WHAT IS ASH DIEBACK?
Ash dieback disease is caused by the fungal pathogen Hymenoscyphus fraxineus (first identified in 2006 as Chalara fraxinea, ash dieback is still commonly referred to as Chalara). The disease was first notified in the UK in 2012, but it had undoubtedly been here much longer – perhaps 20 years – without anyone recognising it. The twin sources of ash dieback are considered to be a combination of imported, infected nursery trees and spores blown over from Northern Europe.
Windborne spores land on ash leaves, damaging them with the toxic chemical viridiol. Dark blotches on the leaves show where the spores have entered. Viridiol is then transmitted further into the tree, eventually constricting and killing the vascular tissue so the tree can no longer draw nutrients and water up to the crown. By now there will be many areas of foliage that have wilted, turned brown
SIGNS OF ASH DIEBACK
The most obvious signs that a tree is infected with ash dieback will be a progressive thinning of foliage from one year to the next, tips of twigs dying (above) and large areas of foliage turning brown and later black. Also look for diamondshaped lesions around the base of side shoots, but this is not always seen. As the tree dies, the remaining foliage tends to display a ‘pom-pom’ form (right), where the tree’s defences are trying to suppress the incoming disease. and eventually black (below). Where live foliage remains it creates clumps or ‘pom-poms’ of leaves – these are the battlegrounds where the tree’s defences meet the incoming disease. Necrosis of stems, which often turn a purplishbrown (below centre), show how far the disease has advanced; the archetypal signs are diamond lesions around the base of side shoots (below left).
In summer the reproductive phase of the fungus appears as tiny, light-buff apothecia (spore-bearing bodies) on the rachis (leaf stem) of fallen leaves (below right). Millions of microscopic ascospores are then released to the wind to begin the cycle again.
There is no cure for ash dieback, and trees will die over several years, in the latter phases also becoming susceptible to aggressive fungi such as Armillaria, which attack the root collar and destabilise the dying trees.