The impact of Ash dieback

Alex McKelvie, Senior lecturer arboriculture Myerscough College, helps us see the wood for the trees.

Figure 1. Spores on early spring leaves.

Ash dieback is fungal disease affecting the common ash tree (Fraxinus excelsior) and other Fraxinus species. It is caused by a fungus called Hymenoscyphus fraxineus which is native to eastern Asia. The disease was first found in England in 2012, although research has shown that it is likely to have been present since at least 2005. 

Ash trees make up 12% of Great Britain’s broadleaved woodland, and are often found in parks, gardens, hedgerows and roadside margins. Ash dieback is present in most parts of England, although the severity of the disease varies locally.

Ash is one of our most useful and versatile native tree species, providing valuable habitat for a wide range of dependent species. It can grow in a variety of soils and climatic conditions. The ‘airy’ nature of its foliage allows light to penetrate to the woodland floor, encouraging ground plants and fauna. A number of insects, other invertebrates, lichens and mosses depend wholly on ash for habitat.

Identifying and symptoms

Ash dieback has the potential to cause significant damage to the UK’s ash population, with implications for woodland biodiversity and ecology, and for the hardwood industries. Experience in continental Europe, which is now being seen replicated in the UK, indicates that it can kill young and coppiced ash trees quite quickly. However, older trees can resist it for some time until prolonged exposure, or another pest or pathogen, such as Armillaria (honey fungus), attacking them in their weakened state eventually causes them to succumb.

Figure 2. New growth near centre of canopy.

Infection can lead to leaf loss and dead branches throughout the crown of ash trees, and clumps of new growth towards the centre of the crown. In areas of high infection, the fungus can cause lesions at the base of the tree, making it more susceptible to secondary infections, such as by Armillaria fungi.

It is recommended that you familiarise yourself with the symptoms of ash dieback so you can assess the health of your ash trees and the severity of the infection in your area. The best time to survey is in the summer, when the symptoms are more obvious.

• Spores from the fungus travel on the wind and infect foliage, causing brown patches spreading towards the central leaf stalk and premature leaf loss (see figure 1). In some cases, the fungus will go on to colonise the branch.
• From the foliage, the fungus can colonise the branch structure of the tree, causing dieback. Trees may respond by producing new growth towards the centre of the crown (see figure 2).
• Lesions are caused as the pathogen colonises branches of a tree. They are typically diamond shaped (see figure 3 on page 38).
• Among the first symptoms that an ash tree might be infected with H. fraxineus is blackening and wilting of leaves and shoots (see figure 4) in mid to late summer (July to September).
• Particularly in woodland, secondary pathogens such as honey fungus can colonise trees weakened by ash dieback and cause root or stem rot (see figure 5).

Figure 3. Blackened diamond shaped lesions on trunk. © Richard Buggs

Because ash trees have many genetic variants and occur right across the UK, they come into leaf at different times in the spring: sometimes as late as the end of May. Trees in the colder north flush later than trees in the warmer south. Some ash trees will break-bud, or flush, earlier than others, and some buds will produce flowers rather than new shoots. Some variation will be more apparent in older trees. So, if an ash tree does not have any leaves in April and May, it does not necessarily mean that it is diseased or dying, but by mid-June all healthy ash should be in full leaf.

In the autumn, you might see clumps of sometimes dark-coloured ash keys, or seeds (see figure 6 on page 42), retained on the trees after the leaves have fallen. This is quite normal, but from a distance they can be mistaken for the blackened leaves which can be a symptom of the disease.

Figure 4. Effected leaves mid to late summer.

The implications

You are not legally required to do anything if you have ash trees that have ash dieback. However, the fundamental legal requirements under tort law, ‘a duty of care’, requires tree owners to identify what practical tree management measures they should take to meet their duty of care, should a tree fail or partially fail.

Public safety must be the priority, so there are implications for those managing trees which become infected. The disease spreads and weakens branches and stems, which will start to fall, meaning that a tree owner may be negligent in their responsibilities if they do nothing.

...This is highly likely with trees affected by ash dieback, and with the number of visitors and workers on a golf course, the implications of not managing ash dieback is a concern to clubs in the sports turf industry. 

Figure 5. Root rot in ash tree.

Along with legal implications of ash dieback, the impact on our landscape will be severe. Studying the impact of the disease experienced in Europe, the spread of the disease in the UK will mean that a significant number of ash trees will be lost from our urban and rural areas over the next twenty years. Estimates forecast that up to 75% of our ash tree population will be lost from UK landscapes overall. 

Golf courses play a vital role in sustaining wildlife in wooded areas. The ecological implications of the loss of ash trees will change wider biodiversity and habitat composition. The impact on populations of other plant and animal species such as bryophytes, invertebrates and lichens that use ash trees for feeding, breeding or as habitat will be hugely damaging.

What do you do about it?

There are financial and practical implications relating to this disease that will need to be addressed. It is therefore vital that people and organisations responsible for managing ash trees and woodlands containing ash understand the implications and take timely, site specific and proportionate action to prepare for this.

Figure 6. Healthy autumn clumps of seed.

Infected trees that are in areas with high levels of public access, or areas of the golf course where players stand waiting e.g. tees and greens, need to be monitored carefully for risks to safety. There is currently no cure for ash dieback, so a responsible tree owner or manager needs to assess the risk and prescribe remedial tree work to take place, such as felling or pruning of dead and dying trees.

Before undertaking any works, remember to check if you require a felling licence. You will need one if the tree is:

1. a) Protected by a tree preservation order (T.P.O.)
2. b) Located in a conservation area
3. c) If the tree, or trees felled, amount to 5 cubic metres of timber by volume

It is important, when recommending works, that a competent and qualified surveyor is appointed to undertake a formal tree survey of high-risk areas, and work is then undertaken by qualified arborists.

It just leaves me to thank Alex for his time and insight into what has to be one of the most impactful issues related to our industry at the moment and in the future.