Ulmus × intermedia Elowsky is a natural hybridelm occurring across Nebraska and several other Midwestern states, derived from the crossing of Ulmus rubra and Ulmus pumila.[1] As Red Elm U. rubra is far less fertile, and highly susceptible to Dutch elm disease (:DED), it could eventually be hybridized out of existence by U. × intermedia.[2] The hybrid was first reported from the wild in the Chicago region in 1950 and was provisionally named U. × notha Wilhelm & Ware in 1994.[3]
The horticulture industry made a number of artificial crosses of the two species, such as 'Coolshade' and 'Fremont', in an attempt to create ornamental trees resistant to DED.[1]
Description
Ulmus × intermedia is significantly different from both parental species, except in biometrics such as leaf length to width ratios, number of teeth, petiole length, and pollen size. The leaves are 4.5–12 × 2.5–6 cm, petiole 0.3 × 1.2 cm, ovate to lanceolate, apex acuminate to acute, base oblique. The samarae are 11.5–21.0 × 10.0–20.0 mm, cream to white.[1][4]
Atypically, fertility of the hybrid seed is high, occasionally in excess of 90%,[5] while Collins found in artificial hybridization no reduction of fertility in the F1 or F2 generations.[6]
Pests and diseases
Morton Arboretum report the hybrid susceptible to DED and Elm Yellows (Elm phloem necrosis).[7]
Cultivation
Specimens of unnamed Ulmus × intermedia ( = Ulmus × notha) stand (2015) in Morton Arboretum, Illinois, received as Ulmus pumila from North Platte Agricultural Experiment Station, University of Nebraska.[8][9][7]
^Kaul, R. B. (1995). Natural and fertile hybrids of the native red elm, Ulmus rubra, with the introduced Siberian elm, U. pumila, are now producing hybrid swarms in Nebraska. Proc. Nebraska Acad. Sci. 105: 28–29.
^Collins, P. E. (1967). Hybridization studies in the genus Ulmus. Ph.D dissertation, Univ. of Minnesota, Minneapolis.
