Chalara dieback of ash, commonly known as ash dieback, caused by the Hymenoscyphus fraxineus fungus, has been confirmed at Westonbirt, The National Arboretum, this autumn. The disease was first spotted in Gloucestershire earlier this year, and so it was no surprise to find it in the arboretum.
Mark Ballard, Curator at Westonbirt, tells us more…
The risk of pests and disease is the same with any collection of trees, big or small, and after Chalara has been found to be present in woodland in and around Gloucestershire this year, we were awaiting the inevitable.
Our team first spotted ash trees with possible symptoms during our routine health and safety tree inspections, which was then confirmed to be Chalara by laboratory sample analysis very recently.
The symptoms themselves only became apparent this year and we believe the infection to be very recent. The affected ash trees at present are all in Silk Wood; they are mostly native common ash (Fraxinus excelsior), and are situated within areas of deciduous woodland.
However, through this, Westonbirt is playing a key role in supporting research into Chalara-tolerant or resistant ash trees through surveys and laboratory samples, which you can read more about below.
What are our plans for the future? We don’t expect this will have a major impact on the Westonbirt landscape. The ash trees are within deciduous woodland, and the change should be very gradual. We expect there to be lots of natural regeneration, but we will continue to assess the affected areas as time goes on.
1. How did the disease get here?
It most probably arrived in the form of spores of the fungus blown by the wind from other infection sites.
2. Who found it?
Arboretum staff first spotted ash trees with symptoms during routine inspections, and the disease was confirmed by laboratory analysis of samples.
3. How long has it been here?
Despite continual surveying at Westonbirt, symptoms only became apparent this year and it is believed that the infection is very recent.
4. Has it been found at Westonbirt before?
No, this is the first time it has been observed anywhere in the arboretum.
5. Where are the affected ash trees?
At the moment the affected ash trees have all been found in areas of Silk Wood.
6. How much ash woodland is there at Westonbirt?
Out of the 240 hectares that Westonbirt covers, there are about 50 hectares (123 acres) of mixed deciduous woodland in which ash trees are prominent.
7. Are the affected trees young or old, planted or regenerated?
The affected trees are native common ash (Fraxinus excelsior) in areas of mixed deciduous woodland within the arboretum, and most are younger, naturally regenerated (self-seeded) trees. However, one Manna ash cultivar (Fraxinus ornus ‘Savar’) has also tested positive for Chalara. We will continue to closely monitor other specimen ash trees, and send further samples for analysis as and when appropriate.
8. What are you doing about it?
Westonbirt is playing a key role in research into Chalara-tolerant or resistant ash trees. We expected that the disease would affect the arboretum eventually, so we surveyed and documented the health of all our ash trees in 2013, before Chalara affected them.
We have 27 species of ash tree, and a total of 45 taxa (species, sub-species, varieties and cultivars). We are in a strong position to monitor how each one reacts to the disease, which in turn will help us to formulate advice for woodland owners.
In addition, a significant amount of scion material (detached shoots or twigs containing buds) from our ash trees has been grafted on to rootstocks, and these will be planted out in eastern England, where the disease is most prevalent. This will aid the on-going research into tolerant or resistant ash trees. The Royal Botanic Gardens at Kew, Edinburgh and Wakehurst have also provided scion material for this project.
9. Is the disease harmful to humans or animals?
No, there is no risk to people or animals.
10. Will you be restricting public access to stop the spread of the disease? What should the public do?
No, we will not be requiring restrictions on access to the arboretum in general, or to woodland areas, but we do encourage visitors to all woodland to help limit the spread of disease by taking some simple biosecurity measures. These include not taking plant material away from affected sites, brushing soil and plant material off their footwear, clothes, wheels and dogs etc before they leave areas with infected trees, and washing these items at home before visiting another park, garden or woodland.
11. What biosecurity precautions are taken at Westonbirt to prevent the spread of Chalara or other plant diseases?
We have regular plant health training days, and the health of our trees is checked regularly. Our working vehicles, machinery and equipment are not taken off site, and contractors who come on to our site must comply with regulations for clean vehicles and equipment. We also have a quarantine facility for screening potentially infected material arriving here.
12. Is this likely to impact on the new Treetop Walkway?
No, the new Treetop Walkway is situated in an area of the arboretum which is away from ash woodland, and is next to only a few single ash trees. Any ash trees in the vicinity will continue to be closely monitored and managed, as with all nearby trees.
13. What does the future hold? Will Westonbirt lose all its ash trees?
It remains to be seen how the disease will progress. We know that older, larger and well established trees can survive exposure to the disease for some time, especially those standing in open, drier locations. We expect that the different species and sub-species represented at Westonbirt will vary in their reactions to the disease. So the overall impact should be gradual, and differ from area to area over time.
It is our intention to allow other species of deciduous trees to naturally form a mixed stand without replanting, in line with our tree management objectives. However, if a closed stand does not form, we will consider regenerating the woodland by planting alternative species in line with relevant guidance and best practice.
14. Are there any treatments or cures available for Chalara ash dieback?
There is no cure, but some chemical treatments are being investigated for their potential to protect high-value individual trees or groups of trees. However, any treatment would have to be perpetually repeated, because the fungus will remain in the area and continue to re-infect trees.
Our Forest Research agency and other research institutions are working to identify the genetic factors which enable some ash trees to survive exposure to the disease, so that we can breed Chalara-tolerant or resistant ash trees for the future.
15. Aren’t dead and dying ash trees unsafe for people?
We regularly check on the safety of our trees, especially those which border paths, buildings and other areas which people use, and prune or remove any which are becoming unsafe.
16. Why don’t you just fell/remove all the dead and dying trees?
Provided they are not becoming unsafe, we want to keep the ash trees for as long as possible to study their reaction to the disease. This is because our collection of ash species and varieties from around the world is a valuable resource for research into the disease.
In addition, all trees are important for wildlife, for example as sources of food for insects and lookout perches for birds of prey, and they continue to play an important role in a diverse landscape.
Mark Ballard, Curator