This American elm displays the classic V-shape
In John Steinbeck’s last novel, “The Winter of Our Discontent,” the main character describes the shadows of elm leaves dancing on his bedroom wall and ponders the age of the stately giants that have lined the street on which he has lived since a child. This book, set in 1960, was about the time Dutch Elm Disease (Ophiopstoma novo-ulmi) was starting to spread across North America and ravaging our native American elms (Ulmus americana). Midwest cities suffered great losses in the 1960s and 1970s as elms, the most common street tree at that time in most cities, were removed.
Dutch Elm Disease (DED) is caused by a non-native fungus spread by the elm bark beetle. DED was, and still is, so destructive to American elms because they had never been faced with this particular fungus before it reached North America in 1928 on imported timber. Elm trees native to Asia, where the fungus originated, have evolved a natural resistance.
Commonly described as elegant and graceful, American elms made beautiful street trees.
Elm bark beetles act as a vector, transporting the disease from tree to tree. As the beetle burrows tunnels beneath the bark to lay eggs, fungal spores on its body are rubbed off. As the fungus grows inside the water-conducting tissues known as xylem, the tree attempts to block off its advance by closing off those essential tissues. The tree quickly loses limbs as xylem is blocked off and the fungus advances.
The fungus is very aggressive and American elms are not familiar with it. Small trees may die very quickly, while larger trees may take several seasons.
The system of tunnels burrowed by the elm bark beetle, the vector of DED.
American elm was a very popular street tree, largely due to it arching, vase-like growth form and upright, graceful appearance. Cross-grained limb attachments mean the American elm is extremely resistant to limb breakage during high winds or heavy snow and ice. Tolerance to almost any adverse condition, including drought, poor soils and road salt, made this tree an excellent choice next to a city street.
Today, large American elms are rare, especially in city settings. Key specimens are usually kept alive by injecting them with a specific fungicide every three years to prevent DED infection. Small and medium-sized trees are more common in rural areas, though these trees usually succumb to DED fairly quickly.
American elm is a very adaptable species, growing quickly even on poor sites. This largely attributes to the presence of American elm despite how devastating DED is.
There are many ongoing research projects to find and propagate naturally resistant American elm trees, as well as crossbreed American elm, with other, more resistant elm species such as Siberian elm (Ulmus pumila). A few breeds now exist that show moderate resistance to the disease.
KEITH O’HERRIN is the City Forester for the City of LaPorte. He can be found at the Park and Recreation Office at 250 Pine Lake Ave. or reached at 326-9600.
As the infection spreads downward, leaves wilt off of branches and the tree is defoliated.
