Conkers under threat… The trees could be wiped out in 15 years… FOREIGN invaders could see the end of British conker trees which are facing extinction… Conkers under threat as vicious European bug spreads across the nation…Diseased horse chestnut trees must be cut down to save others…

These are just a few of the alarm-raising headlines that have appeared in the popular press over the last few years. Yes, there are arboreal problems. While many of the horse chestnut trees are being weakened by various pests/pathogens – leaf mining moth, Guignardia leaf blotch, wood rotting fungi and horse chestnut scale insect  – only the rapidly-spreading bleeding canker, a bacterial disease caused by the Gram negative Pseudomonas syringae pv  aesculi, can kill outright when it infects trees.

In 2007 thousands of horse chestnut trees surveyed across Britain exhibited symptoms of bleeding canker: bleeding, gummy liquid oozing from cracks in the bark located on the main stem and branches, dead vascular tissue under the outer bark, dieback of young shoots and larger branches, leaf discolouration, and premature leaf drop sometimes leading to tree death.

Now considered to be widespread across Britain, bleeding canker is having a major impact on both urban and rural landscapes. Unfortunately, at present little can be done to prevent a tree from becoming infected and there is no known cure for the disease which has already infected over 50 per cent of the horse chestnuts in Britain. Does this mean we should be panicking, felling all affected trees and bracing ourselves for life in a landscape with no horse chestnuts?

Instead of taking all the piercing, high-pitched headlines seriously, it might now be time to carefully listen to some of the experts, the scientists and caretakers who are on the frontline of bleeding canker research and horse chestnut preservation and hear what they think.

Listen to the experts

Stephen Woodward, a Professor at the Institute of Biological and Environmental Sciences at the University of Aberdeen and Editor-in-Chief of Forest Pathology, says, “I get rather worried by stupid headlines like these. This is not a battle between foreign and native, between good and evil. The horse chestnut is just as much a foreigner as the bacteria. And neither one of them is good or evil.”

As Woodward points out, contrary to widespread opinion, the deciduous broadleaved horse chestnut – now a familiar feature in many British settings – is not even a native of the British landscape. It is, in fact, an alien. Indigenous to Greece and the central Balkan Peninsula, it is believed to have been introduced into Western Europe from Turkey around 1576.  Eventually reaching England in the late 16th or 17th century (and there is some debate about the date), it was planted almost exclusively for its ornamental value and rapidly became naturalized.

While classified as threatened or likely to be extinct across most of its native range, outside its native range it seems to be doing better – quite a bit better. The National Woodland Inventory of Woodland Trees estimates there are 470,000 horse chestnut trees in Great Britain with most of them found in non-woodland sites.

Fortunately, these numbers do not scream ‘immediate annihilation’ or ‘instant destruction’.  In reality, many, if not most, of the horse chestnuts in Britain are probably not even accounted for in any national tree census, since they are commonly planted or self-sown and flourishing along streets and grand avenues, in parks, public and private gardens, golf courses and hedges. 

Removing affected trees could be counter-productive

For the many horse chestnuts still standing, it is now necessary to find a cure for bleeding canker. Woodward’s primary interests lie in how host trees respond to infection, with a view to finding provenances and individual genotypes that may be less susceptible to pathogens and could be used in the repopulation of some of our forests and woodlands. He observes that “since some trees will almost certainly survive and possibly help us figure out how to solve the problem, removing affected trees can be unnecessary and counter-productive in some cases.

“Each case should be considered in isolation: How is the tree responding to the infection? Is the tree vigorous? Is it a threat? Could this tree be used for further research? What will it hit if it fell? If we want to solve this problem, we have to look at this from a scientific standpoint and not from a whipped-up-inaccurate-scare-mongering-scenario where trees could be felled willy-nilly.”

The general advice according to Joan Webber, the Principal Pathologist and Head of the Tree Health Research Group at Forest Research, is that “unless there is a pressing reason to remove trees affected with bleeding canker, they should be left in place and monitored and any severely diseased branches removed as necessary, particularly where they represent a health or safety risk.”

