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U. davidiana var. mandshurica Skvortsov
Whatever the taxonomic viewpoint adopted for it, the Ulmus davidiana complex is extremely important in the search for DED-resistant elms. One of the best selected clones, ‘Morton’ (Accolade), is a hybrid between U. davidiana var. japonica and U. wilsoniana – the latter recently sunk into U. davidiana var. japonica (Fu et al. 2003), so the hybrid now fits entirely within the new definition of U. davidiana var. japonica, as do several other named cultivars (discussed on p. 880). Ulmus davidiana var. davidiana is much less familiar in horticulture than these taxa, but shows great potential. It is said to have been introduced to cultivation in 1895, but only one specimen is recorded by TROBI, a 7 m tree with spreading branches planted in 1965 at Stanmer Park, Essex (Johnson 2003). When he measured this in 2002, Owen Johnson noted that it was ‘smothered in pink flowers’. Var. davidiana has only a sparse representation in other European collections, but there are wild-origin trees from China and South Korea at Edinburgh. Seed was obtained by the Morton Arboretum from the Botanical Garden of Forestry, Harbin, China in 1980, under the name U. davidiana var. mandschurica, and the resultant trees are now magnificent specimens there, up to about 12 m in 2006 and forming dense dark crowns, either rounded or vase-shaped. The leaves are quite small but a deep glossy green – described by George Ware (2000) as ‘a splendidly meritorious attribute’. Many seedlings from the Morton trees have been distributed in the Midwestern states, and it seems to be very suitable for propagation from seed (Ware 2000). Although the degree of resistance to DED varies slightly between clones, in general the U. davidiana complex is perhaps the most promising of all for the selection and breeding of new cultivars (Smalley & Guries 2000).