AA #10: Dieback Disease - Devastating Demise
The cache, a camo-taped preform tube, is hidden at a corner of Hallam Lane, the farm access lane off the main West Lane under a huge, but dying, specimen of this iconic tree species, many of which can be seen along this Amble.
Ash dieback or chalara will kill up to 80% of ash trees across the UK. At huge cost, the effects will be staggering. It will change the landscape forever and threaten many species which rely on ash.
It can affect ash trees of all ages. Younger trees succumb to the disease quicker but in general, all affected trees will have these symptoms:
- leaves develop dark patches in the summer. They then wilt and discolour to black. Leaves might shed early.
- dieback of the shoots and leaves is visible in the summer.
- lesions develop where branches meet the trunk. These are often diamond-shaped and dark brown.
- inner bark looks brownish-grey under the lesions.
- new growth from previously dormant buds further down the trunk. This is known as epicormic growth and is a common response to stress in trees.
Ash dieback is caused by a fungus (Hymenoscyphus fraxineus) which originated in Asia. It doesn’t cause much damage on its native hosts of the Manchurian ash (Fraxinus mandshurica) and the Chinese ash (Fraxinus chinensis) in its native range. However, its introduction to Europe about 30 years ago has devastated the European ash (Fraxinus excelsior) because our native ash species didn't evolve with the fungus meaning it has no natural defence against it.
The fungus overwinters in leaf litter on the ground, particularly on ash leaf stalks. It produces small white fruiting bodies between July and October which release spores into the surrounding atmosphere.
These spores can blow miles away, landing and sticking onto leaves, penetrating into the leaf and beyond, then growing inside the tree, eventually blocking its water transport systems, causing it to die. The tree can fight back, but annual re-infections will eventually kill it.
The fungus wasn’t formally described until 2006, but it has been known in Europe for about 30 years. In the UK, ash dieback has had the most impact in the south-east of England, where it was first recorded, in 2012. It is unknown how long the fungus had existed before symptoms appeared.
The disease is developing throughout the UK (see BBC article here). However, as the epidemic is still in its early stages, the full impact will not be known for a while. The slow progress of the disease exacerbates this, so the sometimes subtle changes brought about by ash dieback will need to be carefully tracked.
The spores of this fungus can travel in the wind so it is possible that it arrived in the UK naturally however it was also inadvertently imported on ash saplings, 1,000s of which were imported from infected parts of Europe until a 2012 ban. This undoubtedly sped up the spread of the disease within the UK because it was able to spread from areas of new planting via wind to mature trees.
There will likely be significant loss of the ash populations in the UK which will have a devastating impact on the landscape and woodlands biodiversity, as well as a major loss in connections between habitats as we lose hedges and individual trees outside woods.
The predicted cost of managing the diseases is high, including the practical expense of clearing up dead and dying trees, and loss of its environmental benefits such as air purification. Ash dieback is predicted to cost British society around £15 billion.
There is some hope on the horizon as initial findings estimate that 1-5% of trees may show reasonable tolerance to ash dieback, but none show complete resistance. The population could eventually recover over time and studies are ongoing to investigate this.
This tolerance is complex because several factors are involved including genetic traits, the general health of the tree, its local environment, and the number of ash dieback spores in the atmosphere.
See short videos here (winter signs), here and here (disease signs in summer), here (fascinating explanation of efforts to combat the disease), here (the fight against dieback - BBC) and here (fungal life cycle and disease symptoms).
