Ash dieback: The devastating fungus

Deep in your shed, your favourite hammer might have a handle carved from an ash tree.

Ash (Fraxinus excelsior) is one of Britain’s 32 native species of trees. As our third most common tree, they are a vital part of the ecosystems in our woodlands and hedgerows as well as a durable wood found in all our homes.

However, it's threatened by the ash dieback fungus, or Hymenoscyphus fraxineus; a highly infectious, devastating disease. 

Since 1992 when it was first described in Poland, the disease has quickly travelled across Europe, most likely by wind carrying the fungal spores and also human movement.

In 2012, it was recognised to have arrived in England from an import of nursery trees, but scientists soon found evidence that ash dieback has actually been in the UK since 2002. 

How does the disease work?

The fungus blocks water and nutrients moving in the tree's vascular system, causing a restriction in water movement that leads to a significant leaf loss, bark lesions and then the dieback in the crown of the tree itself (hence the name).

You might first spot dark blotches on wilted leaves that start to shed uncharacteristically in the summer.

That leaf decay (necrosis) is caused by the fungus during its first asexual phase, where it produces the chemical viridol. The infected leaves fall and rot on the forest floor, and a year later tiny mushrooms start to sprout  and the spores spread.  

The branches and trunk might start to show necrotic spots that grow into cankers shaped like a diamond. When peeled, the bark might reveal a grey-ish unhealthy-looking wood. 

As the infection spreads, the lesions creep along the bark, eventually cutting off the supply of nutrients from the roots. They can force the tree to dry and crack.

Young trees  particularly susceptible; older trees may be able to withstand the exposure for longer but become vulnerable to attacks from other pests or diseases.

What does this mean for the UK?

We rely on ash more than we know.

Its durability and malleability make it a great material in building tools, firewood and beautiful furniture. Before a 2012 import ban, huge quantities of ash saplings were brought into the country for industry (and consequentially brought an increased risk of infection).

Ash is part of our biodiverse woodlands and arboreta, providing food and homes for a multitude of species, including mammals, birds and insects. The dappled cover it provides allows enough light to allow ground plants like moss to flourish. 

Ash trees provide important ecosystem services, like flood prevention and water filtration, which would be expensive to replace. Huge expense would also come when felling dangerous trees and replanting. One study in 2019 has estimated the full economic cost could be as high as £15bn in Britain.