A new fungus, Chalara fraxinea, has devastated ash populations in northern and eastern Europe, and has now been confirmed in British trees. There are fears that it could be the worst crisis for our trees since Dutch elm disease hit in the Seventies.
The import of ash trees was banned last Monday, but there are fears that this may be too late. Though large woodlands are likely to be worst affected, the disease could also pose problems to gardeners. With the help of Guy Barter, the RHS’s chief horticultural adviser, we compiled this 10-point guide to help you recognise and deal with the problem.
1 Which varieties of ash are most vulnerable?
Common ash (Fraxinus excelsior), is particularly susceptible, and so is ‘Pendula’, the ornamental variety. Also at risk is Fraxinus angustifolia and F. nigra. Mature trees are more resistant than saplings and younger trees, but tend to succumb eventually.
2 What are the symptoms of Chalara fraxinea?
The tree’s leaves develop dark patches, which then spread to twigs and branches. Cankers then emerge on branches. However, all of these symptoms could be caused by other problems, so final diagnosis should be made by an expert.
3 Who should be notified?
Suspected cases should be reported to the Forest Research Tree Health Diagnostic and Advisory Service (01420 23000; [email protected]), the Forestry Commission Plant Health Service (0131 314 6414; [email protected]) or the Fera Plant Health and Seeds Inspectorate (01904 465625; [email protected]).
4 Who disposes of diseased trees and how?
According to the Forestry Commission, a Statutory Plant Health Notice is served on owners, who are required to remove and destroy affected plants by burning or deep burial on site.
5 If my sick tree does not have Chalara fraxinea, what is most likely to be wrong?
The two most likely causes are honey fungus and tree decay fungi. There is another disorder, confusingly known as “ash dieback” but unrelated to Chalara frax inea, that has been widespread in Britain for years. Affected trees typically suffer severe crown dieback, but there may be recovery growth in secondary shoots and “epicormic” shoots (which previously lay dormant underneath the bark). Scientists think the cause is mainly physical, rather than biological. Cankers on ash stems can also be caused by the common fungal pathogen Nectria galligena. The ash bark beetle Leperisinus varius can also cause bark to die off. The activities of the ash bud moth Prays fraxinella in spring can also be mistaken for Chalara fraxinea. Moth larvae mine into the base of shoots,causing them to wilt and die. Frost and drought can cause similar damage.
6 Are there any resistant varieties of ash?
Fraxinus ornus and Fraxinus pennsylvanica are less susceptible. Least affected are F. americana and F. mandshurica, which display leaf wilting but only minor bark necrosis and limited dieback of shoots/ twigs when infected.
7 What should I grow in place of a diseased ash?
True ash is too big and thirsty for most gardens, so replacing it with a smaller tree more suited to a garden is sensible. Sorbus, including natives such as mountain ash and whitebeam, are good choices with similar foliage in some cases. Field maple and hazel are two other alternatives.
8 How can I be sure that I am not spreading the disease inadvertently?
It is possible that moving ash seedlings and timber around the country will become regulated. So far this has not happened, but it seems unwise to do so, and gardeners should be very careful when buying ash seedlings.
9 If I live in an affected area, should I destroy healthy ash trees to be on the safe side?
There’s no need yet for mass destruction. But this may change. Owners of ash trees should keep an eye on developments, as well as on their trees. The RHS website will be kept up to date by Wisley’s plant pathologists.
10 Are there any other horrible diseases on the way to the UK that we should know about?
Unfortunately, yes. There are many diseases (and pests) that could be very damaging to British trees. One example is chestnut blight, which was first identified in Britain last year. It causes cankers and then death in sweet chestnut trees. The disease caused terrible damage in the United States, where it was responsible for the eradication of American chestnut in the first half of the 20th century. In Britain, sweet chestnut is dominant in around 12,000 hectares of woodland.
For more information visit the RHS website (rhs.org.uk) or the Forestry Commission website (forestry.gov.uk). An app has been launched to help gardeners tag cases of the disease. Available on iOS and Android, Ashtag allows users to submit photos and locations of sightings, which will then be passed on to the Forestry Commission, which is working with Defra to stop the spread of the fungus.