Chalara dieback of ash is a serious disease of ash trees caused by a fungus called Chalara fraxinea (C. fraxinea), including its sexual stage, Hymenoscyphus pseudoalbidus (H. pseudoalbidus). The disease causes leaf loss and crown dieback in affected trees, and usually leads to tree death.

Threat

It is potentially a very serious threat. It has caused widespread damage to ash populations in continental Europe, including estimated losses of between 60 and 90 per cent of Denmark’s ash trees. We have no reason to believe that the consequences of its entering the natural environment in Britain would be any less serious. Experience on the Continent indicates that it kills young ash trees very quickly, while older trees tend to resist it for some time until prolonged exposure causes them to succumb as well.

Susceptible species

Chalara fraxinea is especially destructive of common ash (Fraxinus excelsior), including its ‘Pendula’ ornamental variety. Narrow-leaved ash (Fraxinus angustifolia) is also susceptible. Chalara dieback of ash is particularly destructive of young ash plants, killing them within one growing season of symptoms becoming visible. Older trees can survive initial attacks, but tend to succumb eventually after several seasons of infection.

Spread

Local spread, up to some tens of miles, may be by wind. Over longer distances the risk of disease spread is most likely to be through the movement of diseased ash plants. Movement of logs or unsawn wood from infected trees might also be a pathway for the disease, although this is considered to be a low risk.

Symptoms

Because ash trees have many genetic variants and occur all across the UK, they come to leaf at different times. In general, they come into leaf later in spring than many other trees, often as late as the end of May. So if an ash tree does not have any leaves on it 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. The leaf symptoms of Chalara dieback of ash are best observed in August and September because in autumn infected leaves can be confused with leaves that are naturally changing colour.

Ash trees are starting to break-bud and will produce new growth some time in May, and we expect the vast majority to develop normally.

Ash is traditionally one of the last tree species to flush, sometimes taking as long as six weeks to do so. Some ash trees will break-bud, or flush, earlier than others, and some buds will produce flowers rather than new shoots. Trees in the colder north flush later than trees in the warmer south. Some variation will be more apparent in older trees.

Some shoots on ash trees will fail to flush altogether, while others will flush normally before showing signs of ill-health or dieback later. These events might mean that the trees are damaged in some way, but shoot death and dieback in ash trees can have a number of causes.

Reporting suspected cases

If you think you have spotted the disease, please visit the Forestry Commission's Tree Alert page.