Ash trees are under threat from the Ash Dieback fungus Hymenoscyphus fraxineus, which produces premature leaf-drop and lesions on the stem and leaves. It may also allow other infections such as Honey Fungus Armillaria mellea to take hold and weaken the tree further.
There is no known cure for Ash Dieback although a few trees do show a limited resistance. Infected trees die, and younger saplings generally die quicker than mature Ashes.
Up to 90% of Ashes in the UK may become infected and weakened enough to require removal. This means that landowners and local authorities may decide to cut down a diseased tree while it is still safe for a tree surgeon to do so. Branches from Ash trees can split off fairly easily anyway.