Preliminary Assessments of Shoot Cold Tolerance for American Elm Bred for Enhanced Tolerance to Dutch Elm Disease

Although Dutch elm disease (DED) is the primary threat to American elm (Ulmus americana L.), we hypothesized that shoot freezing injury may also limit tree productivity and survival in the north. We assessed shoot cold tolerance and field winter injury of American elm bred for DED tolerance planted in Lemington, Vermont. We tested for differences in cold tolerance associated with date, maternal DED tolerance sources, paternal sources from plant hardiness zones 5a, 6a, and 6b (determined using data from 1996 to 2005), and the interactions of these. Cold tolerance was greatest in the winter, followed by fall and then spring. For all dates, cold tolerance never differed between maternal DED tolerance sources. However, in mid-winter, paternal sources from zone 5a (coldest zone) were significantly more cold tolerant than sources from zone 6b (warmest zone), and sources from zone 6a were intermediate. Field freezing injury confirmed that shoots were only marginally cold tolerant relative to ambient temperature lows.