Ash Dieback

What is Ash Dieback?

This is a serious tree disease caused by the fungus Hymenoscyphus fraxineus, previously called Chalara fraxinea. Introduced into Europe around 30 years ago, the spread of the disease has devastated our native European ash species, causing leaf loss and crown dieback and leading to the death of many infected trees. However, please don’t panic. Instead, read on for advice on what to do, and what not to do!

Ash Dieback Symptoms

All trees which are infected by the ash tree fungus will have these symptoms:

  • Blackened leaves, developed through leaf spots in the summer months which wilt and potentially shed early.
  • Lesions, which are often dark brown and diamond-shaped, will develop in infected branches where they meet the trunk. Bark underneath these lesions will look a brownish-grey colour
  • New growth will form from previously dormant buds further down the trunk - a common response by trees to stress which is known as epicormic growth.

How can we combat the disease?

Don't Panic!

The initial rate of death from infection throughout the UK may seem rapid and alarming. This is because the young and highly susceptible trees will die, and trees that are already stressed succumb fast. Trees avoiding this initial wave may linger for decades, and this will give woodland managers some breathing space to initiate strategies for the replacement of affected ash trees and stands.

It is worth keeping as much of the current population of ash trees as possible to maintain a diverse genetic resource. Prime specimens, such as those above average size with larger crowns tend to survive best.

For ash trees outside forest settings (e.g., parkland, urban, pasture) the situation is quite different, as only a small proportion are severely affected, and large old trees cope well with the disease.

It's not all bad!

The much-quoted statistic, in the UK, that more than 90% of Denmark’s ash trees have been killed by the infection is WRONG! In fact, 90% of Denmark’s ash trees in forests show signs of ash dieback, but it is mainly young stands that die.

What's the risk?

This vulnerability of woodland ash is because the cool, moist conditions in the UK are more conducive to ash dieback infection. The presence of honey fungus often kills the trees as a secondary infection leaving its host unable to absorb nutrients and water.

Young trees are by far the most at risk. This is because of their smaller girth and thinner, smoother bark, which is more vulnerable to infection around the root collar.

Once a forest tree is affected by ash dieback, it will cease to add any significant girth to the trunk, because all its energy will go into replacing the canopy and fighting honey fungus.

Diseased trees may be left standing, but once epicormic growth appears on the main stem/trunk, the fungus will rapidly enter and stain the wood of the trunk, causing economic loss for the forest owner.

What should we do (or not do)?

In order to identify truly tolerant individuals, it is necessary to allow for at least 10 years of incessant infection pressure in affected trees. Before this time, trees may show a varying degree of tolerance, but most will eventually develop dieback symptoms and decline.It is important to retain ash stands until the few per cent truly tolerant trees stand out, so they can be selected for breeding.

Veteran ash trees (other than in-cycle pollards) should never be cut without an overwhelmingly good reason. Carrying out tree surgery puts the tree under stress. If the tree is susceptible to ash dieback the ‘pathway’ for the disease to the main stem is shortened when the tree has been cut. The new shoots are worst affected, making it difficult for the tree to recover from the intervention. By not undertaking surgery, some veteran ash might undergo very severe mechanical failure. However, ash can survive quite well after such a failure.

Re-pollarding previously neglected pollards (or veteran coppice stools) should not be undertaken as it can place too much stress on the tree when it is also under infection pressure.

Dead wood is an important resource in woodland ecology, and a proportion of dead wood ash should be retained (standing and fallen). If it is likely to impede woodland management, it can be moved, but should still be retained.

Learn more about Ash Dieback

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