American Elm UPDATED

Elms are loved for their graceful, stately shape, with branches like spreading fountains, and green leaves that turn gold in fall. The American elm was the most popular tree to plant in the booming cities of the 19th century, so that by the 20th century many streets were lined with only elms and were shaded in summer by a cathedral-like ceiling of their branches. Sadly, for many years, the American elm (Ulmus americana) could not be recommended because it is vulnerable to a devastating pathogen called Dutch elm disease. When Dutch elm disease (which actually originated in Asia) spread to the US in the 1950s, it was able to mow down elm after elm through their grafted root systems or with the help of a beetle. However, due in part to research at The Morton Arboretum, other species and hybrids that are more resistant to the disease are now available for planting.

american elm

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For over 80 years, U. americana had been identified as a tetraploid, i.e. having double the usual number of chromosomes, making it unique within the genus. However, a study published in 2011 by the Agricultural Research Service of the USDA revealed that about 20% of wild American elms are diploid and may even constitute another species. Moreover, several triploid trees known only in cultivation, such as 'Jefferson', are possessed of a high degree of resistance to DED, which ravaged American elms in the 20th century. This suggests that the diploid parent trees, which have markedly smaller cells than the tetraploid, may too be highly resistant to the disease.[4][5]

U. americana is also the most susceptible of all the elms to verticillium wilt,[17] whose external symptoms closely mimic those of DED. However, the condition is far less serious, and afflicted trees should recover the following year.

In 2005, approximately 90 'Princeton' elms were planted along Pennsylvania Avenue in front of the White House in Washington, D.C. The trees, whose maintenance the National Park Service (NPS) manages, remain healthy and are thriving.[34] However, it has been noted that U. americana cultivars are not recommended for more than singular plantings as they have unresolved DED and elm yellows concerns.[35]

It has also been noted that monoculture plantings of U. americana cultivars, such as those along Pennsylvania Avenue, have disproportionate vulnerabilities to disease.[35] Further, long-term studies of 'Princeton' in Europe and the United States have suggested that the cultivar's resistance to DED may be limited (see Pests and diseases of 'Princeton').

The National Elm Trial evaluated 19 elm cultivars commercially available in the United States in scientific plantings throughout the nation to assess and compare the strengths and weaknesses of each. The trial, which started in 2005, lasted for ten years. Based on the trial's final ratings, the preferred cultivars of U. americana are 'New Harmony' and 'Princeton'.[36]

Thanks to the plant cloning work of tree geneticist Alden Townsend, American elm tree clones resistant to Dutch elm disease became a reality and the prognosis for Ulmus americana is now good. In the late 1990s, after 25 years of work with U. americana, Townsend announced he'd succeeded with two new strains, U. americana 'Valley Forge' and U. americana 'New Harmony.' Townsend's clones are now on the market.

I. Ulmus americana (American elm); hardy tozone 3-4 (depends on cultivar); height and width vary with cultivar, but theAmerican elm hybrids all have the classic, vase-shaped form; native to easternand central U.S. and Canada; available cultivars are listed below

American elms were the dominate tree species along city and suburban streets across the Eastern United States through the first half of the 20th century. These graceful, towering trees with their vase-shape form created cathedral-like canopies. The American elm, Ulmus americana, also known as white elm, was prized for its fast growth that provided quick shade and for its ability to tolerate poor soil conditions along city streets.

American elm (Ulmus americana), also known as white elm, water elm, soft elm, or Florida elm, is most notable for its susceptibility to the wilt fungus, Ceratocystis ulmi. Commonly called Dutch elm disease, this wilt has had a tragic impact on American elms. Scores of dead elms in the forests, shelterbelts, and urban areas are testimony to the seriousness of the disease. Because of it, American elms now comprise a smaller percentage of the large diameter trees in mixed forest stands than formerly. Nevertheless, the previously developed silvical concepts remain basically sound.

