Courtesy Jim Sherald

History (Jefferson American Elm)

Dutch elm disease first was observed on the National Mall around 1950 and, by 1994, it had claimed over 200 trees. It was during this time that National Park Service plant pathologist Horace V. Wester, recognized that several of the elms retained their fall foliage almost a month later than the almost 600 other surrounding elms. Wester postulated that the extended period of foliation was associated with a more healthy specimen, one possibly immune to Dutch elm disease. Through further research, scientists from the National Park Service Center for Urban Ecology, US National Arboretum, and Michigan State University Department of Crop and Soil Science determined that the tree that came to be known as the National Mall Jefferson Elm was indeed resistant to Dutch elm disease.

In conducting DNA analyses, scientists discovered that the Jefferson Elm is a triploid, meaning the tree’s cells have two sets of chromosomes in the nucleus. This is significant because elms are naturally diploid or tetraploid, meaning they have two or four sets of chromosomes in the nucleus. Scientists believe this may be the reason that the Jefferson Elm is resistant to Dutch elm disease. Moreover, the Jefferson elm is noteworthy because of its unique physical characteristics, including its significantly larger leaf buds and U-shaped versus V-shaped branch junctions which provide a stronger connection between the branches and the trunk. For these reasons, in February 2005, the National Park Service and the US Department of Agriculture jointly released the Jefferson Elm cultivar to the commercial nursery trade. While the organizations hope that the Jefferson American Elm will help to reintroduce the elm into the American landscape, it is important that research continues as the Jefferson may too one day become susceptible to a new strain of Dutch elm disease.