It has been estimated that ash dieback will kill approximately 80% of the UK’s ash trees.
Ash dieback isn’t the name of a disease, it is the name of a fungus, Hymenoscyphus fraxineus. It originated in Asia, where it doesn’t cause much damage to the Manchurian ash and the Chinese ash, which are its main hosts in its native range.
The fungus seems to have arrived in Europe about thirty years ago, possibly spreading under its own steam, possibly introduced with imported timber; we don’t really know. It is believed to have been introduced into the UK with sapling trees, at a time when the rules governing the movement of trees around Europe were more lax than they are now.
The European ash (Fraxinus excelsior), our native ash species, has no natural defence against Hymenoscyphus fraxineus and the effect of this alien incomer has been devastating.
The fungus overwinters in leaf litter, particularly on leaf stalks beneath an ash tree. It produces small white fruiting bodies between July and October and the microscopically tiny spores can travel many miles in the wind. If they land on the leaves of an ash tree in the right conditions, they germinate, producing a filament that penetrates the leaf and grows into the tree through the leaf stalk. As the fungus blocks the cells that transport water through the stalk to the leaves, they die and blacken. The fungus colonises more and more of the tree until it blocks the long xylem cells that form the tree’s water transport system. The tree can fight back, but re- infections, year after year, will weaken and eventually kill it.
Our landscape and our skyline will be changed in much the same way that Dutch elm disease changed them. But the biodiversity of our woodlands will be changed too, by the loss of habitat, at a time when we can ill-afford to lose anything.