Black Ash-American Elm-Red Maple (Type 39) appears throughout the Northern Forest and into the Boreal Forest in Canada, and throughout the Lake States and into the northern edge of the Central Forest. In this type the most common associates, other than the type species, are as follows: In the Lake States and Canada, balsam poplar (Populus balsamifera), balsam fir (Abies balsamea), and yellow birch (Betula alleghaniensis); in Ohio and Indiana, silver maple (Acer saccharinum), swamp white oak (Quercus bicolor), sycamore (Platanus occidentalis), pin oak (Quercus palustris), black tupelo (Nyssa sylvatica), and eastern cottonwood (Populus deltoides); in New England and eastern Canada, sweet birch (Betula lenta), paper birch (B. papyrifera), gray birch (B. populifolia), silver maple, and black spruce (Picea mariana); and in New York, white ash (Fraxinus americana), slippery and rock elms (Ulmus rubra and U. thomasii), yellow birch, black tupelo, sycamore, eastern hemlock (Tsuga canadensis), bur oak (Quercus macrocarpa), swamp white oak, and silver maple.

American elm is attacked by hundreds of insect species including defoliators, bark beetles, borers, leaf rollers, leaf miners, twig girdlers, and sucking insects. The carpenterworm (Prionoxystus robinae) bores into the sapwood and degrades the wood. Among the insects that defoliate elm are the spring cankerworm (Paleacrita vernata), the forest tent caterpillar (Malacosoma disstria), the elm leaf beetle (Pyrrhalta luteola), the whitemarked tussock moth (Orgyia leucostigma), the elm spanworm (Ennomos subsignaria), and many other leaf-eating insects that attack elm and other hardwoods. The elm cockscombgall aphid (Colopha ulmicola) forms galls on the leaves but does little damage to the tree. Several scale insects attack elm and may cause damage. Both the elm scurfy scale (Chionaspis americana) and the European elm scale (Gossyparia spuria) are widely distributed. Among the leafhoppers, the whitebanded elm leafhopper is classed as a serious pest since it is the vector for phloem necrosis (15).

A few horticultural forms have been recognized. These are Ulmus americana columnaris, a form with a narrow columnar head, U. americana ascendens, with upright branches, and U. americana pendula, with long pendulous branches.

Dutch elm disease (DED) is a lethal vascular wilt disease of American elm (Ulmus americana) that is caused by Ophiostoma novo-ulmi and O. ulmi. While once widespread in the region, O. ulmi has been displaced by the more aggressive O. novo-ulmi and is now believed to be uncommon to rare in the region. Both O. novo-ulmi and O. ulmi are non-native to North America and Europe. Overland spread of DED from diseased to healthy elms is facilitated by the native elm bark beetle (Hylurgopinus rufipes) and European elm bark beetle (Scolytus multistriatus). The disease also spreads from diseased to healthy trees through root grafts when elms are in close proximity to one another.

The American elm (Ulmus americana L.) was once one of the most common urban trees in eastern North America until Dutch-elm disease (DED), caused by the fungus Ophiostoma novo-ulmi, eliminated most of the mature trees. To enhance DED resistance, Agrobacterium was used to transform American elm with a transgene encoding the synthetic antimicrobial peptide ESF39A, driven by a vascular promoter from American chestnut. Four unique, single-copy transgenic lines were produced and regenerated into whole plants. These lines showed less wilting and significantly less sapwood staining than non-transformed controls after O. novo-ulmi inoculation. Preliminary observations indicated that mycorrhizal colonization was not significantly different between transgenic and wild-type trees. Although the trees tested were too young to ensure stable resistance was achieved, these results indicate that transgenes encoding antimicrobial peptides reduce DED symptoms and therefore hold promise for enhancing pathogen resistance in American elm.

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Nice idea for a project. I'll just add that Ulmus americana and Ulmus rubra aren't particularly closely-related within the Ulmus genus and in that sense differ in many easy to learn ways. Despite this they are frequently confused in online posts and official articles, typically with Ulmus americana photos being labeled as "Ulmus rubra". Once one is familiar with both species, can contrast their differences to another species couplet: e.g. Ulmus rubra and the European Ulmus glabra which look fairly similar in terms of leaves and fruit 041b061a72