The American Elm -- Better Days Ahead?
by Edward Richardson • Spring 1999 (Volume 27, no. 1)
In 1998, while on a Connecticut Botanical Society big tree walk in Edgerton Park, on the New Haven/Hamden line, I pointed out several specimens of the cultivar, Ulmus americana 'Delaware'. These trees, planted in the early 1990s, are now 5 to 6 inches in diameter, nearly 20 feet high, and appear to be in fine condition. This cultivar was released by the U.S. Department of Agriculture (USDA) in the 1980s, and is touted as being highly resistant to Dutch Elm Disease (DED).
By now, most botanically aware people know that DED is a fungus which arrived in the United States in 1930 on a shipment of logs from Europe. Since then American elms have been virtually wiped out in much of the country, a loss estimated as at least 70 million mature trees. On the Society's Notable Trees List there are only twelve elms that exceed 4 feet in diameter. These specimens were probably protected by their relative isolation. We now lose one or two every year, probably due to aging. Experts estimate that only one tree in a hundred thousand has natural resistance to DED.
Before DED arrived in this country, the American elm was regarded as the very best urban tree. It grew fast, and had a long, clear trunk with high, overarching limbs. It tolerated smog, drought, salt, heat and cold, root compaction, and mowers. Stately colonnades of elms once overhung many city streets, providing a paradise for orioles and welcome shade for humans.
This popularity resulted in overplanting, and helped seal the tree's fate. Not only did this crowding make it easy for the elm-bark beetle (the carrier of the fungus) to get from tree to tree, but it also resulted in root grafts which transmitted the disease directly.
After many years of work on elm genetics, the USDA recently announced two new American elm cultivars that are resistant to DED. These are called 'Valley Forge' and 'New Harmony'. They are not immune to DED, but they can tolerate the fungus without wilting and dying back, even when injected with a million spores of DED. In natural circumstances, the average tree needs only 10 to 100 spores to ensure its demise.
The 'Valley Forge' is of unknown parentage, its seed one of thousands collected by the USDA in the 1960s. It is tough, and 11-foot specimens injected in 1992 are now 19 feet tall and show no sign of DED. The 'New Harmony' is only about one-fifth as tolerant of DED as the 'Valley Forge', but about equal to the cultivar 'Princeton', which Princeton Nurseries of Allentown, NJ has been selling for years.
The USDA released 'New Harmony' to add to plant diversity, since its injections are more demanding than trees face in nature. Both new cultivars are being grown in commercial nurseries and should be available to the public in 2000.
It is of interest that DED may have originated in Asia, where many of the elms are somewhat resistant to the fungus. Efforts were made to cross Asian and American elms, but the results were poor. This may be because the American elm has 56 chromosomes, while all other elms (some 60 species in the Northern Hemisphere) have only 28. So, not only is Ulmus americana unique in its handsome vaselike shape, but it is also unique in its genetic code.
When the new elms come to market, let us hope that they will be planted with restraint. By now we should have learned the value of diversity.