More on American Elm
Dutch Elm Disease
The American elm was once widely distributed throughout the eastern United States and was a preferred tree for use along city streets and in the yards of many homeowners. The Dutch elm disease (DED) fungal pathogen, Ophiostoma ulmi, was introduced into the United States in Cleveland and Cincinnati, OH in 1930, and spread to destroy millions of American elm trees in urban and forested landscapes. The DED fungus is vectored by the elm bark beetles Scolytus multistriatus and Hylurgopinus rufipes from diseased trees. In addition, American elm trees can form root graftsthat allow the fungus to spread from diseased trees to nearby healthy trees. In the 1940s in Illinois, a new strain of DED fungus, O. novo-ulmi, appeared that is more virulent than O. ulmi and destroyed an additional tens of millions of trees. More recently, a new elm bark beetle from Asia, Scolytus schevyrewi, first detected in CO and UT, is now in 22 states; however, this pest primarily attacks the Siberian elm. For several decades research was focused on means of controlling elm bark beetles, generating and/or finding DED-resistant American elm strains, and phytosanitation techniques to limit the spread of DED. Of these efforts, only development of phytosanitation methods was largely successful. A practical means of controlling elm bark beetles was not developed and no American elm with resistance to DED was found. However, several strains American elms that are tolerant of DED were identified; these were released to the nursery industry for sale to the public and are also being used in efforts to restore the American elm to forested landscapes as described in the pages linked below.