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Ash dieback disease

Friends of the Earth’s campaign to double UK tree cover is taking place against the background of the most serious disease to affect Britain’s trees since Dutch elm disease arrived in the 1970s. Ash dieback disease was first found in the UK in 2012, and over the last three years it has spread to all parts of Greater Manchester (see this map for more information).

This disease is spread by a fungus (Hymenoscyphus fraxineus) and mainly affects the European ash Fraxinus excelsior, although it has also been found in species of ash from the Mediterranean and North America.  It appears that roughly 90% of trees affected by ash dieback will die as a result of the disease, but studies have found that some trees are naturally resistant and will survive being infected.

What are the signs of ash dieback? The first sign is usually lesions on the stems, often in the form of long strips of discoloured bark.

Lesion at later stage
Lesion at early stage

These extend beneath the surface and cut off the supply of sap to the leaves, which thus turn yellow, brown and then black and finally wilt and fall.

Affected leaves starting to change colour and wilt

This process tends to start at the top of the tree and near the tips of branches. Younger trees (those of roughly head height) are killed rapidly, whereas mature trees are less badly affected and survive much longer.

The older trees at the rear appear healthy, whereas many of the young trees in the foreground have died
Affected ash tree showing dead upper branches

What can I do if I notice an ash tree is affected? This depends on where the tree is growing. If an ash tree is growing in a street it will need to be felled if infected as it will pose a risk to the public. Ask the council to replace it with another native species, preferably one of similar size. If a group of ash trees are growing together in an open space, it is always possible that one or more of them will turn out to be resistant, and there is no need to fell all the trees as a preventive measure. This leads on to the final question…

How can I stop the spread of ash dieback? Again, this depends on the location of the tree. In practice there is little you can do if the tree is growing in a garden or open space. This is because the fruiting body of the fungus grows on fallen leafstalks in leaf litter, which would all have to be removed. Street trees are therefore at an advantage as street cleaning should remove the leaf litter, but this would be very hard work in a garden. You should, however, take care not to move any leaf litter from near an affected tree yourself as this might run the risk of spreading the disease.

If you would like more information on ash dieback, City of Trees have a leaflet with further advice and links to publications by the Forestry Commission.

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