Dutch elm disease
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Dutch elm disease is a fungal disease of elm trees, originally native to Asia. It has been accidentally introduced into America and Europe, where it has devastated native populations of elms which had not had the opportunity to evolve resistance to the disease. The name 'Dutch' refers to the first scientific discovery and study of the disease in the Netherlands.
The causative agent is the fungus Ophiostoma ulmi, spread by a bark beetle as the vector for infection. The fungus blocks the water conducting vessels within the tree; the first symptom of infection is usually an upper branch of the tree with leaves starting to wither and yellow in summer, months before the normal autumnal leaf shedding. This progressively spreads to the rest of 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 may put up small suckers. These may grow up for some years into small elm trees, but after a decade or so the new trunks become large enough to support the bark beetles, and with their inevitable arrival the fungus returns, and the new tree dies.
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Dutch Elm disease in Europe
Dutch Elm disease was first noticed in the Netherlands in about 1920, and spread slowly, reaching Britain in 1927. This first strain was a relatively mild one, which only killed a small proportion of elms, more often just killing scattered branches. In about 1967, a new, far more virulent strain arrived in Britain on a shipment of Rock Elm logs from North America, and this strain proved both highly contagious and lethal to all of the European native elms. By 1990-2000, very few mature elms were left in Britain or much of northern Europe. One of the most distinctive English countryside trees the English Elm (see e.g. John Constable's painting The Hay Wain) is particularly susceptible. Thirty years after the epidemic, the magnificent specimen trees are long gone. The elms often still hang on in hedgerows, as the roots are not killed and send up root sprouts ("suckers"). However, these suckers rarely reach more than 5 m tall before succumbing to a new attack of the fungus.
Dutch Elm disease in the United States
The disease was first reported in the United States in 1926, with the beetles believed to have arrived in a shipment of furniture. The disease spread slowly from New England westward and southward, reaching the Chicago area by 1960 and Minneapolis by 1970.
Treatment
The first fungicide used for treatment of Dutch Elm Disease was Lignasan BLP (carbendazim phosphate), which was introduced in the 1970s. This had to be injected into the base of the tree using specialized equipment, and was never especially effective. It is still sold under the name "Elm Fungicide". Arbotect (thiabendazole hypophosphite) became available some years later, and is somewhat more effective. Either product must be injected every year or two to provide ongoing control; the disease generally cannot be eradicated once a tree is infected.
Alamo (propiconazole) has become available more recently and shows some promise.
Treatment of diseased trees is costly and at best will prolong the life of the tree, perhaps by as many as five or ten years. It is usually only justified when a tree has unusual symbolic value or occupies a particularly important place in the landscape.
Resistant trees
Research to select resistant cultivars and varieties has been underway since the disease became endemic in the U.S. Early efforts involved the hybridization of the Chinese Elm with the American Elm, and produced a resistant tree that lacked the beauty, traditional shape, and landscape value of the American Elm. Few were planted.
Three major groups of resistant cultivars are commercially available now:
- The Princeton Elm, a cultivar selected in the 1920s for its landscape value. By happy coincidence, the trees exhibit moderate resistance to Dutch Elm Disease. Because mature trees planted in the 1920s still remain, the properties of the mature plant are well known.
- The Liberty Elm, a set of five cultivars produced through selection over several generations starting in the 1970s. Marketed as a single variety, nurseries selling the "Liberty Elm" actually distribute the five cultivars at random. Two of the cultivars are covered by patents.
- The Valley Forge elm, and some related cultivars, have been shown to have the highest resistance to Dutch Elm Disease in controlled tests.
Even resistant cultivars can become infected, particularly if the tree is under stress from drought and other environmental conditions, and if the disease pressure is high. With the exception of the Princeton Elm, no trees have yet been grown to maturity. The oldest liberty elm was planted in about 1980, and the trees cannot be said to be mature until they have reached an age of sixty years.
Possible earlier occurrences
A less devastating form of the disease, caused by a different fungus, had possibly been present in Britain for some time, as this passage in Richard Jefferies' 1883 book, Nature near London, shows:
- There is something wrong with elm trees. In the early part of this summer, not long after the leaves were fairly out upon them, here and there a branch appeared as if it had been touched with red-hot iron and burnt up, all the leaves withered and browned on the boughs. First one tree was thus affected, then another, then a third, till, looking round the fields, it seemed as if every fourth or fifth tree had thus been burnt. [...] Upon mentioning this I found that it had been noticed in elm avenues and groups a hundred miles distant, so that it is not a local circumstance.
This suggestion remains largely speculative, and there is no proof that it was caused by a fungus related to Dutch Elm Disease.