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 Christine Buisman and Marie Beatrice Schol-Schwarz, 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.
Overview
Taxonomic range
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.
Pathogens
Dutch elm disease is caused by ascomycete microfungi.Ascomycetes: Phylum Ascomycota , Biology of Plants, Seventh Edition, W. H. Freeman and Company, 2005. Three species are now recognized:
Ophiostoma ulmi, which afflicted Europe from 1910, reaching North America on imported timber in 1928.Clinton, G. P., McCormick, Florence A., Dutch elm disease, Graphium ulmi; New Haven, 1936 Ophiostoma himal-ulmi, a species endemic to the western Himalaya. Ophiostoma novo-ulmi, an extremely virulent species from Japan, first described in Europe and North America in the 1940s, has devastated elms in both continents since the late 1960s.
Insect vectors
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
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.
Mechanism
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.
Symptoms
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.
Disease range
Europe
Beginnings
Dutch elm disease was first noticed in continental Europe in 1910, and spread slowly and eventually extended to all other countries except Greece and Finland.Clouston, B., Stansfield, K., eds., After the Elm (London, 1979) Barendina Gerarda Spierenburg compiled records of trees displaying symptoms from 1900 - 1905 onwards in the Netherlands and her publication of this information in 1921 was one part of the start of extensive research and practical measures to try to halt the disease. In addition the fungus that caused the disease was isolated in 1921 in The Netherlands by Bea Schwarz, a pioneering Dutch phytopathologist, and this discovery would lend the disease its name. Following this, in the 1920s and 30s Christine Buisman, working in the Netherlands and USA, identified the sexual stage of the fungal pathogen and also developed methods for experimental infections of elm seedlings that led to selection of resistant trees.Heybroek, H. M. and Nijboer, R. (2013). Christine Johanna Buisman in Italy. p. 4–6. Private publication, Netherlands. In Britain, the disease was first identified in 1927 by T.R. Peace on English elm in Hertfordshire.Harris, E. (2017). The European White Elm, Ulmus laevis Pall. Quarterly Journal of Forestry, Vol. 111, No. 4, October 2017. p.263. Royal Forestry Society.
Epidemic
In around 1967, a new and far more virulent strain arrived in Britain, apparently via east coast ports on shipments of rock elm U. thomasii logs from Canada destined for the small-boat industry, confirmed in 1973 when another consignment was examined at Southampton Docks. This strain proved both highly contagious and lethal to European elms; more than 25 million trees died in the United Kingdom, while France lost 97% of its elms. The disease spread rapidly northwards, reaching Scotland within 10 years.
Aftermath
By 1990, few mature elms were left in Britain or much of continental Europe. One of the most distinctive English countryside trees, the English elm U. minor 'Atinia', is particularly susceptible as it is the elm most favoured by the Scolytus beetles. Decades after the outbreak of the epidemic, nearly all these trees are gone. The species survives in hedgerows, as the roots are not killed and send up suckers. These often reach a height of 10 m before dying off from a new attack. However, established hedges kept low by clipping have remained apparently healthy since the onset of the disease.
The largest concentrations of mature elms in Europe are now in Amsterdam and The Hague. In 2005, Amsterdam was declared the "Elm City of Europe": the city's streets and canals are lined with at least 75,000 elms, including several generations of research-elms (see below: Resistant trees).Amsterdamse Bomem Some 30,000 of the 100,000 mature trees in The Hague are elms, planted because of their tolerance of salty sea-winds. Since the 1990s, a programme of antifungal injections of the most prominent 10,000 elms, and of sanitation felling, has reduced annual elm losses in The Hague from 7% to less than 1%. The losses are made up by the planting of disease-resistant cultivars. The largest concentration of mature elm trees remaining in England is in Brighton and Hove, East Sussex, where of the 30,000 elms in 1983Research on Dutch Elm Disease in Europe, ed. D. A. Burdekin (London, 1983) 15,000 still stand (2005 figures), several of which are estimated to be over 400 years old. Their survival is owing to the isolation of the area, between the English Channel and the South Downs, and the assiduous efforts of local authorities to identify and remove infected sections of trees immediately when they show symptoms of the disease.Brighton and Hove Council page on the city's elm collection (viewed 2 June 2010) Empowered by the Dutch Elm Disease (Local Authorities) (Amendment) Order 1988, local authorities may order the destruction of any infected trees or timber, although in practice they usually do it themselves, successfully reducing the numbers of elm bark beetle Scolytus spp. Sanitary felling has also, to date, preserved most of the 250,000 elms on the Isle of Man, where average temperature and wind speed inhibit the activity of the beetles, which need a temperature of at least 20 degrees to fly and a wind speed of less than five metres per second.
As of 2016, Edinburgh city council states that 15,000 trees survived the epidemic. A policy of sanitary felling kept losses in the city to an average of 1,000 a year, or fewer. Elm was the most common tree in Paris from the 17th century; before the 1970s there were some 30,000 ormes parisiens. Today, only 1,000 mature elms survive in the city, including examples in the large avenues (Avenue d'Italie, Avenue de Choisy, Boulevard Lefebvre, Boulevard de Grenelle, Boulevard Garibaldi) and two very old specimens, one in the garden of the Tuileries in front of the l'Orangerie and another in the Place Saint-Gervais in front of l'hôtel de ville de Paris. Losses are now being made up with disease-resistant cultivars, especially the Dutch-French research elm 'Nanguen' (), named for the ancient Roman name for the city: Lutetia.
North America
Dutch elm disease was first reported in the United States in 1928. The beetles were believed to have arrived in a shipment of logs from the Netherlands destined for use as veneer in the Ohio furniture industry. Quarantine and sanitation procedures held most cases within of metropolitan New York City until 1941 when war demands began to curtail them.Life, 11 September 1944, p. 58 The disease spread from New England westward and southward, almost completely destroying the famous elms in the "Elm City" of New Haven, Connecticut, reaching Detroit in 1950, Chicago by 1960, and Minneapolis by 1970. Of the estimated 77 million elms in North America in 1930, over 75% had been lost by 1989.
The disease first appeared on the planted rows of American elm trees (Ulmus americana) on the National Mall in Washington, D.C., during the 1950s and reached a peak in the 1970s. The United States National Park Service used a number of methods to control the epidemic, including sanitation, pruning, injecting trees with fungicide and replanting with disease-resistant American elm cultivars (see Ulmus americana cultivars). The NPS combated the disease's local insect vector, the smaller European elm bark beetle (Scolytus multistriatus), by trapping and by spraying with insecticides. As a result, the population of American elms planted on the Mall and its surrounding areas has remained intact for more than 80 years.
Dutch elm disease reached eastern Canada during World War II, and spread to Ontario in 1967, Manitoba in 1975 and Saskatchewan in 1981. In Toronto, 80% of the elm trees have been lost to Dutch elm disease; many more fell victim in Ottawa, Montreal and other cities during the 1970s and 1980s. Large elm trees that were commonly scattered on farms and pastures across eastern Canada through the mid-20th Century have since become rare. Quebec City still has about 21,000 elms, thanks to a prevention program initiated in 1981. Alberta and British Columbia are the only provinces that are currently free of Dutch elm disease, although, in an isolated case, an elm tree in Wainwright, Alberta, was found diseased in June 1998 and was immediately destroyed. The presence of Dutch elm disease was monitored in this area during subsequent years but was not seen again. Today, Alberta has the largest number of elms unaffected by Dutch elm disease in the world.