Chalara Fraxinea




Ash dieback (Hymenoscyphus fraxineus) is a fungus 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 did not evolve with the fungus and this means 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 tens of miles away. They land on leaves, stick to and then penetrate into the leaf and beyond. The fungus then grows inside the tree, eventually blocking its water transport systems, causing it to die.

The tree can fight back, but year-on-year infections will eventually kill it.



In the UK, ash dieback has had the most impact in the south-east of England. This is where it was first recorded in the UK back in 2012. It is unknown how long the fungus was in the area before the symptoms became apparent, perhaps some years.

We see evidence of the disease throughout the UK. However we are still at the beginning of the epidemic, so we won’t know the full impact for a while. The slow progress of the disease exacerbates this, so we need to track the sometimes subtle changes brought about by ash dieback.

It’s thought that we are going to lose up to 95% of our ash trees in the UK. This is going to have a devastating impact on the landscape and the biodiversity of our woodlands, as well as a major loss in connections between habitats as we lose hedges and individual trees outside of woods.

The predicted cost of managing the diseases is high. It includes the practical expense of clearing up dead and dying trees, to the loss of its environmental services such as air purification.



There is hope on the horizon. Initial findings suggest that we might have some trees that are tolerant to ash dieback, meaning that the population could eventually recover over time (likely over 50 years).

However, tolerance to the disease is complicated because a number of factors play into it including genetic traits, the health of the tree and the number of ash dieback spores in the atmosphere.

As a Company we are dedicated to helping stop the spread of Ash Dieback and will do all that we can to protect future trees.


When faced with the impact of ash dieback, a landowner could be forgiven for thinking that the best way to manage the problem is to remove all of the ash trees as soon as possible. However, during an epidemic, there will be a proportion of trees that will survive, and it is these trees that go on to build a tolerant future ash population. Leaving as many symptomatic and asymptomatic ash trees as possible in the landscape will lead to greater future resilience to this disease. In addition, both standing and recumbent dead and decaying wood provides a very important conservation function in a wood and is extremely important for many species.

Woodland Trust