Dutch Elm Disease
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Dutch Elm Disease (DED) is a fungal disease that affects the UK’s main elm tree species including the English Elm (Ulmus procera), Wych Elm (Ulmus glabra) and the Smooth Leaved Elm (Ulmus minor). The fungus is transmitted by two notable European bark beetles: the large elm bark beetle Scolytus scolytus; and the small elm bark beetle Scolytus multistriatus. Fungal spores stick to the beetles and are introduced to the tree when the beetles bore into the bark to feed. The fungus infects the vascular system of the tree, blocking water transport and eventually the tree wilts and dies. The first recorded outbreak of Dutch Elm Disease was in the Netherlands in 1910; giving rise to the name of the disease. However, the actual origin of the disease fungi is still uncertain. In 1920 the Forestry Commission (FC) began monitoring its rapid spread into the UK. By the 1940s the first wave of the disease had died down, but it had caused considerable losses of trees in Britain. This first epidemic was caused by a fungus called Ophiostoma ulmi (O. Ulmi) but in the late 1960s a new outbreak of the disease occurred; this time caused by a new, more aggressive species of fungus called Ophiostoma novo-ulmi (O. novo-ulmi). |
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Dutch Elm Disease |
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A detailed study of early infections in West Yorkshire revealed the role of imported infested elm logs in the spread of the disease. Within a decade of the second outbreak 20 million elms were dead out of an estimated UK population of 30 million.Pockets of isolated elm trees survived the outbreak, as did many small hedgerow elms. Suckers grew from surviving roots of these elms and new seedlings took root. However FC research predicted that the disease would return in cycles as the remaining elms grew large enough to support beetle breeding. This prediction proved accurate and by the mid 1990s substantial tracts of the elm re-growth and hedgerow elms between 3m and 12m tall were dead or dying. |
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Large elm beetle Scolytus scolytus
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Subsequent research showed that the 90s’ epidemic coincided with the return of the large elm beetle Scolytus scolytus, which probably migrated back from neighbouring parts of Britain where it had survived.
The large elm beetle is a very effective carrier of the fungus and re-emerged when trees grew bigger providing larger diameter breeding material.
Although the small elm beetle returned to trees first, making use of smaller diameter branches for breeding, it is a far less effective carrier of the fungus so the disease was not always triggered by its presence.
In late summer you may still see the effects of DED in hedgerows as elm leaves turn prematurely brown and yellow and wilt. The best advice in conserving our British elms is to keep them trimmed, because the larger trees are more likely to attract bark beetles to feed.