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Invaders from the Far East: Ash dieback

Ash dieback disease has destroyed European ash tree populations, but Genomics research at Earlham Institute suggests the threat could be even worse.

April 23, 2018

Ash dieback disease has caused rampant destruction across Europe over the last few decades, finally reaching the shores of the UK in 2012. Now, research at Earlham Institute has unearthed an even more daunting threat from beyond Europe. Could a single spore travelling from the far east of Russia be enough to spell doom for Europe’s remaining ash trees?

It was September 2012 when, in a Norfolk woodland, Anne Edwards (who received a British Empire Medal) discovered the first signs that ash dieback - an unrelenting destroyer of ash trees on the European continent - had reached British shores.

A whole twenty years previously, in 1992, a time when Manchester was still mad for it, the Fresh Prince was still shooting b-ball outside of the school, Blackburn Rovers helped kick off the English Premier League for the first time and Nirvana still had two years left to smell like teen spirit - the ash dieback pathogen landed in Europe for the first time, in Poland.

Fast forward ten years or so and this disease had spread through Lithuania to Latvia, south to Bulgaria and through East Germany and Norway to Denmark. Come 2010, there was hardly a European region unblemished by ash dieback - which had taken hold from Finland down to Slovenia and Italy, and up through to Belgium and France.

It was a disaster in waiting for British ash trees.

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Norses for courses.

So, back to the present, and Blackburn Rovers are now just about surviving in League 1, Alan Shearer is a pundit on Match of the Day, the Fresh Prince of Bel Air is 48 and the Haçienda is now a bunch of apartments in central Manchester …

… In this time, following in the footsteps of the Vikings 1200 years ago, ash dieback disease has journeyed across the North Sea and managed to quickly colonise Britain.

The introduction of the ash dieback pathogen, a fungus by the name of Hymenoscyphus fraxineus (which was previously known as Chalara), triggered a fast early response network to be born, known as the Nornex consortium. The Norns are guardians of the sacred tree Yggdrasil in Norse mythology, and thus the Norwich and Exeter-based collaboration hoped to provide a scientific stance against the advance of ash dieback.

The consortium aimed to quickly understand both the ash dieback pathogen itself, as well as a possibly sacred saviour from Denmark. With a suite of the most modern genome sequencing equipment, as part of Nornex Earlham Institute was in a perfect position to help respond as rapidly as possible.

We can’t possibly speculate as to whether “Tree 35” - a Danish ash tree that showed signs of resistance to ash dieback - is from the same legendary lineage as Yggdrasil, but among a group of institutes throughout the UK and Denmark we did hope to glean from within this plant the secrets that might help stem the rampant tide of ash dieback.

Several years on, we know a lot more about the ash dieback pathogen and of “Tree 35”, with well annotated genome sequences telling us a lot about how we might identify native ash trees that show some resistance to the disease, or indeed accelerate the process of breeding some much needed resistance into new trees to help transplant some further defence against the onslaught.

That numerous trees show resistance to the pathogen is a positive sign for many, indeed, some suggest that within native British populations there may be greater resistance than in Denmark.

However, new understanding of how the disease might spread, as well as where it came from in the first instance, is of cause for much greater concern, and might spell doom for even those trees that have, thus far, managed to hold out.

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At the heart of this threat is sex, and the possibility of gene flow from Eastern populations into the already-rampant European varieties.

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An even more severe threat, borne on the wind.

Ash dieback, now prevalent in Europe, is thought to have arrived from far away in East Asia, perhaps Eastern Russia or Japan, where the fungus has been thriving alongside native ash trees there for millennia.

Indeed, the native European fungus Hymenoscyphus albidus, which looks almost exactly the same as the disease-causing H. fraxineus and is being displaced by the ash-dieback pathogen, is completely harmless to European ash trees. In Europe, H. albidus lives like a normal decomposer fungus (a saprophyte) should, simply thriving on breaking down dead ash leaves.

As with many examples of invasive species, in its native environment the ash dieback fungus doesn’t pose as much of a threat to Asian ash trees, which have presumably equipped themselves with the appropriate evolutionary arms to deal with it.

It is not dissimilar to the example of Varroa mites, which have been implicated in the “colony collapse” syndrome of European honey bees, which have no natural defense to the destructive parasites. Japanese honey bees, which have evolved alongside this nuisance, have developed behavioural strategies to cleanse their colonies and negate the threat.

