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What is Ash Dieback Disease?

The three words that any landowner dreads to hear: Ash Dieback Disease. But what exactly is it?


Ash Dieback Disease, also known as Chalara Ash Dieback, is caused by a fungus called Hymenoscphus fraxineus, which originates in Asia. This disease is incredibly destructive, especially to the UK’s native species of Common Ash (Fraxinus excelsior).


First discovered in the UK in 2012, Ash Dieback Disease has spread across the whole country, making it extremely common and difficult to control. In fact, the disease is now categorised as an endemic. It has the potential to kill up to 95% of ash trees across the UK, changing the landscape forever and threatening the many species which rely on this tree for survival.


Ash is one of our most versatile, useful trees, Not only can it grow in a variety of soils and climates, its airy foliage allows light to reach the forest floor below, nurturing the flora and fauna that grows there.


Widely produced for its timber, ash proves to be flexible, strong and durable, making it an attractive option for uses such as furniture, tool handles and rowing oars. With the swift decline of this species, the above industries may have to consider alternative materials for their goods.

So, how does it spread?


The fungal disease lies dormant in leaf debris on the ground in winter months, especially on the stalks of ash leaves. Between July and October, the fungus shows itself in the form of white, mushroom-like fruiting bodies on the dead leaves and stalks, which release spores into the atmosphere. These spores can spread up to 10 miles if blown around in strong gusts of wind, and we all know how windy it can get in the UK!


The spores then land on healthy leaves, adhering to them and then infecting them with the disease. The fungus spreads down through the leaves, infecting the branches until it eventually reaches the trunk. It gradually inhibits the water transport systems, causing the tree to die.


Some trees are more resistant than others, such as manna ash (F. ornus), Chinese ash (F. chinensis), and Manchurian ash (F. mandshurica), with only their foliage seemingly affected by the disease.



Ok, how can I identify an ash tree suffering from Dieback Disease?


Where Ash Dieback Disease is present, you are most likely to notice a significant thinning of the canopy and increased amount of deadwood within the crown.

A common symptom is the blackening and wilting of leaves and shoots, around July to September time. It’s easy to get these signs confused with the natural change in colour of the leaves as autumn approaches, so it’s important to keep a watchful eye out from July when summer is in full swing.


Most of these leaves drop prematurely, leaving the fungus to fester amongst the debris on the ground. However, the disease can spread through the leaf and into the tree, as we’ve already mentioned. Signs of this include blackened twigs, and dark lesions to form on the bark of the tree trunk. These lesions, also known as cankers, are characterized by their elongated diamond shape, and are mostly located where branches join the main trunk of the tree.


Usually, the lesions spread upwards and downwards from the joint, as the disease spreads in both directions. In severe cases, the cankers can girdle the whole trunk, which effectively cuts off the tree’s supply of fluid and nutrients from the roots.


You’ll also want to look out for cracks in the tree trunk, as the lesions can dry out and crack open over time. Sometimes, the wood underneath the surface of the lesion is stained a brown/grey colour, signifying that the disease has penetrated the trunk.



Is there a cure to Ash Dieback Disease? What if my ash trees are infected?


Unfortunately, there is no known cure to Ash Dieback Disease, although some fungicides have proved to be effective in suppressing it somewhat. This can enable some trees to be saved, such as those with high heritage or cultural value. However, this treatment will need to periodically re-applied, which can become costly.


Remember, that legally, you are responsible for managing trees affected by Ash Dieback Disease. If your diseased tree poses risk to someone else’s property or plants, you could find yourself in a tricky situation if you do not take reasonable action to limit the spread of disease. As a landowner, you have a responsibility to take action if you know your trees are affected.

The best thing you can do is closely monitor the progress of the disease. We don’t like to promote the felling of living trees if we can help it, although we can help you with this if it’s something you would like to do. There is evidence that infected trees can recover from Ash Dieback; it’s not an automatic death sentence.


We do advise that you keep an eye on the base of the tree for basal lesions, which will weaken the trunk and make the tree more prone to toppling over. We recommend full removal for trees exceeding 50% Ash Dieback Disease. If you have an ash tree, or several, which have concerning symptoms, do not hesitate to call The Arb Team. Our experienced tree surgeons will be more than happy to help you, and fell the tree in a safe and professional way.



How can we stop the spread of Ash Dieback Disease?


There are a number of small, simple things that you can do to help us minimise the spread of this devastating disease.


When you visit forestry, woods, parks, and public gardens, it’s good practice to brush any soil, twigs and leaves off your footwear and wheels (pram, wheelchair, bikes & cars) before leaving the location.


Wherever possible, park your car on a hard surface, such as concrete, tarmac or compact gravel. We also encourage cyclists & mountain bikers to wash down their wheels before and after they leave the site, which significantly lowers the risk of transmitting the disease to healthy areas.


If you have ash trees on your land, you can help slow the spread of the fungus by collecting up all of the fallen leaves and burning them, if permitted. If you’re unable to dispose of the leaves via a bonfire, then composting them is a great eco-friendly alternative. Ensure that you bury the leaves with at least 4 inches of soil, or a 12 inch layer of other plant material, and leave the compost heap undisturbed for about a year. This will help prevent spore dispersal and will encourage the swift decomposition of the leaves.



There you have it! If you’ve spotted a potentially infected ash tree near you, there are plenty of resources available to determine the course of action. The Forestry Commission regularly updates its advice and figures, and as ever, we are here to provide any advice and assistance that you need.


Dealing with an outbreak of Ash Dieback Disease? Call the professional tree surgeons at The Arb Team.