Emerald ash borer is a saproxylic beetle native to Asia which feeds on Ash. Photo: U.S. Department of Agriculture via Flickr (CC BY)
The spreading canopy of an ash tree. Photo: Sarah via Flickr (CC BY).
Ash trees are the third most common tree in the UK - but are decline after disease has taken a toll on populations. Photo: Photo Kent via Flickr (CC BY-NC-ND)
Ash dieback disease (Hymenoscyphus fraxineus) in Kocherwald (Bad Friedrichshall, Germany). Photo: Sarang via Wikimedia Commons (Public Domain).
The fungus and the beetle: ash trees face wipeout from disease double whammy
Steve Woodward & Eric Boa
29th April 2016
Britain's ash woods are under threat from a fast-spreading 'dieback' disease, write Steve Woodward and Eric Boa. With 3% of ash trees resistant to the fungus, the species should just be able to survive. But now scientists fear the arrival of the Emerald ash borer beetle, already infesting forests in the US and mainland Europe. Could the two combine to push our ash trees into extinction?
It is now highly likely that the Emerald ash borer will gradually spread across Europe towards the UK, too. The ash trees remaining after the effects of H. fraxineus will certainly provide suitable food sources for the borer in the UK.
Ash dieback is back in the news, even though it never really went away.
The first confirmed reports were from a few woodlands in East Anglia in late 2012. Urgent surveys soon established a much wider distribution in other parts of England as well as Scotland and Wales
Since then, the disease has spread relentlessly and few areas with ash appear unaffected. The worrying feature of ash dieback is that most trees appear susceptible, dying within a few years. Some survive for longer, but the long term prognosis for ash in the UK is not good.
Even as we celebrate the survivors, an insect pest introduced into Europe from Asia is heading our way. Already established in North America, the emerald ash borer has ripped through ash trees in the northern states in the US and threatens to do the same in the UK.
The paper that recently returned ash dieback to the media spotlight did so in a general review of the ecological importance of the common ash (Fraxinus excelsior) in northern Europe.
The author's prediction that ash would be driven to near extinction by a fatal combination of dieback and emerald ash borer was not new, even if media reports suggested otherwise: the same prediction was made in the UK in 2012 and in other European countries from the turn of the century.
Finding disease-resistant trees key to ash tree survival
Ash is a keystone species in the UK, providing a habitat for almost 1,000 other organisms (excluding bacteria). Over 100 species are either completely or highly dependent on ash for survival. So it's not only ash that is threatened by extinction - if indeed this is what is going to happen.
While many reports suggest that the ash tree will eventually disappear from British forests, all is not totally lost.
Researchers working together as NORNEX have identified a 200-year-old tree in Norfolk - named 'Betty' - that could hold the key to fighting dieback. The tree's strong tolerance to the disease has helped scientists identify three genetic markers which can help to predict a tree's susceptibility to the disease.
Tolerant trees still get infected by the disease popularly known as 'Chalara', but more correctly as Hymenoscyphus fraxineus. In our experience, all common ash appear susceptible to infection, but their responses differ.
A rate of 3% of trees showing tolerance doesn't sound high, but ash does seed prolifically so perhaps survivors will eventually fill some of the gaps left by more susceptible individual trees.
Too little, too late?
At the first sites where ash dieback was reported around 20 years ago in Lithuania, many healthy looking ash were left standing, when the badly diseased mature trees were felled. Sadly, almost all of the trees that were left in those first fellings are now either dead or showing severe dieback.
The latest research in the UK strongly suggests that Hymenoscyphus fraxineus was already present some years before its official confirmation in 2012. Given the extent of damage caused by the disease, why was it not discovered earlier? A possible reason is that it takes time for a pathogen population to build up sufficiently to become strikingly obvious.
It is too late to start agonising about a failure to detect ash dieback. A disease spread by the wind to widely dispersed trees is impossible to stop. We need to think more carefully about research priorities and what we want to achieve. A first priority should be to maintain healthy ash trees in the wider environment in the UK and elsewhere in Europe. This means paying equal attention to the pathogen and the host - and what happens between them.
There is good news for the UK, however: the government has already recognised the need for even more and sustained research to avoid such damage to forests. Alongside NORNEX, much research emphasis has been on understanding the fungus that causes the disease and looking for resistance in the host.
The pathogen that causes dieback orginates in Asia, where it invades the tree's leaves, but doesn't cause disease - nothing to get concerned about there. When the fungus came to Europe (we're not sure how), however, it also spread to the stem, killing branches and eventually whole trees.
A new pest could be moving in
The NORNEX report mentions the second threat on the ash tree's horizon: the emerald ash borer, an insect which is present on the continent of Europe, having spreading westwards from Moscow. The bright green bug is native to the same areas of east Asia as H. fraxineus, but spread to North America - probably in the late 1990s/early 2000s - where it is devastating all native species of ash.
Unfortunately, it is now highly likely that the Emerald ash borer will gradually spread across Europe towards the UK, too. The ash trees remaining after the effects of H. fraxineus will certainly provide suitable food sources for the borer in the UK. It is essential, therefore, that the rewarding efforts made to understand ash dieback are continued with emphasis on the emerald ash borer.
Salvation may be possible, but not overnight. There are some promising paths to pursue but we have to consider whether Asian ash species will provide suitable habitats for the same dependent species as native ash - and that's an ecological question that is hard to answer.
Strengthening resistance using Asian ash species holds considerable promise for the future - and not only for ash. The genetic methods employed by NORNEX are relevant to different tree species threatened by other diseases and insects. There is hope ahead to dampen the doomsday scenario that brief bursts of media interest fuel.
It's not good by any account, but there are glints of hope to lighten the path ahead.
Steve Woodward is a Professor and the Personal Chair at the University of Aberdeen.
Eric Boa is a Research fellow at the University of Aberdeen
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