Ash dieback

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Ash tree showing signs of ash dieback in the crown. Credit Fera

Chalara, or ash dieback, is a disease of ash trees caused by a fungus. It was first detected in the UK in 2012 and is spread locally by wind. Ash dieback has been confirmed in all BBOWT woodland nature reserves.

Ash dieback is caused by Hymenoscyphus fraxineus a fungus that was previously called Chalara fraxinea. Fungal spores infect the tree through leaf stems, and fruiting bodes set spores from decaying leaf litter. Chalara infection causes leaf loss, crown dieback and bark lesions. Once a tree is infected by the disease it is likely to prove fatal.

The full impact of ash dieback on the population of ash trees in the UK cannot be predicted. In Europe there are studies looking into the tolerance of ash dieback in some trees. Ash trees make up a large proportion of woodland canopy cover; therefore the impact of ash dieback is likely to result in significant loss of ash trees and visible changes to woodland.

What BBOWT is doing

BBOWT's position statement gives details of what we are doing on our reserves.

Infected ash trees will not be felled unless they represent a health and safety risk. It is important to retain as many ash trees as possible to provide a chance that tolerant trees will produce viable tolerant, or even resistant, offspring.

Retaining ash trees also prolongs the availability of habitats for ash-associated biodiversity, and allows a chance for adaptation in those species. Coppicing ash trees will continue, but only with trees that are already in an active cycle of woodland management.

Decaying and dead trees and fallen wood offer beneficial habitats for wildlife. These will be left on site, unless they pose a risk to health and safety of visitors to the reserves.


Taking simple biosecurity measures is part of best practice in woodland management, and helps to reduce the risk of transferring the disease between sites. BBOWT will advise contractors and teams working on sites that are infected.

Because all BBOWT woodland sites are infected it is not essential for BBOWT staff, volunteers and members to take biosecurity measures when they visit our reserves.

However, anyone visiting another woodland, or working on a site, where ash dieback has not been confirmed must take simple measures before entering the woodland site to help prevent this disease spreading.

These include: cleaning outerwear and footwear, and cleaning vehicle wheels and wheelarches, tools and equipment. The Forestry Commission provides information about preventing diseases spreading.

Reporting ash dieback

If you are concerned about the possibility of ash dieback affecting trees, please check the Forestry Commission's website which has details of symptoms and how to report suspected cases.