Editing Ulmus americana (section)

==== Dutch elm disease ====
{{Further|Dutch elm disease}}
Dutch elm disease (DED) is a [[fungus|fungal]] [[disease]] that has ravaged the American elm, causing catastrophic die-offs in cities across the range. It has been estimated that only approximately 1 in 100,000 American elm trees is DED-tolerant, most known survivors simply having escaped exposure to the disease.<ref name="DEDresistance">{{cite web|url=http://www.ars.usda.gov/is/AR/archive/jul96/elms0796.htm|title=New American Elms Restore Stately Trees|publisher=Agricultural Research Service, United States Department of Agriculture|date=July 1996|access-date=December 18, 2014|archive-url=https://web.archive.org/web/20141219051331/http://www.ars.usda.gov/is/AR/archive/jul96/elms0796.htm|archive-date=December 19, 2014|url-status=live}}</ref> However, in some areas still not infested by DED, the American elm continues to thrive, notably in [[Florida]], [[Alberta]] and [[British Columbia]]. There is a notable grove of old American elm trees in [[Manhattan]]'s [[Central Park]]. The trees there were apparently spared because of the grove's isolation in such an intensely urban setting.{{citation needed|date=February 2019}}

The American elm is particularly susceptible to disease because the period of infection often coincides with the period, approximately 30 days, of rapid terminal growth when new springwood vessels are fully functional. Spores introduced outside of this period remain largely static within the [[xylem]] and are thus relatively ineffective.<ref name=Smalley>{{cite journal|last=Smalley|first=E. G.|year=1963|title=Seasonal Fluctuations in Susceptibility of Young Elm Seedlings to Dutch Elm Disease|journal=Phytopathology|volume=53|issue=7|pages= 846–853}}</ref>

The American elm's biology in some ways has helped to spare it from obliteration by DED, in contrast to what happened to the [[American chestnut]] with the [[chestnut blight]]. The elm's seeds are largely wind-dispersed, and the tree grows quickly and begins bearing seeds at a young age. It grows well along roads or railroad tracks, and in abandoned lots and other disturbed areas, where it is highly tolerant of most stress factors. Elms have been able to survive and to reproduce in areas where the disease had eliminated old trees, although most of these young elms eventually succumb to the disease at a relatively young age. There is some reason to hope that these elms will preserve the genetic diversity of the original population, and that they eventually will hybridize with DED-resistant varieties that have been developed or that occur naturally. After 20 years of research, American scientists first developed DED-resistant strains of elms in the late 1990s.<ref name="DEDresistance" />

Elms in forest and other natural areas have been less affected by DED than trees in urban environments due to lower environmental stress from pollution and soil compaction and due to occurring in smaller, more isolated populations.

Fungicidal injections can be administered to valuable American elms, to prevent infection. Such injections generally are effective as a preventive measure for up to three years when performed before any symptoms have appeared, but may be ineffective once the disease is evident.