Photo By Nayelli Guerrero | A researcher drills a hole into a tree to inoculate it near River Falls, Wisconsin,...... read more read more
The American elm (Ulmus americana) was once a common sight on the Upper Mississippi River, but Dutch elm disease, or DED, has killed many trees.
DED is an invasive fungal pathogen that is spread by elm bark beetles and root grafts between healthy and infected trees. When DED infects an elm, it prevents water from reaching the tree’s leaves, causing it to wilt and die. DED affects many species of elm trees, but American elms are particularly susceptible due to their low disease resistance. The abundance of American elms on the river floodplain has meant that DED has disproportionately impacted these forests. Although there are still some American elms left on the Upper Mississippi River, DED kills most elms before they can grow larger than 12 inches in diameter.
Over the years, DED transformed forests composed of maple, elm, and ash into forests dominated by maple and ash. The arrival of the invasive insect emerald ash borer has only increased the urgency of finding ways to combat DED. Emerald ash borer has decimated the ash trees in the Upper Mississippi’s maple-ash dominated forests. Without American elms to replace the dying ash trees, the gaps in the Upper Mississippi’s floodplain forest canopy are converting to areas dominated by invasive reed canary grass and other herbaceous species.
For high value trees in urban landscapes, preventive fungicide treatments can be used to preserve susceptible elms, however, there are not many options to protect large elms in the wild. The long-term solution to DED in the natural environment is the development of disease-tolerant elms. While no elm is completely immune to DED, tolerant elms can block the spread of the disease and survive to a larger size.
“The loss of American elm and green ash as viable tree species in the Upper Mississippi River floodplain has had a significant impact on forest composition,” Andy Meier, St. Paul District forester, said. “The breeding and selection of disease-resistant elm varieties could provide a critical component of forest restoration, giving us the potential to restore
one species that has been lost.”
The St. Paul District and the U.S. Forest Service are part of the effort to identify American elm specimens that are DED-tolerant. The two agencies are collaborating on a study to compare DED resistance in wild American elm populations to a disease tolerant population. Data from the study will help researchers understand the heritability of DED resistance and develop new sources of American elm that are more tolerant to DED. The seeds of the trees that are most resistant to DED will be used in habitat restoration plantings along the Upper Mississippi River and throughout
the eastern United States.
The study is located at a 1.75-acre wetland mitigation site owned by the Corps near Ellsworth, Wisconsin. In 2014, the district helped the Forest Service plant 640 young trees at the site. The planting includes the “enriched” saplings of elms descended from crosses between clones with known DED tolerance. The wild elm population is represented by saplings grown from wild American elm seed collected from Corps sites near Spring Valley, Wisconsin, and La Crescent, Minnesota, and a Forest Service site near White Lake, Wisconsin.
Corps and the Forest Service employees recently returned to the site to inoculate around 200 saplings against DED. On a hot summer day in early June, the Forest Service’s Charlie Flower, Melanie Moore, Linda Haugen and Milcah Puliyelil began inoculating trees.
The researchers drilled a hole about one inch into each tree to penetrate the xylem, the water-conducting tissue of a tree. They then inoculated each tree with 60,000 spores of a local isolate of the DED fungus, dabbed petroleum jelly over the hole and tied a flag around it. Researchers only inoculated trees one inch or more in diameter to ensure they could survive standard inoculation methods and DED infection and still have enough canopy to show leaf variation in response to DED.
The Forest Service returned to the site four weeks after inoculation and again at eight weeks to perform crown ratings, a process that evaluates the amount of dieback in the crowns of DED-treated trees. They will return for a final crown rating in one year. The data from the crown ratings will help researchers determine differences in response and survival between the wild saplings and the enriched saplings with DED-resistant parents. The researchers plan to return to the site in a few years to complete a second round of inoculations on trees that were not at the right stage for inoculation this year.
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| Date Taken: | 09.15.2021 |
| Date Posted: | 09.15.2021 11:29 |
| Story ID: | 405304 |
| Location: | RIVER FALLS, WISCONSIN, US |
| Web Views: | 234 |
| Downloads: | 0 |
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