An unfortunate case of ash dieback

Recently we have stumbled across a disease of ash trees in our woodland sites called ash dieback, also known as Chalara. This is a fatal disease which mainly effects the growth of young ash trees, and is caused by a fungus called Hymenoscyphus fraxineus, previously called Chalara fraxinea. Older trees are also impacted however these are better able to withstand it, despite being weakened. The disease has never hit as hard as this before in the Cotswolds and as someone who is new to this job I have found it quite striking to see how much it has spread and its detrimental effects on young ash trees. As ash is the second most common tree in the UK this disease could have an even bigger impact than the Dutch Elm Disease in the 1970’s, which led to the loss of around 15 million elm trees.

Ash dieback was first spotted in the UK in 2012, after spores spread from a group of infected trees in a nursery sent from the Netherlands, presumably by wind. It is thought to have started in Poland in 1992 and has since spread throughout Europe. It effects both common and narrow-leaved ash species, which are identified as having compound leaves with pinnately arranged leaflets and smooth, grey bark:

Leaflets on an ash tree. Source: Forestry Commission, 2015.

Impacts of the disease which can not be cured include leaf loss, crown dieback and bark lesions. Below is a classic example of leaf loss on a young ash tree, which myself and Ranger Tim spotted last week in Popes Wood. Not only were the leaves dead, but the stems had turned brown where the disease had spread meaning the branches were easily snapped off.


An infected young ash tree at Popes Wood, with leaf loss and brown stems. Not to be confused with the changing colours of autumn. Photo taken by Ranger Tim.


The agent which causes ash dieback is considered a quarantine organism and so any sighting of the disease needs to be reported. I sent a report for Popes Wood to the Forestry Commission using their online Tree Alert form, so that they can better monitor the spread of the disease.

The worry is that next spring we will be seeing less ash trees in our woodlands due to the loss of young trees, leaving mainly sycamore and beech trees to thrive here in the Cotswolds. However, government scientists have been working to identify genetic factors which tolerate and resist the infection, so the hope is that in the future we will have a new breed of tolerant ash trees in our woodlands.

Further info:
Forestry Commission website:
Guardian article

Ranger Ellie


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