Dutch Elm Disease (DED)
DED was accidentally introduced to the UK from the USA in the late 1960s on imported elm logs. It is caused by the fungus Ophiostoma novo-ulmi and is spread by the elm bark beetle. Since the 1960s, it has devastated native populations of elms that aren’t resistant to the disease (in France, an estimated 90% of the elm population was destroyed).
Originally believed to be native to Asia, the name “Dutch Elm Disease” refers to its identification in 1921 in the Netherlands by two Dutch phytopathologists.
The disease is spread by beetles breeding in dead and dying elms, including those killed by the disease. The larvae tunnel in the bark and outermost wood, forming galleries. The fungus produces sticky spores in these galleries, which contaminate the newly hatched adult beetles as they emerge. The beetles fly to healthy elms, where they feed on young bark and introduce the pathogen into the conducting tissue of the tree. The fungus grows in this tissue and blocks vital water flow and causing rapid wilting and death.
The beetles tend to prefer mature trees of 20 years old or more. After the first wave of the disease in the early 1970s there followed a period of fewer cases while the trees regenerated from suckers, but these regenerated trees have in turn succumbed to the disease.
How do I recognise it?
The first sign of infection is usually seen in the upper branches of the tree with leaves starting to wither and yellow before the normal autumnal leaf shedding. This progressively spreads to the rest of the tree, with further dieback of branches. Over a period of time the roots will also die as they no longer receive nutrients via the leaves. Often, not all the roots die completely; the English elm for example puts up suckers which grow and mature for approximately 15 years, but unfortunately these also become infected.
You may also notice that peeling off the bark from affected branches reveals brown streaks in the outer wood, which appear as a broken or continuous brown ring in the outer growth ring if the branch is cut across.
Unfortunately, control of the disease using chemicals is not feasible. When the outbreak was in its early stages, protectant fungicides were injected into trunks but this process needed to be carried out annually and soon abandoned as it was just impractical.
Any attempts to prevent the spread of DED have now been widely abandoned, with the exception of continued treatments to trees in the Brighton and Hove area. However, dead trees are a safety hazard and should be felled promptly.
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