Many urban areas in the mid-20th century had closed canopy streets lined with American elms. By the end of the century, many of those areas had complete canopy loss due to the Dutch elm disease (DED) fungus. The elm project at the University of Minnesota aims to re-establish the tree that had garnered the admiration of past generations through selective breeding of trees that are tolerant or resistant to the pathogen. Native cultivars are expected to show increased fitness across the northern United States. Since the arrival of the Dutch elm disease fungus Ophiostoma ulmi in Minnesota, and the more aggressive strain O. novo-ulmi, millions of American elms (Ulmus americana), red elms (U. rubra), and rock elms (U. thomasii) have been lost to the disease.
American elm is a particularly hardy and resilient tree. It has the ability to tolerate the stresses of an urban environment making it an excellent choice for planting along boulevards and in parks. Elm also plays an important ecological role in Minnesota’s forests as a dominant species in many cover types. Trees that survived in areas where the disease had a significant impact are of particular interest. With the help of State Park managers and staff as well as arborists, urban foresters and private residents from throughout the state, we have been able to locate and identify large trees that may have inherent tolerance to the disease.
However, these trees need to be propagated and rigorously tested by means of inoculation to determine if this is the case. Identification, propagation, and screening takes a significant amount of time, and for this program to be successful, long term research is essential. Young elms are propagated through cuttings or cleft grafting onto American elm (U. americana) or Siberian elm (U. pumila) rootstock. Greenhouse inoculations are a tool for preliminarily assessing if a genotype of interest has some tolerance to DED. Trees destined for field inoculation trials will need at least two growing seasons before they are of size to reliably test. For proper statistical analysis of each genotype a minimum of six clones are needed.
The goal for the conclusion of this study is to obtain a diverse selection of native elms from Minnesota that are genetically different, but all show resistance to DED, alongside optimal growth and hardiness characteristics for use in commercial propagation.