Ash Dieback, Hymenoscyphus fraxineus, is a fungal disease spread via airborne spores that is having a lethal effect on Britain’s Ash trees. The disease was first identified in the UK in 2012. It now covers nearly 80% of the UK and is showing little sign of abating. In central Europe, the disease has affected 70% of all Ash trees, most notably European Ash Fraxinus excelsior, which makes up 12% of UK woodland. The total UK Ash population is estimated at 125 million trees with 60 million of these outside woodland in hedgerows, parks and gardens. 80% of all these trees will probably be killed by the disease. Contrary to popular belief and very sadly Ash Dieback can affect trees of all ages with mature trees taking slightly longer to succumb.
Mid-summer is the ideal time to identify infected trees. Ash usually put on a mass of growth with healthy long stems in late spring and early summer. In an infected tree this new growth will start to wilt and turn black or not grow at all leaving dead sticks pointing up from the canopy. The trees will try to mitigate the effect of the disease by throwing up lots of vigorous fresh stems (epicormic growth) from normally dormant buds on the inner branches and main stem.
As the disease progresses the tree declines further with the remaining clumps of leaves exhibiting a classic “pom-pom” effect as shown below. The tree will not recover from this point but may take several more years to die. In young trees, wilted foliage and diamond shaped lesions on the bark are the classic signs of the disease.
As mature trees decline Ash dieback causes trees to become brittle. Limbs and branches, particularly those with existing weaknesses such as cavities or areas of decay, are very likely to fail. The disease also weakens the tree making it easier for other pathogens such as Honey Fungus to take hold.