North Dakota State University Research Foundation, US
The American elmcultivarUlmus americana 'Lewis & Clark' (trade name Prairie Expedition) is a development from the North Dakota State University (NDSU) Research Foundation breeding programme, released in 2004 to commemorate the 200th anniversary of the eponymous expedition.[1] The cultivar was cloned from a tree discovered in 1994 along the Wild Rice River south west of Fargo, North Dakota, where all those around it had succumbed to Dutch elm disease; the tree remains in perfect health (2008). Prairie Expedition proved only moderately successful in the US National Elm Trial, averaging a survival rate of 62.6% overall, potentially due to environmental factors rather than susceptibility to Dutch elm disease.[2] Nevertheless, Prairie Expedition is considered the hardiest of the American Elm cultivars, able to survive in Zone 3 (-40 to -34 Celsius). [3]
Prairie Expedition was introduced to the UK in 2008 by Hampshire & Isle of Wight Branch, Butterfly Conservation, as part of an assessment of DED-resistant cultivars as potential hosts of the endangered White-letter Hairstreak.[4]
Description
Foliage, midsummer
Foliage, October
Prairie Expedition is distinguished in maturity by its broad umbrella crown and dark green foliage.[5] However, the tree was judged to have little ornamental value in the National Elm Trial . [3] The clone grows moderately quickly, averaging an increase of > 1 m in height per annum when young, reaching a height of <17 m in 25 to 30 years.[6] The leaves are < 13 cm in length by 9 cm broad, coarsely-toothed, and with a 4 mm petiole.
Pests and diseases
Possession of an innate resistance to Dutch elm disease was suggested after inoculation with the causal fungus at the NDSU,[7] however replication of the tests is considered too limited to be conclusive. No other specific information available, but the species as a whole is highly susceptible to Elm Yellows; it is also moderately preferred for feeding and reproduction by the adult Elm Leaf Beetle Xanthogaleruca luteola,[8] and highly preferred for feeding by the Japanese Beetle Popillia japonica[9][10] in the United States.
U. americana is also the most susceptible of all the elms to verticillium wilt.[11][10]
