Dutch elm disease

Dutch elm disease is a fungal disease of elm trees which is spread by the elm bark beetle. Although believed to be 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 elm disease refers to the first scientific discovery and study of the disease in the 1920s in the Netherlands; the disease is in no way particular of the Dutch Elm hybrid.

Overview

The causative agents are micro-fungi. Three strains are now recognized, Ophiostoma ulmi, which afflicted Europe in 1910, reaching North America on imported timber in 1928, Ophiostoma himal-ulmi, a strain endemic to the western Himalaya, and Ophiostomi novo-ulmi, the extremely virulent strain that has devastated Europe's elms since the late 1960s. O. novo-ulmi is now considered to be a hybrid between O. ulmi and O. himal-ulmi. The new strain was widely believed to have originated in China, but a comprehensive survey there in 1986 found no trace of it, although elm bark beetles were very common.

The disease is spread by two species of Scolytus beetles which act as vectors for infection. The tree reacts to the presence of the fungus by plugging its own cambial tissue in an attempt to block the fungus from spreading further. As the area around cambium (the vascular tissue) is crucial for delivering nutrients and water to the rest of the plant, this plug prevents them from travelling up the trunk of the tree, eventually killing it. 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.

Disease range

Europe

Dutch elm disease was first noticed in Europe in 1910, 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, and had largely died-out by 1940. It was isolated in Holland in 1921 by Marie Beatrice Schwarz, a pioneering Dutch phytopathologist, and this discovery would lend the disease its name.^*

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. More than 25 million trees died in the UK. The elms often still hang on in hedgerows, as the roots are not killed and send up root sprouts ("suckers"). These suckers rarely reach more than 5 m tall before succumbing to a new attack of the fungus. However, established hedges kept low by clipping have remained apparently healthy throughout the nearly 40 years since the onset of the disease in the UK.

In 2005, researchers from Spain announced in Nature the results of a survey of genetic diversity of elms in Spain, Italy and the UK. ^* They state that most elms in the UK are clones, genetically identical to a single tree they theorise arrived with the Romans.

The United States

The disease was first reported in the United States in 1928, with the beetles believed to have arrived in a shipment of logs from the Netherlands destined for the Ohio furniture industry. The disease spread slowly from New England westward and southward, reaching the Chicago area by 1960 and Minneapolis by 1970.

Canada

Dutch elm disease reached Eastern Canada during the Second World War, and spread to Manitoba in 1975 and Saskatchewan in 1981. In Toronto, Ontario, as much as 80% of the elm trees have been lost to Dutch elm disease, and many more have fallen victim to the disease in Ottawa and Montreal and other cities during the 1970s and 1980s. Alberta and British Columbia are the only provinces that are currently free of Dutch elm disease, although an elm tree in southeastern Alberta was found diseased in 1998 and was immediately destroyed before the disease could spread any further. Thus, this was an isolated case. Today, Alberta has the largest number of elms unaffected by Dutch elm disease in the world. Aggressive measures are being taken to prevent the spread of the disease into Alberta as well as further progression of the disease in other parts of Canada. The City of Edmonton has banned elm pruning from March 31 to October 1, since fresh pruning wounds will attract the beetles during the warmer months.

Treatment

The first fungicide used for preventive 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 it has been proven effective. Arbotect must be injected every 2 to 3 years 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, though several university studies show it to be less effective than Arbotect treatments. Alamo is primarily recommended for treatment of Oak Wilt.

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 began in the Netherlands in 1928, and in the USA since the disease became endemic there. Initial efforts in the Netherlands involved crossing varieties of U .minor and U. glabra, but later included the Himalayan or Kashmir Elm U. wallichiana as a source of anti-fungal genes. Early efforts in the USA 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 1922 by Princeton Nurseries for its landscape value. By happy coincidence, this cultivar was revealed to be highly resistant in inoculation studies carried out by USDA in the early 1990s, 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 demonstrated resistance to Dutch elm disease approximately equal to that of the Princeton elm cultivar, in controlled USDA 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.

The Dutch research programme ended in 1992, after the release of two complex hybrids 'Columella' and (Lutèce ™) found to be actually immune to the disease when inoculated with unnaturally high doses of the fungus. In Italy, research is continuing at the Istituto per la Protezione delle Piante, Florence, to produce a wide range of disease-resistant trees using a variety of Asiatic species crossed with the early Dutch hybrid 'Plantyn' as a safeguard against any future mutation of the disease. Two trees with very high levels of resistance, 'San Zanobi' and 'Plinio', were released in 2003. Both feature the Siberian Elm U. pumila in their ancestry. In 2001, English Elm was genetically engineered to resist disease in experiments at Abertay University, Dundee, by transferring anti-fungal genes into the elm genome using minute DNA-coated ball bearings ^*. However, there are no plans to release the trees into the countryside.

Trees in the genus Zelkova, closely related to elms, are also planted as resistant substitutes for susceptible elms. Zelkova serrata, the Japanese Zelkova, the most commonly planted Zelkova tree, is similar to the American Elm in size and the vase-shaped crown.

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.

From analysis of pollen in peat samples, it is apparent the elm all but disappeared from Europe during the mid-Holocene period about 6000 years ago. Examination of sub-fossil elm wood has suggested that Dutch elm disease may have been responsible.

References

Brasier, C. M. (1996). New horizons in Dutch elm disease control. Pages 20-28 in: Report on Forest Research, 1996. Forestry Commission. HMSO, London, UK.

External links

Tree diseases | Ascomycetes

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