Dutch elm disease

Dutch elm disease is caused by an Ascomycete fungus that kills elm trees, and is spread by elm bark beetles. Believed to be originally native to Asia, the disease was accidentally introduced into America, Europe, and New Zealand. In these regions it has devastated native and exotic populations of elms that did not have resistance to the disease. The name "Dutch elm disease" refers to its identification from 1921 onwards in the Netherlands by the Dutch phytopathologists Bea Schwarz and Christine Buisman, who both worked with Johanna Westerdijk. The disease affects multiple species in the genera Ulmus and Zelkova, and is carried from tree to tree by any of several species of beetle, especially bark beetles of the genus Scolytus.

Multiple types of treatment or prevention have been attempted. The use of insecticides against the beetle vectors was tried in America in the 1940s and 1950s, causing significant harm to other organisms including insects and birds. Several fungicides are effective at protecting individual trees by injection every two or three years, as long as the root system has not been infected. A vaccine, Dutch Trig, uses a different fungus, Verticillium albo-atrum, to induce an immune response in individual trees. Breeding of elms resistant to Dutch elm disease has been attempted in the Netherlands, America, and Italy.

Dutch elm disease affects multiple species in the genera Ulmus and Zelkova. Ulmus species affected include the English elm, Ulmus procera, which was devastated by outbreaks in the 20th century. Smooth-leaved elm, Ulmus minor, has a measure of resistance. Wych elm, Ulmus glabra, is largely avoided by the beetle that carry the disease.

Dutch elm disease is caused by ascomycete microfungi. Three species are now recognized:

In Europe, the elm bark beetles Scolytus scolytus, S. triarmatus, S. multistriatus and S. laevis are vectors of the disease. S. scolytus is the species that most often carries the pathogen Ophiostoma novo-ulmi . Other reported vectors include S. sulcifrons, S. pygmaeus, Pteleobius vittatus and Р. kraatzi. In North America, vectors include the native elm bark beetle, Hylurgopinus rufipes.

Field resistance is the ability of varieties of elm to survive once released into an environment where Dutch elm disease occurs. The European white elm (Ulmus laevis) has little or no genetic resistance to Dutch elm disease, but is distasteful to the vector beetles and has thus often avoided serious harm. Elms such as U. laevis, U. minor, and U. pumila synthesize a range of triterpenes, including alnulin, β-amyrin, friedelin, ilexol, lupenone, lupeol, methyl betulinate, and moretonol, which to varying degrees render the bark distasteful to vector beetles.

In an attempt to block the fungus from spreading farther, the tree reacts by plugging its own xylem tissue with gum and tyloses, bladder-like extensions of the xylem cell wall. These plugs prevent water and nutrients from travelling up the trunk of the tree, eventually killing it.

The first symptom of infection usually appears in an upper branch of the tree. Here, the leaves start to wither and yellow during summer, which is months before the normal autumnal leaf shedding. This morbidity spreads in a progressive manner throughout the tree, with further dieback of branches. Eventually, the roots die, starved of nutrients from the leaves. Often, not all the roots die: the roots of some species, especially the English elm (formerly Ulmus procera), can repeatedly put up suckers, which flourish for approximately 15 years before dying off.