Similarly, ash trees in East Asia exhibit certain characteristics that perhaps keeps them one step ahead of the fungus. Certain species shed their leaves earlier than usual, for example, before a fungal infection can really take hold.

Potentially due to the import of a Manchurian ash tree into a European nursery, or increasingly more likely a wayward spore floating high above the silk road, the Asian fungus has taken hold in Europe, causing the rampant spread that we have witnessed over the last quarter of a century.

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Lead author Dr. Mark McMullan of Earlham Institute has fronted a comprehensive genome sequencing effort to understand the H. fraxineus pathogen genome, tied to in-depth population genomics.The project points to a genetic bottleneck representing the equivalent of just two individual spores which established themselves in Europe from East Asia.

Pathogens are often successful when a large population confers upon them lots of genetic diversity, and therefore adaptability, paving the way for a successful strain to overcome plants that are defenceless against them.

With a greater armory, so to speak, there is a greater chance that the pathogen can overcome resistance in a host, or find a back door.

Alarmingly, the two individuals of the ash dieback fungus, with all of their limited genetic diversity, have managed to rampage through Europe’s ash trees so efficiently that only around 5% of native trees are thought to have any resistance at all.

Analysis of 43 isolates of European H. fraxinus compared to 15 from a single wood in Japan, which covers just part of its native range, is what really sets the alarm bells ringing.

Strains of ash dieback covering the whole of Europe had eight times lower genetic diversity than was seen in 15 individuals from a single wood in Japan. If the diversity between different Asian varieties of the fungus is as staggeringly different as the diversity seen between European and Japanese isolates, then we might have even more to worry about.

At the heart of this threat is sex, and the possibility of gene flow from Eastern populations into the already-rampant European varieties.

In simple terms, the ash trees of Europe have barely been able to cope with H. fraxineus as it is (over 80% of young Norwegian ash trees died between 2009 and 2016), leaving the majority of native ash trees under threat of being wiped out. Most of the genetic weaponry that might infect them is yet still to travel to Europe from East Asia, which it could do at any time. If just two spores can inflict this much damage, imagine what another two successful invasions could do.

Dr Matthew Clark, formerly Head of Technology Development at EI and now a Research Leader at the Natural History Museum, added: “The risk is that if the pathogenic fungus gets the chance to mate and reproduce with just one more strain, the resulting progeny could gain the ability to kill these remaining ash trees.”

The stark reality is, as Dr McMullan points out, “the introduction of one more spore from Asia could introduce 90% more diversity to Europe, which is probably bad news for the ash trees that currently show reduced susceptibility to the disease.”

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It is hard to really put an estimate on how many trees will be lost, as well as what trees, in the end, are able to resist this rampantly destructive pathogen.

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Credit: Visualpower/Shutterstock.com

Ash tree fact

Ash Dieback has destroyed more than 80% of young ash trees in Norway

Ash dieback percentage

of European ash trees are under threat from Ash Dieback

Ash tree facts

There are 90 million ash trees in the UK

Ash tree fact

More than 1,000 species rely on the ash tree

The dieback bores through.

More devastating still, another invasive species, the Emerald ash borer, has nestled its way into European Russia, a scourge which can only add to the demise of the European ash trees, and has proved deadly to tens of millions during its steadfast jaunt through North America.

The Emerald ash borer might be enough, on its own, to threaten the existence of the billions of native ash trees of North America. In America, as in Europe, ash trees represent a major native species, on which many other species rely.

One big fear is that without the fundamental species, such as ash, which make up a large constituent of native ecosystems, the opportunity for invasive plants to become established and spread is further increased, which threatens yet more native biodiversity.

It is difficult to talk about mortality when it comes to ash dieback because it simply takes so long for an ash tree to die. It’s a painstaking downward spiral for the tree, which succumbs slowly over years - first showing signs of disease on the leaves, spreading eventually to the stems, followed by severe crown dieback - until any semblance of resistance is finally thwarted, often due to secondary infection from another species of fungus, in the end.

It is hard to really put an estimate on how many trees will be lost, as well as what trees, in the end, are able to resist this rampantly destructive pathogen - although the signs from the European continent don’t point to a happy ending.

However, what is very clear is that the bountiful diversity present within Asian strains of the ash dieback pathogen presents an alarming threat to the existence of Europe’s remaining ash trees, and their fate may lie in nothing more than the fateful gust of an Easterly breeze.

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Article author

Peter Bickerton

Scientific Communications & Outreach Manager

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