Is this mainstay of the British landscape doomed? (Image: Tim Graham/Getty)

A fungus deadly to ash trees has just reached Britain and Ireland, after emerging 20 years ago in Poland. Already it has devastated ash trees in mainland Europe, sweeping through more than 20 countries powerless to prevent its spread.

How did this fungus develop? And what, if anything, can be done to stop it in countries like the UK, where ashes account for around a fifth of all trees? By the sound of it, the outlook is not good. New Scientist investigates.

Just how many ash trees have been killed?

Although there are no official figures, ash trees have effectively been wiped out in Poland, where the disease first made its appearance in 1992. In Lithuania, 99 per cent of the ashes are gone; in Denmark, 90 per cent. Elsewhere, the impact has been mixed, with some but not all ashes succumbing.


And now it’s reached the UK?

Yes, hence the current panic. Since February, the disease has been spotted in several English nurseries. The outbreaks were traced to trees and seeds imported from countries that are already affected, so the response has remained low-key, although 100,000 nursery trees and saplings have been destroyed.

The alarm bells really started ringing last month when the disease was spotted in wild ash trees in East Anglia, one of the regions of England that is closest to mainland Europe. The likelihood is that the fungus must have spread here naturally.

How did it do that?

It probably blew in from the European mainland; the fungal spores can travel great distances by wind. Alternatively, it may have been brought here by contaminated birds, or even vehicles and people.

How does the fungus kill ash trees?

Fungal spores land on leaves, germinate and begin invading tissue. It starts with the leaf, then moves into the leaf stalks. Ultimately the fungus spreads into the tree’s trunk. As it spreads, the fungus chokes off all water channels in the tree, so in its wake tissues wither and die. Eventually, the tree succumbs.

Can it be stopped?

Apparently not. Most countries where it has taken hold simply gave up. Part of the problem is that the fungus does not spread from infected trees themselves, but from infected leaves shed in the autumn. The fungus grows on the leaves and leaf stalks as they decay, then produces copious spores in summer which spread to uninfected ash trees, completing the life cycle.

“There’s very little you can do,” says Ottmar Holdenrieder of the Swiss Federal Institute of Technology in Zurich, who in 2010 helped uncover the fungus’s complete life cycle. “It’s a waste of time to chop down trees.” The infective material is all on the forest floor and cannot be removed or eradicated with fungicides without destroying countless other forms of forest life.

Which fungus causes the disease?

This is a long story. It begins in 2006, when Tadeusz Kowalski of the University of Agriculture in Krakow, Poland, identified a newly discovered fungus, Chalara fraxinea, as the cause of the disease (Forest Pathology, DOI: 10.1111/j.1439-0329.2006.00453.x).

This did not solve the puzzle. Fungal species often exist in two forms: one that reproduces itself asexually, and one that multiplies sexually by producing spores. It turned out that Chalara fraxinea is asexual, so the real killer remained at large.

By 2009, Kowalski had found what he thought was the sexual form of C. fraxinea: Hymenoscyphus albidus, which produces spores from tiny toadstool-like growths on ash leaf litter (Forest Pathology, DOI: 10.1111/j.1439-0239.2008.00589.x). But this was something of a red herring. H. albidus has been growing on the decaying leaves of Europe’s ash trees for centuries, so was unlikely to be the culprit. It has been known in the UK, for example, since the mid-19th century.

The real killer was unmasked a year later, and produced toadstools identical to those of harmless old H. albidus. Using painstaking genetic analysis, Holdenrieder, Kowalski and others found that this doppelganger was actually a different, and lethal, species. They named it Hymenoscyphus pseudoalbidus (Forest Pathology, DOI: 10.111/j.1439-0329.2010.00645.x).

How did this killer emerge, apparently out of the blue?

Holdenrieder and his colleagues are still investigating that, and should reveal their results next year. For now, their hunch is that it came from Asia, either via the wind or accidentally brought in on imported ash trees. There is circumstantial evidence: ash trees in Asia are immune to the disease.

The ideas that H. pseudoalbidus evolved from its close European relative, or that climate change made European ashes more vulnerable to a pathogen that was already there, have both been ruled out. In short, the fungus is new to Europe, and the serious money is on it arriving from the east.

Does our identification of the culprit help in combating the disease?

Sort of. The most important thing is that it may now be possible to breed or develop ash trees that are immune to the fungus. These have already emerged in Lithuania, says Holdenrieder, where 99 per cent of the original ash population died out. He says that the offspring of survivors are proving resistant.

Alternatively, it might be possible to develop a vaccine, as was developed to protect elm trees against Dutch elm disease, but this is a distant goal.

Meanwhile, now that the fungus is in Europe, it is locked in an “arms race” with the ash trees. The fungus has the whip hand because it breeds far faster.

Can we do anything to stop the fungus evolving?

Newly published work by Holdenrieder’s colleague Andrin Gross suggests that the key is to avoid importing any further variants of the fungus, as these help the fungus to continue evolving and overcoming resistance in the trees (Fungal Genetics and Biology, DOI: 10.1016/j.fgb.2012.08.008). “Ash trees will never be able to adapt if we constantly introduce new variants of the fungus,” says Gross.

What’s the best hope for UK trees?

Earlier this week, the UK government banned any further imports of ash trees. It also initiated a huge survey of forests in East Anglia to establish how far the disease may have spread. “Once we have a handle on how big or small this issue is, we can decide whether to go for eradication or containment,” says a spokesman for the UK Forestry Commission.

The best hope is that any fungus is present only in small pockets that can be cleared to prevent further spread. The worst case is that it is everywhere, in which case it’s probably goodbye to the English ash.

“If that’s the case, there’s nothing they can do about it,” says Jim Briercliffe, business development manager of the Horticultural Trades Association (HTA) in Reading, UK, which represents suppliers of plants, seeds and gardening equipment. The only option will be to replace dead ash with other kinds of tree.

The HTA warned the government in 2009 that the disease was rife in Danish nurseries and could easily reach the UK. “It is annoying that our warning was ignored,” he says.

How is mainland Europe coping?

“My gut feeling is that the whole of Europe will have to live with the disease in the long term,” says Holdenrieder. “Ash tree populations will be reduced to less than 10 per cent what they were originally.”

Meanwhile, researchers are pooling their resources to see what can be done. A pan-European group of Chalara specialists called FRAXBACK is meeting for the first time in November in Uppsala, Sweden.