Since the 1930s, when a fateful shipment of infected French logs arrived in Cleveland, millions of American elms—the quintessential Main Street shade tree—have withered, yellowed, and died from Dutch elm disease. But the American elm may yet make a comeback, thanks to the discovery of two new varieties that can withstand the disease. Wholesale nurseries began producing them in early 1997.
Geneticist Alden Townsend of the U.S. Department of Agriculture’s National Arboretum lab in Glenn Dale, Maryland, spent nearly three decades screening tens of thousands of trees for tolerance to the disease, which is caused by a fungus called Ophiostoma ulmi. The fungus infiltrates the trees’ xylem, or water-transport system, producing countless spores. The cells surrounding the xylem form protrusive growths, and together the protrusions and spores clog up the waterworks and dry out the tree.
As researchers have done since the 1930s, Townsend combed forests for the healthiest-looking elms, collected cuttings and seeds, and nursed them into trees. We inoculated thousands of trees with the fungus, he says. Then the most tolerant were picked, propagated, and reinoculated. After a series of tests over a number of years, we narrowed our candidates down to the two trees that showed the least amount of dieback.
Townsend isn’t sure why the two cultivars—Valley Forge, which one of his predecessors grew from a seed collected in the northern United States, and New Harmony, which Townsend found in an Ohio forest—tough out the fungus better than other elms. There are all kinds of theories, he says. One is that these trees produce a chemical in response to the fungus that suppresses sporulation and growth. We do know that when we inoculate these trees, we can still later recover the fungus. That’s why we emphasize that they are not immune or resistant to the fungus—they tolerate its presence.
By 2000, the New Harmony and Valley Forge trees now growing in nurseries will be large enough to sell. By then, however, Townsend will have tested other promising clones—including crosses with the two—and may be ready to release another, even more tolerant tree.