Ash Tree Dieback

As the disease is moving quickly throughout the local area, we thought we’d give you some info on what is happening.

What is it?

Ash dieback is a disease of Ash trees (Fraxinus Excelsior) caused by the fungus Chalara Fraxinea. The disease causes leaf loss and crown dieback in affected trees and leads to the hosting trees death.

Ash is a very important tree in the UK both ecologically and culturally so this disease is causing great concern about the damage it will do.

The proportion of trees with a high level of natural resistance seem to be very low, probably less than 5%.

Trees show a 10 – 15% decline in the canopy in a single year and anecdotal reports from areas of the UK currently infected by ash dieback support this as a typical rate of decline. However, some individual trees (depending on their health and condition) could decline much more rapidly.

How does it affect the tree?

The disease is spread by spores from the fruiting bodies of the fungus produced on fallen ash leaves. These airborne spores can disperse naturally via wind over tens of kilometres. It can also occur at the base of trunks (the root collar) and through root infection.

Evidence suggests young trees are killed quickly while many mature ash trees can resist infection for some time until eventually dying or becoming weakened and succumbing to attack from another pest or pathogen.

Root-attacking fungi such as honey fungus can hasten the death of woodland trees affected by ash dieback causing the tree to become unstable, shed branches and eventually succumb to the disease.

Symptoms

Dark lesions – often long, thin and diamond-shaped – appear on the trunk at the base of dead side shoots

– The tips of shoots become black and shrivelled

– Blackened, dead leaves – may look a bit like frost damage

– The veins and stalks of leaves, normally pale in colour, turn brown

– Saplings have dead tops and side shoots

– In mature trees, dieback of twigs and branches in the crown, often with bushy growth further down the branches where new shoots have been produced

– In late summer and early autumn (July to October), small white fruiting bodies can be found on blackened leaf stalks

Action

The aim of management, should be to slow the spread, minimise the impact of the disease, and preserve as many Chalara – tolerant ash trees as possible.

We should avoid felling Ash trees that show no sign of the infection as there is a small percentage of trees showing resilience and surviving. These trees might be the key to combating the disease so we should try to retain them.

Trees in areas with high levels of public access therefore need to be monitored carefully for risks to public safety, and some felling or pruning of dead or dying trees is advisable if risk assessments show they are a hazard. When assessing trees’ health, look for signs of lesions (cankers) or honey fungus (Armillaria) near the base of the trunks: these can weaken the trunks and make the trees more prone to falling.