This is especially important since Webber and her colleagues have found that “it is not an immediate death sentence for a tree if it gets infected. There is no reason to panic just yet. While the disease is definitely spreading, some trees appear to recover from episodes of disease, and some never develop any symptoms at all despite being exposed to the pathogen.”

Sarah Green, a forest pathologist, who has studied the bacterium for seven years, suggests “removing an infected tree (in Britain) will not stop the disease from spreading as it is already fairly ubiquitous.” Furthermore, since Pseudomonas syringae pv. aesculi is able to survive independently for extended periods in soil and water, and can tolerate lengthy periods of freezing at very low temperatures, eliminating infected trees will not eliminate the disease vector.

There may be a natural resistence to the pathogen

Green and her colleagues believe that while many trees died when the bacterium first spread across the country, trees do appear to survive following infection, with varying levels of symptoms and, although this has never been fully studied, there may be a natural resistance to the pathogen already present in the horse chestnut population in Britain.

Essentially, like Webber and Woodward, Green’s advice is “to leave any living, infected tree that poses no public risk standing as there is a good chance the tree may survive for years displaying a certain level of symptoms but still with a generally full crown.”

Alan Cathersides, National Landscape Adviser at the conservation body Historic England, also agrees with Webber and says, “to make sure the horse chestnut is part of our future, the possibility of propagating disease-resistant varieties of horse chestnuts must be studied.”

Cathersides estimates that 90% of the registered historic landscapes in Britain contain horse chestnuts and while they are not an historic part of the ancient woodlands, according to Cathersides, “if the horse chestnuts disappear it would be a tragedy for historic designed landscaped areas.” 

In fact, so worried were Historic England (previously English Heritage) that they contributed towards a Forest Research project to study the possibility of propagating disease-resistant varieties of horse chestnuts. 

Cathersides continues, “In order to do this, of course, it is necessary that large-scale felling of all infected trees not be undertaken and that any decision to fell any tree is only taken after the detail of the site and level of infection is fully understood and appropriate consultation has been undertaken.

“The potential benefits of any trees not infected or better able to withstand infections should be considered since they could be the very key to saving the horse chestnut from decimation. Except for obviously dangerously sick trees it is more important to spend time and money on getting a fuller understanding of the pathogen with the hope of finding a way to reduce/stop its spread or control it than to cut down every tree which displays the first signs of disease.”

We need long-term scientific studies of the problem

 Robert Jackson, a Professor of Molecular Microbiology at the University of Reading and a senior editor at Molecular Plant Pathology, echoes these views. Jackson, who is working to search for ways to kill the bacteria – possibly using a phage or virus to destroy it, says “one of the main stumbling blocks to finding a solution to the horse chestnut problem is that long-term scientific studies are needed and that takes funding and, unfortunately, very few grants are given to tree research for long-term studies.”

Looking at the newspaper headlines and listening to the experts it would seem that two very different scenarios are being painted here.

So who should we listen to?

Whilst everyone seems to agree that some horse chestnuts are obviously being decimated by and dying from bleeding canker disease, it is not an automatic and immediate death sentence for all horse chestnut trees since there are signs that some infected trees appear to be surviving for many years or even recovering after being attacked.

Furthermore, ridding the landscape of affected trees and not using them for valuable research could very well be counterproductive if we want to find a cure for bleeding canker disease. In addition, time and money might be better spent on necessary research rather than on eliminating non-dangerously affected trees.

Quite simply, horse chestnut Armageddon is not looming on the horizon as some headlines and an assortment of popular news articles would have us believe. And fortunately for us, this gloriously beautiful valuable tree – an alien that has given much to the British landscape – is not headed for immediate extinction, just as long as we follow the expert’s advice.

This Author

Dawn Starin is an anthropologist. Her articles have appeared in both peer-reviewed journals and in popular publications as varied as Al Jazeera, the Ecologist, The Humanist, New Internationalist, New Statesman, The New York Times, Philosophy Now, and Scientific American amongst others.