American elm is a large, native tree species that has been devastated in recent years by Dutch Elm Disease. Leaves are alternate and simple, with double saw-toothed margins. They are 3-6” long with rounded bases and unequal sides that meet at a sharply pointed tip. The upper surface is dark green, while the underside is paler and slightly hairy. The leaves are distinguished by their parallel lateral veins. Flowers are unremarkable, appearing as drooping clusters of 3-5 individual reddish green flowers. Female flowers develop into small, flat samaras in late spring. The bark is light gray when young, becoming deep furrows and a darker gray color with age.
American elm wood is coarse, heavy and strong, and has been used for small wood products, like planes and carriage hub caps. Its primary use has been as an ornamental street tree. It was a very popular boulevard tree throughout the United States, and the Minneapolis Parks and Recreation Board even released its own cultivar, ‘Minneapolis Park’ that was especially well-suited to boulevards.
Dutch Elm Disease (DED), a fungal pathogen carried by an Asian beetle, became a serious problem of many elm varieties in Europe and North America in the first part of the 20th century. DED reached Minnesota in 1961, and has now spread throughout the United States and Canada, killing millions of trees. The disease can be caused by either Ophiostoma ulmi or Ophiostoma novo-ulmi, two fungal pathogens that enter trees through various types of elm bark beetles. The fungus then spreads throughout the tree through its vasculature, and eventually to other nearby trees through root grafts, or through other beetles.
The University of Minnesota has become a leader in DED research, including researching treatment options and finding or breeding resistant cultivars. A recent study from the University’s Department of Plant Pathology and Department of Forest Resources field-tested remaining elm trees at the Minnesota Landscape Arboretum and the World War II Memorial in St. Paul. Research into the mechanism and treatment of the disease, as well as resistant cultivars continues.
There are many fewer American elms in the Twin Cities now, and the oldest trees that remain are either resistant cultivars or are treated with fungicide. Due to the work of the University and other researchers, cities and institutions are beginning to plant new, resistant varieties of American elm or hybrid Asian elms.
Wetwood is a bacterial disease that can lead to wilting or dieback. Elm yellows, a fungal disease, causes long, yellowish stains on elm bark. Dutch Elm Disease is a major issue for elms in North America, Europe, and New Zealand.