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Ash Dieback Disease

Ash dieback is a serious disease of ash trees caused by the fungal pathogen Hymenoscyphus fraxineus (previously known as Chalara fraxinea).

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Ash dieback is a serious disease of ash trees caused by the fungal pathogen Hymenoscyphus fraxineus (previously known as Chalara fraxinea). It has spread rapidly across much of Europe.

It was first noted in Republic of Ireland in October 2012 on plants imported from continental Europe. The disease is now prevalent throughout most of the island of Ireland.

The disease can affect ash trees of any age and in any setting. The disease can be fatal, particularly among younger trees.

What are the symptoms?

The wide range of symptoms associated with ash dieback includes:

  • Foliage wilt
  • Foliage discolouration (brown / black discolouration at the base and midrib of leaves)
  • Brown / orange discolouration of bark
  • Dieback of shoots, twigs or main stem resulting in crown dieback
  • Necrotic lesions and cankers along the bark of branches or main stem
  • Epicormic branching or excessive side shoots along the main stem

Foliage wilt Shoot discolouration / dieback Elongated angular stem lesions
Note, symptoms similar to the above may be caused by other factors, e.g. frost.

How can it spread?

Infection first makes its way into a tree when the spores of the fungus are carried in the air and land on healthy leaves over the summer months. The fungus then grows into the leaves and down into the leaf petiole (or rachis), and progressively into twigs, branches, and the stem.

The infected leaves gradually wilt and blacken, but may remain on the tree for some time. These infected leaves then fall to the ground over the autumn and early winter months and the fungus produces a characteristic blackened ‘pseudosclerotial plate’ on the rachises. These blackened rachises harbour the disease overwinter.

In the sexual reproductive stage of the fungus, which takes place over the course of the summer and autumn months (June to October), very small mushroom-like fruiting bodies develop on the blackened rachises and decaying leaf litter from the previous autumn and winter. When mature, these tiny fruiting bodies release large quantities of microscopic spores into the air, some of which will land on the leaves of both healthy and previously infected ash trees to begin the cycle again.

Where the disease is already present in a locality further local spread is likely to be caused by spores borne on the wind, each year travelling many kilometres from the original source. There is also a risk of introducing the disease into a locality where it is not yet present (and where that locality is at a considerable distance from an existing source of infection) by bringing already diseased ash seeds or plants into that area for the first time.

However, movement of larger diameter ash logs from infected areas is considered to be much lower risk as long as certain phytosanitary measures are properly implemented. These include ensuring the larger diameter logs have no evident signs of the disease (e.g. lesions or staining) and that all leaves and foliage (whether living or dead) are completely removed on site before transportation.

Further information:

Relevant publications: