Disease-Resistant Elms in Minnesota: The Search Continues

February 27, 2015

by Daniell Ringle

The story of planting American elm trees has become much more complicated since Dutch elm disease (DED) arrived in Minnesota. Prior to the arrival of the disease, elm saplings were often dug up from the wild and transplanted into cities and communities. Now we have to go through a rigorous process to find disease resistant elm cultivars, because anything we plant from the wild has a high likelihood of becoming infected with DED. There are some disease-tolerant elm cultivars being planted in Minnesota, but they are not from the state (Ed. As of this writing, the St. Croix™ Ameican elm did not have wide commercial availability. St. Croix™ is the first disease-resistant American elm selected from a Minnesota-native parent tree. It is currently being produced and marketed by Bailey Nurseries and is now available in retail and wholesale nursery and garden center outlets.) Team Tree is trying to find resistant elm trees in each quadrant of the state so that we can have Minnesota grown cultivars adapted to our unique climate. We are specifically looking for mature elms that have lived since DED was found in Minneapolis in the 1960’s and likely have natural resistance to the disease. After identifying a specific mature elm that we would like to replicate, we collect fast growing branch tissue during the winter dormancy period.

collecting elm cuttings

Collecting elm cuttings in Mankato, MN

scion material

Branch material for experimentation

When we are ready to use the dormant branch material, last year’s new growth is grafted or processed as stem cuttings to make a clone genetically identical to the tree it came from. To make a rooted cutting, we cut a section of branch material several inches in length and plant it in a rooting medium. To encourage root establishment, we dip the cutting in a concentrated hormone solution that originates from a growth regulator that plants produce to initiate the growth of new roots.

Rooting cuttings (of cottonwood, not elm)

Rooting cuttings (of cottonwood, not elm)

Modified cleft grafting, however, is best cloning technique if we want quick growth, because the tree can allocate all its energy into one small grafted branch, instead of having to focus on developing new roots. When grafting, a wild elm seedling (rootstock) is used for the base of the new tree and a few inches of potentially resistant branch material (scion) is grafted to it. We do not allow the rootstock material to form leaves and branches, so the resulting tree will be completely from the scion wood. The purpose of the rootstock, as you could likely guess from its name, is solely to be the root system of the tree.

Grafted elm clone

Grafted elm clone

Successfully leafed out elm in its first season post-graft

Successfully leafed out elm in its first season post-graft

Mature, disease resistant elm

Mature, disease resistant elm

 

Because we usually graft in the wintertime, we keep the newly grafted trees in a greenhouse so they have a head start on the growing season. We then move them out to our nursery for the summer. From this time until the tree reaches 3 to 5 years of age, the main focus is on growing a large healthy clone. Once the tree is large enough, it is inoculated with Dutch elm disease to test for resistance. Any trees that have shown that they can survive exposure to DED after several rounds of testing will be used for restoring American, red and rock elm to Minnesota’s landscape. We hope that someday the fruits of our labor will pay off and result in a specimen such as the one posted above.

Team Tree

Team Tree in SW Minnesota

Team Tree

Team Tree in Lake City, MN

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