"The problem with that disease," he adds, "is that the phytoplasma moves to the roots of the tree and kills them rather quickly. As a result, elms usually die within a year after being infected. We have lost many elms on the perimeter of the main campus to elm yellows." This spring, the disease also claimed one of two elms that graced the front corners of Penn State's iconic Old Main building for generations.

The other scourge of the elm tree, Dutch elm disease, "has been in Pennsylvania since the 1930s, slowly killing elms," says Moorman. The disease is caused by a fungus, which is spread by bark beetles. "It is known that trees infected by elm yellows are very attractive to elm bark beetles," he explains. "Many elms are being infected by both elm yellows and Dutch elm disease and there has been an explosion in the elm bark beetle population over the last two years."

Penn state entomologist Greg Hoover has been monitoring elm bark beetle populations over several years, adds Moorman. "Based on Hoover's data, the elms can be sprayed at specific times to suppress peak bark beetle activity and thereby lessen the spread of Dutch elm disease."

While there's currently no cure for these diseases, there is something we can do and that is to grow disease-resistant trees, says Moorman. There are several Dutch elm disease-resistant elms but only one hybrid—Homestead—that is resistant to both Dutch elm disease and elm yellows. "The greatest threat to our trees comes from people introducing non-native pathogens, insects, and mites into our ecosystem. Our native trees and shrubs usually have no resistance to these and there are no natural enemies here to suppress introduced pests, such as the Dutch elm disease fungus, emerald ash borer, the Asian long-horned beetle, and viburnum leaf beetle. These are all in the northeast because of human activity."

Great care must be taken, says Moorman, to avoid pests hitchhiking into our country on imported firewood, logs for lumber, and wooden shipping crates, among other products. After all, he concludes, American trees are not only part of our history and legends—from Johnny Appleseed to George Washington's cherry tree—but are a living national treasure to preserve for future generations.