Editing Elm (section)

==== Dutch elm disease ====
{{main|Dutch elm disease}}
[[File:Iepziekte op goudiep (Ophiostoma ulmi on Ulmus hollandica 'Wredei' 4 May 2009).jpg|thumb|[[Ulmus × hollandica 'Wredei'|Golden elm]] tree with [[Dutch elm disease]]]]
Dutch elm disease (DED) devastated elms throughout Europe and much of North America in the second half of the 20th century. It derives its name "Dutch" from the first description of the disease and its cause in the 1920s by Dutch botanists [[Marie Beatrice Schol-Schwarz|Bea Schwarz]] and [[Christine Buisman|Christina Johanna Buisman]]. Owing to its geographical isolation and effective quarantine enforcement, Australia has so far remained unaffected by DED, as have the provinces of [[Alberta]] and [[British Columbia]] in [[western Canada]].

DED is caused by a micro[[fungus]] transmitted by two species of ''[[Scolytinae|Scolytus]]'' elm-bark [[beetle]]s, which act as [[Vector (epidemiology)|vectors]]. The disease affects all species of elms native to North America and Europe, but many Asiatic species have evolved antifungal genes and are resistant. Fungal spores, introduced into wounds in the tree caused by the beetles, invade the [[xylem]] or vascular system. The tree responds by producing [[tylose]]s, effectively blocking the flow from roots to leaves. Woodland trees in North America are not quite as susceptible to the disease because they usually lack the root grafting of the urban elms and are somewhat more isolated from each other. In France, inoculation with the fungus of over 300 clones of the European species failed to find a single variety that possessed of any significant resistance.

The first, less aggressive strain of the disease fungus, ''[[Ophiostoma ulmi]]'', arrived in Europe from Asia in 1910, and was accidentally introduced to North America in 1928. It was steadily weakened by viruses in Europe and had all but disappeared by the 1940s. However, the disease had a much greater and longer-lasting impact in North America, owing to the greater susceptibility of the American elm, ''Ulmus americana'', which masked the emergence of the second, far more virulent strain of the disease ''[[Ophiostoma novo-ulmi]]''. It appeared in the United States sometime in the 1940s, and was originally believed to be a mutation of ''O. ulmi''. Limited gene flow from ''O. ulmi'' to ''O. novo-ulmi'' was probably responsible for the creation of the North American subspecies ''O. novo-ulmi'' subsp. ''americana''. It was first recognized in Britain in the early 1970s, believed to have been introduced via a cargo of Canadian rock elm destined for the boatbuilding industry, and rapidly eradicated most of the mature elms from western Europe. A second subspecies, ''O. novo-ulmi'' subsp. ''novo-ulmi'', caused similar devastation in Eastern Europe and Central This subspecies, which was introduced to North America, and like ''O. ulmi'', is thought to have originated in Asia. The two subspecies have now hybridized in Europe where their ranges have overlapped.<ref name = Webber>Webber, J. (2019). What have we learned from 100 years of Dutch Elm Disease? ''Quarterly Journal of Forestry''. October 2019, Vol. 113, No.4,&nbsp;p.264-268. Royal Forestry Society, UK.</ref>  The hypothesis that ''O. novo-ulmi'' arose from a hybrid of the original ''O. ulmi'' and another strain endemic to the [[Himalayas]], ''[[Ophiostoma himal-ulmi]]'', is now discredited.<ref name=Brasier,1>Brasier, C. M. & Mehotra, M. D. (1995). Ophiostoma himal-ulmi sp. nov., a new species of Dutch elm disease fungus endemic to the Himalayas. ''Mycological Research'' 1995, vol. 99 (2), 205&ndash;215 (44 ref.) {{ISSN|0953-7562}}. Elsevier, Oxford, UK.</ref>

No sign indicates the current pandemic is waning, and no evidence has been found of a susceptibility of the fungus to a disease of its own caused by [[d-factors]]: naturally occurring virus-like agents that severely debilitated the original ''O. ulmi'' and reduced its sporulation.<ref name=Brasier,2>Brasier, C. M. (1996). New horizons in Dutch elm disease control. Pages 20-28 in: ''[http://www.forestry.gov.uk/pdf/New_horizons_DED.pdf/$FILE/New_horizons_DED.pdf Report on Forest Research] {{webarchive|url=https://web.archive.org/web/20070628155810/http://www.forestry.gov.uk/pdf/New_horizons_DED.pdf/$FILE/New_horizons_DED.pdf |date=28 June 2007 }}'', 1996. Forestry Commission. HMSO, London, UK.</ref>