The American elm was once dominant on the landscape, an iconic symbol of the nation’s landscape, but has unfortunately succumbed to a fungal disease introduced from Europe in the 1900s. The graceful, large elm trees widely planted in the suburban landscape for its shade, tolerance of poor soil conditions and fast growth. However, in the early 20th century, logs infested with fungus carrying bark beetles were transported into the US beginning the epidemic that is Dutch elm disease.
Dutch elm disease spread rapidly and unchecked through the landscape wiping out trees from the streets, parks and cityscapes. Elms were replaced with other trees, but species of elms which resist the disease, but as of yet has not been found. Research and breeding programs, such as those performed by the U.S. National Arboretum and Chicago’s Morton Arboretum, have produced several cultivars with some resistance such as: ‘Valley Forge’, ‘New Harmony’, and ‘Patriot’. Other studies have focused on crossing European and Asian elm species.
A national trial is currently ongoing which explores hybridization for disease resistance and, so far, 17 resistant elm cultivars are being evaluated in the long term in 15 states. Disease resistant elm cultivars are listed by the University of Minnesota and can be viewed at http://www.extension.umn.edu/. The story of the American elm parallels the plight of the American chestnut tree, a once proud, prominent member of the landscape taken down by introduced disease and now being restored through genetic research and breeding efforts. Insect resistant tree species are also being researched, for elm and walnut trees, in particular, due to the insect and disease interactions being so lethal for hardwood trees.
American elm trees which were saved from Dutch elm disease now appear to be especially susceptible to elm yellows. Elm yellows is a phytoplasma, transmitted by leafhoppers, which affects the trees’ phloem tissues. Ash trees are under threat from the emerald ash borer and ash yellows, while red oaks are facing mortality by oak wilt and bacterial leaf scorch. Planting diverse tree species in our city streets, residential landscapes, parks and open spaces can aid in preventing the total wipeout of one species of tree from the landscape and slow the spread of insects and disease.