Editing Dutch elm disease (section)

===Testing for disease resistance===
Elms are tested for resistance by inoculation with the fungal pathogen in late May when the tree's growth is at its annual peak. Clones raised for testing are grown to an age of 3 or 4 years. In Europe, the [[wikt:inoculum|inoculum]] is introduced into the cambium by a knife wound. However this method, developed in the Netherlands, was considered too severe in America, where the principal disease vector is the bark beetle ''[[Scolytus multistriatus]]'', a far less effective vector than the larger beetle [[endemicity|endemic]] to Europe, ''[[Scolytus scolytus]]'', which is unknown in America. In the method devised by the [[USDA]], the inoculum is introduced to the cambium via a 2&nbsp;mm-diameter hole drilled through the bark in the lower third of the tree. This method was further refined by the [[University of Wisconsin]] team, which drilled holes in the branches to simulate natural infection by the bark beetles feeding in the twig crotches, but results from this method were found to exaggerate the genetic resistance of the host. Consequently, tests were conducted on specimens in a controlled environment, either in greenhouses or customized plant chambers, facilitating more accurate evaluation of both internal and external symptoms of disease.

Another variable is the composition of the inoculum; while an inoculum strength of 10<sup>6</sup> spores / &nbsp;ml is standard in both continents, its composition reflects the different ''Ophiostoma'' species, subspecies and hybrids endemic to the two continents. In Italy for example, two subspecies, ''americana'' and ''novo-ulmi'', are present together with their hybrid, whereas in North America, ssp. ''novo-ulmi'' is unknown.<ref name=Mittempergher>{{cite journal|last1=Mittempergher|first1=L|last2=Santini|first2=A|journal=Investigacion Agraria: Sistemas y Recursos Forestales|title=The history of elm breeding|volume=13|issue=1|pages=161–177|date=2004|url=http://www.inia.es/gcontrec/Pub/161-177-(14)-The_history_1161943529015.pdf|access-date=9 February 2017|archive-date=11 February 2017|archive-url=https://web.archive.org/web/20170211155148/http://www.inia.es/gcontrec/Pub/161-177-(14)-The_history_1161943529015.pdf|url-status=dead}}</ref> The differences in method and inocula possibly explain why the American cultivar [[Ulmus americana 'Princeton'|'Princeton']], displaying high resistance in the US, has often succumbed to Dutch elm disease in Europe.<ref name=Brookes>{{cite web|last=Brookes |first=A.H. |title=Disease-resistant elm cultivars, Butterfly Conservation trials report, 3rd revision |date=2013 |publisher=Butterfly Conservation |location=Lulworth UK |url=http://www.hantsiow-butterflies.org.uk/conservation/2013%20Elm%20Report%20Sept%202013.pdf |url-status=dead |archive-url=https://web.archive.org/web/20140529181302/http://hantsiow-butterflies.org.uk/conservation/2013%20Elm%20Report%20Sept%202013.pdf |archive-date=2014-05-29 }}</ref>