Ash Dieback

In the last few years, ash dieback has been well covered in the news and you may well have noticed some patches of blackened leaves hanging on local ash trees, or you may have been concerned to see ash trees being felled when they are near public footpaths in Surrey Wildlife Trust managed woodland. Discovered in 2012, ash dieback, also known as 'Chalara' is a disease of ash trees caused by a fungus called Hymenoscyphus fraxineus.

Ash dieback has caused the widespread loss of ash trees in continental Europe and is affecting countless woodlands, parks and gardens across the U.K, including our nature reserves. But generally, felling is only carried out selectively when there is a risk to public safety and of course only outside the bird-nesting season. Ash trees in the middle of woodland are generally left for nature to take its course, but if the infected tree is in your garden, then regular inspection is necessary.

How can I recognise it?

The main symptoms of ash dieback are:

Blackening of leaves hanging on the tree. The symptoms are often easier to spot in mid-late summer when a healthy ash should be in full leaf.
Discoloured stems, often with a diamond-shape lesion.
Black and shrivelled shoots.
The death of twigs and branches in the crown of mature trees.
Small white fruiting fungal bodies on blackened stalks in early autumn.

The Forestry Commission have a leaflet to help with symptom recognition which you can download here.

It is likely that the majority of our native ash trees will exhibit symptoms of ash dieback, but not all that do will die. A small percentage of ash trees will have a degree of tolerance to the disease and others will exist in locations where they escape the worst impacts. There is some evidence that ash trees growing in open, less humid locations such as streets and hedgerows may deteriorate more slowly or persist indefinitely, although it is not yet clear whether this will be a consistent pattern. Some trees with few symptoms could survive on these sites for many years and a small proportion of trees may have a degree of genetic tolerance to the disease which would bode well for survival and regeneration.

What action should I take?

You are not legally required to take any particular action if you own infected ash trees, unless your county, forestry or plant health authority serves you with a Statutory Plant Health Notice (SPHN) requiring action. This is unlikely.
Where ash trees occur in small numbers, gardeners can help to slow the local spread of ash dieback disease by collecting up and burning (where permitted), burying or deep composting (to get a high temperature decomposition) fallen ash leaves. This disrupts the fungus's life cycle. Do not spread the compost though.
With the exceptions of felling for public safety or timber production, there is a general presumption against felling living ash trees, whether infected or not. This is because there is good evidence that a small proportion will tolerate H. fraxineus infection. There is also the possibility that a proportion of ash trees can become diseased, but then recover to good health. But as any deadwood sections increase in size they are likely to be shed and where vulnerable targets are in the fall path, work will be necessary to control the risk, either by way of whole tree felling or by pruning out dead/ dangerous parts.
Check out www.forestry.gov.uk/biosecurity for advice on good working practices for biosecurity, including:
- When visiting outdoor areas such as woodlands, parks or gardens, drive and park your vehicle only on hard-standing surfaces such as tarmac if possible.
- Clean mud, organic material and water off your boots, bikes and buggies – and the dog before you leave, because fungi, bacteria and insects can live in these materials. Simple steps such as washing your footwear before and after walking in woodland, sticking to footpaths and washing bike and car tyres can all help reduce the risk of spores being spread.

Where can I find more information?

There is lots of information available on government, Forestry Commission, DEFRA, Wildlife Trust websites etc. Forestry Research have good information to start with at https://www.forestresearch.gov.uk/tools-and-resources/fthr/pest-and-disease-resources/ash-dieback-hymenoscyphus-fraxineus/

There is no cure for ash dieback, but some trees are less susceptible to the disease. Investigating this natural resistance could be the best way to secure the future of the UK's ash trees.