Ash dieback is a serious fungal disease that usually causes the death of ash trees. It’s currently decimating the ash population in the British Isles and Europe. It spreads as the fungal spores are dispersed on the wind. There’s currently no way to stop this dispersal and no cure for the disease. Some ash trees have genetic factors that give them some tolerance of the disease and new generations of ash are evolving more resistance. Scientists are working to identify these genetic factors so they can be used to breed new generations of resistant ash trees if natural regeneration alone isn’t sufficient.
What is ash dieback?
Ash dieback is sometimes referred to as Chalara ash dieback because the asexual phase of the fungus’s life cycle was previously known as Chalara fraxinea. The fungus is now called Hymenoscyphus fraxineus, but the original name is still used in some cases to distinguish it from other forms of ash dieback.
Ash dieback is believed to have originated in Asia, where it affects Manchurian ash (Fraxinus mandshurica) and Chinese ash (Fraxinus chinensis). However, these ash species are able to tolerate the disease because they evolved alongside it, whereas European ash (Fraxinus excelsior) hasn’t evolved the same natural defence.
Identifying ash dieback
The first signs of ash dieback are likely to be wilting and blackening of leaves and shoots, and branches dying back. Other ash dieback symptoms can include branches not producing leaves (although, ash comes into leaf later than many other tree species, so leafless branches are not necessarily a sign of ash dieback), diamond-shaped lesions on branches and the trunk, near the base of the tree, and dieback of branches and shoots.
There is great variation in the timings of ash coming into leaf, but if trees aren’t in full leaf by mid-June, there’s likely to be a problem. August and September are good months to look out for ash dieback as healthy ash trees are in leaf but haven’t undergone the foliage colour change and wilting that occur naturally in autumn.
How does ash dieback spread?
Ash dieback spreads via windborne fungal spores which can travel for tens of miles. It can also be spread when infected ash trees are transported around the country or moved between countries.
How does ash dieback cause problems?
Ash dieback is causing many problems. It has the potential to wipe out a large proportion of the ash trees in the UK. As trees develop the disease, they can become liable to dropping branches and falling. This poses public safety issues and has led to many ash trees being felled. The cost of felling unsafe ash trees is estimated to be £4.8bn – only a third of the total estimated cost, which also includes research, replanting and loss of profits for the forestry sector. Ash dieback kills younger trees more quickly, whereas older ash trees may cope with several seasons of infection before they finally die. Weakened trees with ash dieback are also more susceptible to other pathogens such as honey fungus that hasten their decline and death.
Ash is an important native species for woodland biodiversity and ecology. Around a thousand species are associated with ash, including mammals, birds, invertebrates, lichens, mosses, vascular plants and fungi. Over forty of these species solely rely on ash trees for their survival and many others are highly reliant on ash trees. The potential effects on woodland biodiversity could be significant and scientists are studying which other tree species might help fill the habitat gaps left by ash trees.
Ash dieback and the law
There is no legal requirement to take any action if you have infected ash trees in your garden, unless you are served with an SPHN (Statutory Plant Health Notice), which is extremely unlikely to happen. In fact, government guidance is to leave ash trees to grow unless they pose a risk to safety, as some may develop tolerance to the fungus. If you have an infected tree, keep an eye it, watching in particular for cankers or lesions near the base of the tree which might suggest the tree is in danger of falling. If you are in any doubt about the safety of a tree, seek advice from a qualified tree surgeon.
If you find a tree that you think is infected with ash dieback, check the symptoms on the Forestry Commission website. If it’s in an area not currently known to have confirmed cases of ash dieback (see the Forestry Commission map), contact TreeAlert (in Great Britain) and TreeCheck (in Northern Ireland).
How to deal with ash dieback
If you have ash trees in your garden, Forest Research (part of the wider Department for Environment, Food and Rural Affairs (Defra) group) recommends collecting and burning, burying or deep composting ash leaves (where permitted) to help slow the spread of the disease. For deep composting, they suggest covering them with a 10cm (4-inch) layer of soil or a 15-30cm (6-12 inches) layer of other plant material, and leave the heap undisturbed for a year (other than covering it with more material). More information can be found on the Forest Research website.
Frequently asked questions
How did ash dieback get to the UK?
It was thought ash dieback came to the UK in 2012 – the year the disease was first identified in a consignment of infected trees imported from The Netherlands – but scientists have since found evidence that the fungus was already present in the UK by 2006.
Can a tree recover from ash dieback?
Some trees can survive ash dieback and recover. Research suggests around 25 per cent of ash in mixed woodlands could survive, but it’s thought that a lower percentage will survive in woodland where ash is the dominant species.
