Ashes to ashes - Ash dieback disease
The UK’s ash population is under severe threat from a fungal disease, Chalara fraxinea, which (at the time of writing) has been conﬁrmed in 32 locations. A specialist team drawn from the Forestry Commission, Defra, Fera, Natural England and the Environment Agency is looking at ways to safeguard the future of the much-loved tree species. Eoin Redahan spoke to the Chalara team at the UK Forestry Commission about the blight.
Q - The disease was recently confirmed in UK woodlands and forests in Kent and Essex. How extensive is the damage?
Our priority has been to get an overall picture of the distribution of the disease within Great Britain rather than to quantify the extent of the damage it has caused so far, locally or nationally. That is why we (and our partners) undertook a rapid survey of ash trees across Britain and appealed for reports of suspected cases. This will help to inform our long-term disease control strategy.
Q - In what way does C. fraxinea destroy ash species?
Initially, leaves are infected. The fungus grows into shoots and then branches, killing the bark and causing cankers. The impact of foliage loss due to leaf infections occurring year on year, coupled with bark death and penetration of the underlying wood, disrupts the supply of nutrients and water to the crown of the tree and weakens affected trees.
Q - How does the disease work?
The life-cycle is completed as spores are produced from tiny, mushroomlike fruiting bodies that form on the fallen leaves of ash trees that were infected the previous year. These spores are released into the air and blown by the wind into contact with the leaves of healthy ash trees, thereby causing infection. The disease can spread 20–30km a year.
Q - How easy is it to detect the disease spores? What volume of spores does it take to infect a tree?
The tiny fruiting bodies produce the spores during the summer and are barely visible to the naked eye, so it requires microscopy equipment to detect the spores themselves. Millions of spores are released from the fruit bodies after they have formed on stalks of infected leaves lying in the litter layer, and experience in mainland Europe suggests a high dose of spores may be needed for leaves to become infected.
Q - How long does it take to kill a tree?
Once infected, ash trees do not recover. Young trees can die within a year of symptoms becoming visible. However, older individuals can survive many years and might not die directly from Chalara dieback, but from a combination of Chalara and other pests and diseases, especially honey fungus (Armillaria).
Q - How do you halt the spread of the disease?
Unfortunately, once it is established it is probably not possible to completely halt the spread of a wind-blown fungus, but the aim is to slow the disease’s progress by taking the sensible biosecurity measures we are recommending
(see www.forestry.gov.uk/chalara). This can help us to gain time to learn more about the fungus’ behaviour and investigate possible resistant trees, which could help to ensure that we will have ash trees in Britain in the future.
Q - In what way are wood products treated to avoid the spread of the disease?
Ash wood poses only a low risk of spreading the disease. At present it is not permitted to move ash wood from a conﬁrmed infected site that has been served with a statutory Plant Health Notice. At other sites where the disease status is not certain, we advise the sensible precaution of sweeping leaves and similar material off logs and vehicles before they leave the site, because movement of leaves poses the highest risk of spreading the disease. Chalara infection can cause staining of ash wood, reducing its value for some of the end uses for which it is popular, such as furniture, ﬂooring and joinery.
Q - Could the genetic resistance of some trees provide a clue in stymieing the disease?
There is evidence from mainland Europe that a small percentage of ash trees have survived attack by the fungus, suggesting that they might have some degree of resistance. We will be looking for the same thing in Britain, because the seed from such trees could form the foundation of future generations of resistant ash trees in the British landscape.
Q - Does it affect other tree species?
Only ash (Fraxinus) species are known to be susceptible.
Q - Is it difficult to find traces of C. fraxinea?
Some of the symptoms of C. fraxinea infection are quite distinctive from those caused by other ash tree diseases, and are relatively easily recognised by trained surveyors. We sent a number of staff to Denmark to receive training from specialists there, and they have come back and trained more staff here to recognise the disease. We also have two videos and a photographic guide to the symptoms on our website to aid recognition of the disease.
Q - What treatment solution has been proposed?
The long-term proposals for management of the disease will be known when the national Control Plan (an interim plan has been published) has been published. In the meantime, Forestry Commission staff and other tree professionals across Britain remain on high alert for signs of the disease.
Q - What effect has this disease had on imports and exports of wood products?
Legislation introduced by the UK Government on 29 October 2012 primarily applies to planting material such as seeds, seedlings and saplings. It made no changes to the regulations applying to movements of ash wood and wood products, except that shipments found to be infected with C. fraxinea could be ordered to be returned or destroyed. The Irish and Northern Irish authorities, however, have tightened up their import requirements for ash wood, but kilndried or bark-free ash wood may still be exported from Great Britain to Ireland and Northern Ireland.
Q - What are the team’s next steps?
The cross-Government team working on the disease is evaluating all the research and survey material available, as well as drawing from other countries’ experience in mainland Europe, and using this to draft a national disease Control Plan for Great Britain.
130,000 - Hectares of predominantly ash woodland in the UK
40 - Maximum height of an ash tree in metres
150 - Estimated life expectancy in years
For videos and a photo guide to the disease’s symptoms, visit www.forestry.gov.uk/chalara
For the latest interim control plan, visit