Recorded cases of Ash Dieback (2019)
What is Ash Dieback?
Chalara Dieback of Ash (Hymenoscyphus Fraxinea), also known as Ash Dieback is a disease that has spread to the UK from Asia, effecting many Common and Raywood Ash trees.
In Europe, it has been seen to be fatal to its host, with countries such as Poland and Denmark estimating a loss of up to 95% of Ash trees.
The disease was first identified in the UK in 2012 in a nursery in South East England on a tree imported from the Netherlands.
It was soon identified in woodland sites across Norfolk and Suffolk. It now continues to spread across the UK, appearing now in most English counties.
Ash Dieback Biology
Hymenoscyphus fraxineus has been isolated from the roots of symptomatic trees, as well as from leaves, shoots and branch/stem lesions.
The infectious spores (sexual) of the fungus are produced by fruiting bodies (apothecia) and can be wind-blown over long distances (20-30 km).
The apothecia are produced from June to October on ash leaf petioles and rachises (stalks) from the previous year in the leaf litter.
The spores land on leaves or other parts of the trees. From the leaves, the fungus makes its way down the petioles, rachises and stems.
The fungus can also produce asexual spores, but these are not believed to be infectious and can only spread over short distances by water splash.
When it is producing asexual spores, the fungus is known as Chalara Fraxinea, and the disease is therefore sometimes called Chalara dieback or just Chalara.
Blotches and wilting caused by Ash Dieback
How Does Ash Dieback Spread?
The fungus has several pathways of spread over long distances;
It can be spread through the movement of diseased ash plants and logs or unsawn wood from infected trees.
Until the ban was applied on all movement of ash trees and seeds in October 2012, high volumes of ash (F. excelsior) were imported every year either for forestry or non-forestry purposes. Therefore, the potential for entry of the pathogen to the UK was very high.
However, the theory that spores wind-blown from the continent are a common source of entry is now widely accepted, as cases recorded in the wider environment were initially located in the eastern parts of the country.
The density of wider environment infections is still greatest in the east but there have now also been cases recorded in many other areas.
The fungus is also seed-borne.
Typical lesions caused by Ash Dieback
How Can You Identify Ash Dieback?
Ash Dieback is most easily identifiable when the tree is in leaf. It causes black blotches and the wilting to appear on the leaf, although this symptom alone may not indicate Chalara Dieback of Ash.
Small lens-shaped lesions or necrotic spots appear on the bark of stems and branches and enlarge to form perennial cankers.
The infection may girdle the stem and kill it in a single season. If the bark is peeled, the wood underneath has a brownish to grey discolouration. This discolouration extends beyond the bark necrosis.
The affected trees show extensive dieback of shoots, twigs and branches. Trees often have prolific epicormic shoots (shoots produced from previously dormant buds below the bark of the trunk or branches).
Dieback in crown with large amounts of Epicormic growth
How to Control Ash Dieback
There is currently no chemical control available for Ash Dieback.
One non-chemical control recommended is to remove and destroy any leaf litter below the tree.
The fruiting body and sexual stage of the disease occurs on the fallen leaf, so by removing them, spore numbers can be significantly reduced.
It is safe to incorporate leaves collected into leafmould or compost bins.
What is the Future for Ash in the UK?
Studies show that Ash Dieback is often fatal to Ash trees.
Younger trees have been seen to decline rapidly, with older trees being able to tolerate the disease for longer, often succumbing to secondary infections such as attack from Honey Fungus Armillaria Mellea.
With information from public reporting tools, such as Forest Research’s Tree Alert, it has become clear that Ash Dieback is now spreading quicker than expected.
The 2012 ban on imported Ash by Defra may have had some impact, but due to lack of control, Ash Dieback is now widespread.
Estimates of loss in the UK range widely from 70% to 98%. This would cause a huge gap in our tree canopy, a gap that is likely to be filled with pioneer species such a Sycamore.
This would have devastation consequences for many species (as many as 1000) that use Ash for habitat such as wood mice, liverworts, wrens, blue tits, bats, lichens, fungi and beetles.
Bullfinches will eat ash keys in winter when food is scarce. The caterpillars of many kinds of moth feed on ash leaves. Many lichens are hosted by ash.
Lichens are not a parasite and they do not feed off the tree but use it as a support.
King Alfred’s Cakes (Daldinia concentrica) are specific to ash, and a few beetles that live only on this fungus are dependent solely on ash for survival.
Ash also plays an extremely important role in the timber industry. Ash produces some of our best timber and is also an important source of firewood fuel, burning reasonably well when green.
The UK is home to vast amounts of Ash with high cultural and historical significance. Ash has historically played an important role in our folklore and is therefore one of the most identifiable trees on our island.
There are likely to be a small genetic group that is tolerant of Ash Dieback. Currently, focus is being put on identifying these trees, preserving them and studying them with the intention to clone their genetics and redistribute them into the wider landscape, repopulating area where significant loss has taken place.
Large landowner tree policy should not be written to this affect, allowing a tree time to recover rather than instant removal. This is suggested for trees with >50% dieback in the crown in locations of low targets, where it is safe to do so.
This process could take as long as 10 years but may be crucial to the survival of the species in the UK. GraftinGardeners policy on Ash trees is in line with this view.
What GraftinGardeners Do
When giving advice to clients or writing woodland management plans, our experts always advise retention of infected Ash trees with crown dieback >50% when safe and practical to do so.
We no longer plant or import Ash species. An infected tree will be appraised on its location first with a risk-based rating of its surroundings defined by the location or frequency of targets below that could be harmed or damaged in the event of failure.
These ratings would define work recommendations. Leaf clearance works can be completed for trees where required, especially those of a high amenity value for historical, cultural or aesthetic reasoning.
All major infections recorded will be added to the Tree Alert database to help support Forest Research in clarifying the national situation.
In extreme cases, we can provide soil decompaction and soil improvement injections (biochar), to help improve the trees weakened energy resources and resist infection.
Please contact us for more details.
Senior Arboricultural Consultant,
Article was written by Rory Hobbs.
Article Source: http://www.graftingardeners.co.uk/responding-to-ash-dieback/