Ash dieback: what the agencies record

This page reports what the official source says. It is not legal advice: ask the office named here. It describes what the named sources say and is not a diagnosis of any tree. Each section names its source, all read on October 9, 2026. Felling licences, notices and the rules country by country are on the ash dieback regulated status page and are not repeated here. The full list of diseases is on the tree diseases page.

Names and the fungus

Forest Research calls ash dieback a highly destructive disease of ash trees (Fraxinus species), especially the United Kingdom's native common ash, Fraxinus excelsior. It names the causal agent as the fungus Hymenoscyphus fraxineus, of eastern Asian origin. It says the disease is also called Chalara, ash dieback and Chalara dieback of ash, and that using the name Chalara helps separate it from dieback on ash caused by other agents. The asexual phase of the fungus was formerly called Chalara fraxinea and the sexual phase Hymenoscyphus pseudoalbidus, and Forest Research says some older documents use those names. Forest Research lists the disease as present in the United Kingdom and reportable.

Hosts

Forest Research says common ash, including its Pendula ornamental variety, is especially affected, and narrow-leaved ash is also susceptible. Manna ash has been found only with infected foliage and might prove tolerant. It says scientists believe all of the more than 60 ash species worldwide are susceptible, with varying tolerance, and that Chinese ash and Manchurian ash appear able to tolerate infection and show only mild leaf symptoms, having co-evolved with the fungus. It also reports infection in the United Kingdom on mock privet, narrow-leaved mock privet and white fringetree growing close to infected ash. They are in the same plant family as ash, and Forest Research says the findings are unlikely to matter much because those plants are ornamental and not widespread.

Symptoms as the sources state them

Forest Research names blackening and wilting of leaves and shoots in mid to late summer, July to September, as among the first symptoms, and says those are the best months to survey for leaf symptoms because normal autumn colour from late September can be mistaken for them. Most infected leaves are shed early. In some cases infection moves into twigs, branches and the trunk, forming dark lesions or cankers in the bark, often shaped like an elongated diamond centred on branch joints, which can girdle the trunk. Lesions too small to girdle can dry and crack open as the tree grows around them, and Forest Research says they should be visible at any time of year. Scottish Forestry describes progressive leaf loss that leaves only sparse foliage in the crown over time.

Look-alikes the source names

Forest Research lists normal ash behaviour that can be mistaken for the disease. Ash is one of the last trees to come into leaf and can flush as late as the end of May, with trees in the colder north flushing later than those in the warmer south, so a bare ash in April and May is not necessarily diseased, though all healthy ash should be in full leaf by mid-June. Some shoots fail to flush and some buds make flowers instead of shoots. In autumn, clumps of dark ash keys that stay on the tree after the leaves fall can look from a distance like blackened leaves. It adds that shoot death and dieback on ash can have a number of causes.

What happens to trees

Forest Research says experience in continental Europe, seen replicated in the United Kingdom, shows that the disease can kill young and coppiced ash fairly quickly, while older trees can resist for some time until prolonged exposure, or another pest or pathogen such as Armillaria (honey fungus, see Armillaria root rot), kills them in their weakened state. Scottish Forestry says ash dieback has killed ash trees across Europe, is usually fatal in younger trees, and that older trees more often die from the combined effect of the disease and root pathogens such as honey fungus. Forest Research adds that a small proportion of ash can tolerate infection, and some may become diseased and then recover, though it says it is too early to know whether recovery occurs in the British ash population.

How it spreads

Forest Research says wind carrying spores accounts for local spread of up to tens of miles, and that longer distances are most likely through the movement of diseased ash plants. It says movement of logs or unsawn wood from infected trees might also be a pathway, considered a low risk.

Where the sources place it

Forest Research says the disease is present in most parts of the United Kingdom, that its effects are most visible where the fungus has been present longest and conditions suit it best, and that it is widely present in continental Europe and Ireland. It points to an annually updated grid-square map, and notes that the growth of the map over time shows when sites were found and not the rate of spread. Scottish Forestry, on a page published February 22, 2025, says the disease is widely distributed throughout Scotland and that import restrictions apply to ash originating in some parts of Europe and outside Europe.

National measures and reporting

A Forest Research guidance note updated March 19, 2020 says the United Kingdom introduced national measures against ash dieback in 2012 that required imports of ash to come from pest-free areas, so import and movement were essentially prohibited. It says that, because the disease is well established and present in every county, there was no technical case to keep the national measures, and that from January 13, 2020 the national measures against ash imports from the European Union no longer applied and domestic movement restrictions were lifted. It says imports from third countries remain prohibited under the plant health regulation, and that the note gives the main risk to ash as new Asian strains of the fungus and pests such as emerald ash borer, covered on emerald ash borer. For reporting, Forest Research asks for reports from parts of the country where the disease has not been recorded, through TreeAlert in Great Britain and TreeCheck in Northern Ireland, with clear close-up photos. Scottish Forestry says no report is needed where the disease is already known in the 10 km grid square. See report a pest in the United Kingdom.

Text is adapted from Forest Research and contains public sector information licensed under the Open Government Licence v3.0.