Elms Revisited

Until Dutch elm disease (DED) was accidentally introduced into the United States around 1930, the streets in many states were lined with American elms (Ulmus americana). This review highlights the aftermath of DED, and updates readers on the advances in our knowledge of the pathosystem, which consists of a tree, a fungal pathogen, and an insect vector. Conventional breeding has produced new cultivars of American elm that are more disease-tolerant, although still not resistant. Suitable DED-resistant hybrid elms have been bred using species from Europe and Asia. The discovery of diploid populations of American elm may open new opportunities in elm hybridization and genome analysis. Growing knowledge of resistance mechanisms reveals a complex interaction of anatomy, physiology, environmental factors, and tree age. The beetle’s role is largely understood but appears not to be a viable point of attack in the war on DED. The genome of the fungal pathogen has been sequenced, and gene expression studies are well under way. There is a renewed interest in understanding the evolution, genetics, and physiology of the DED pathogen. The genetic engineering of elms has been demonstrated but not with the specificity and vigor as has been reported for genetically engineered American chestnut. Elm yellows, caused by a phytoplasma, are still a deadly problem for elms, although outbreaks are more regional than for DED. Germplasm resources are critical to elm improvement, and the first comprehensive survey of living elm species, hybrids, and cultivars growing in America is presented in tabular form.