Elm clone identification and the conundrum of the slow spread of Dutch Elm Disease on the Isle of Man

Abstract

This study was undertaken as part of a review of Ulmus L. for the forthcoming Flora of the Isle of Man. We have adopted molecular and morphological methods to reassess the identity of Ulmus on the Isle of Man and to explore the possibility that past records of English Elm, a single genotype cultivar called U. minor ‘Atinia’, were misidentifications. First, we utilised seven DNA microsatellite markers to test clone identity in a collection of 23 elms from across the Isle of Man. Sixteen samples were matched to a reference sample of Dutch Elm, U. × hollandica ‘Major’, and one sample was matched to a reference sample of English Elm. In parallel, we appraised a method employed to identify English Elm using a leaf shape metric, and found that leaf width divided by length values > 0.69 are indicative of this clone. Both molecular and morphological results in this study highlight that Dutch Elm has been misidentified as English Elm in the past, and that English Elm may in fact be a rare occurrence on the Island. The susceptibility of U. × hollandica ‘Major’ to Dutch Elm Disease suggests that the unusually slow spread of the disease on the Island is unlikely to be the result of local disease resistance in the elm population. One possibility that we explore is that the Scolytus beetle vector may experience suboptimal climatic conditions that could be limiting its dispersal, thereby reducing the rate of spread of infection.