980s, Dutch elm disease had almost wiped out 77 million American elms," says Alden (Denny) M. Townsend, now retired. He was a plan geneticist for the U.S. National Arboretum that the Agricultural Research Service operates in Washington, D.C. A plant geneticist works with a plant's genes to search for and strengthen--or weaken--certain traits, like its tolerance to cold or the size or sweetness of its fruit. They also work to improve breeding methods and ways to make sure that future generations of a particular plant will have the traits growers want.
Here and there, some trees survived despite the fungus. Now, these survivors--and many years of work by Townsend and other scientists--have ended the nightmare on Elm Street.
In 1996, Townsend and his co-workers released two new American elms, named Valley Forge and New Harmony. The story goes back more than 25 years, when the scientists started searching for and testing thousands of the survivors. Which ones, they wondered, had a natural ability to fight off Dutch elm disease? To start finding out, they cut fresh shoots from as near a tree's top as possible. Back in the greenhouse, they used a safe chemical that would make tiny roots sprout from the shoots. Then they put the fungus on these tiny plants, and waited to see what would happen.
Why bother with all that? Why not just get seeds from the survivors and plant them to grow new trees? After all, the survivors must have what it takes, right? Not so! "It is not uncommon," Townsend explains, "to find a single elm surviving by chance where others have died. Most of the thousands we tested turned out to be just lucky."
Through their tests, the scientists proved that only about one in 100,000 American elm trees are really disease-tolerant. Valley Forge and New Harmony are not completely immune. But they come very close!
In 2005, scientists released another American elm, named Jefferson, which has even more disease tolerance. It was cloned from one of the trees on the National Mall that survived the attack. You can even pay these trees a visit. They’re right by the old Smithsonian building and are at the arboretum. The arboretum, just a few miles from the National Mall, is a place where you can hike, ride a bike, have a picnic and learn about different types of plants in its 40 collections and gardens.
People can buy these super trees from nurseries to plant in their yards. Thanks to help from ARS scientists, your kids and your grandkids can hang from tire swings and build tree houses in a good old American elm.
--By Hank Becker, formerly Agricultural Research Service, Information Staff