Dutch Elm Disease and Botanical Variation in English Elm

Abstract

THE severe outbreaks of Dutch elm disease (Ceratocystis ulmi) in parts of southern England have not followed the pattern of earlier occurrences, and continue to cause concern. Features of particular interest are the possible effects of the disease in reducing the range of botanical variation in the English elm, and the extent to which the distributions of elm species have themselves influenced the intensity and extent of the outbreaks. It is generally agreed that the major division of the English elms is between Ulmus glabra Huds. (Ug), native in the north of England and more sporadic in the south, and the Ulmus minor and Ulmus procera complex. Until recently, U. minor has been referred to as U. carpinifolia Glad., and, together with U. procera, is probably native to southern Europe. The presence of the two species in England, where they seldom set seed, is most easily explained by human introduction from late pre-Roman time onwards. U. minor and U. procera are usually distinguishable in England. Hybrids between U. glabra and U. minor are well known, and Richens^1,2 has suggested that hybrids may also occur between U. procera and U. glabra as well as between U. minor and U. procera.