Tuesday, October 30, 2012

More on ash die back disease

My mail box continues to be crowded with injunctions to help stop the spread of ash die back disease, Chalara fraxinea.  As someone who has done many years of study on our native trees and has the greatest possible affection for them I do not want to be seen as not caring about the ash.

However, I do think we have to be realistic.  It seems to me there is little chance of stopping a fungal disease that is spread by spores and is widespread all over Europe right up to the Channel coast from advancing into the UK (as the Forestry Commission suggest it already may have done) by perfectly natural means as well as on imported plants.

Prompted by this recent emergency I was particularly noticing ash trees on my way into Hastings this morning, a journey of about 10 miles.  There must be thousands, many the young trees that appear to be particularly susceptible, in hedgerows, on embankments as well as in the edges of the woodlands that I could see.  It would be very difficult to survey all these trees carefully for die back and, if the disease were found the prospect of the damage to the countryside that would result from unearthing them from hedges etc. is alarming.

Where the A21 enters Hastings there are steep embankments covered in young ashes on either side of a road bridge.  If the disease were to occur here, trying to deal with it (bearing in mind that it might be in the roots as well as in the top hamper) would involve major engineering works and probably significant road closure.

It occurs to me that the disease has probably been identified in plant nurseries and on Forestry Commission and Woodland Trust properties because young trees in these places get much more detailed scrutiny than elsewhere.  Many woods have  thousands of young ashes as well as older ones, often in rarely visited spots and systematic survey and destruction of the infected would involve an impossible amount of people hours and cost.  There will no doubt be extra vigilance in well-visited woods, but there are many places where ashes grow on farmland and other private property where detailed scrutiny is unlikely to take place.  Indeed, one of the characteristic habitats of the ash is on vertical limestone and other cliffs ...

While the present government does not seem to be particularly pro-wildlife, blaming them for outbreaks of ash die back in the wild as a consequence of not preventing imports of the trees sooner than they did seems like blaming the American government for hurricane Sandy.

If ash die back becomes widespread in the UK, as seems likely based on the experience in other European countries, it might be better to look for trees that do not have it rather than those that do.  As I said in yesterday's post, there seem to be some chalara-resistant strains of ash and these will need to be cherished.

Monday, October 29, 2012

Ash die back disease, Chalara fraxinea

Recently I have read and listened to many items about the fungal infection Chalara fraxinea or ash die back disease that kills the trees it attacks.  While I am sure no one would welcome any serious threat to our native ash trees, it does seem to me that much of the commentary is ill-founded and not very helpful.

The general story from the media is that nearly all the ash trees in Denmark have died, that a few cases have been found in Great Britain and that imports of ashes from nurseries abroad have been banned.

A good way of getting a more balanced understanding of the situation is by reading the Forestry Commission's fact sheet on the issue:


According to the FC the disease was first recorded in Poland in 1992 (other sources say 1990) and infected trees have been found widely across Europe, including several places from Kent to central Scotland in the UK.  Infected trees are widespread in north western France with up to 80% affected in some areas and the fungus has also been reported from Belgium where "eradication measures have sought unsuccessfully to stop the spread of the disease". 

In a recent Guardian article George Monbiot said “The only way the fungus can arrive in this country is through imports of infected saplings.”  However, as it is a fungus presumably it spreads by spores that can travel in all sorts of ways. Indeed, the FC in their fact sheet say that "It is believed to have entered Great Britain on plants for planting imported from nurseries in Continental Europe. However, now that we have found infected older trees in East Anglia with no apparent connection with nursery stock, we are also investigating the possibility that it might have entered Britain by natural means. These include being carried on the wind or on birds coming across the North Sea, or on items such as footwear, clothing or vehicles of people who had been in infected sites in Continental Europe." (I wonder how they are going to investigate these possibilities).

The banning of imports might, I suppose, slow the spread of the disease, but it looks rather like shutting the stable door after the horse has gone. If it can spread for some distance by spores from nursery stock it can, presumably, find its way across the Channel.  Most of us remember how fast the horse chestnut leaf miner moth spread: stopping imports of its host would have made little difference.

So far as containment is concerned the FC say "We are treating C. fraxinea as a ‘quarantine’ plant pathogen, which means that we may use emergency powers to contain or eradicate it when it is found. This is being done in the form of Statutory Plant Health Notices which we serve on affected owners requiring them to remove and destroy affected plants by burning or deep burial on site. Equivalent measures are being taken on land managed by the Forestry Commission. This is the only available treatment."

If the same situation prevails here as in mainland Europe landowners are going to be faced with an impossible and probably worthless task.  If infected trees were found, for example, not only the top but the roots would have to be burnt or deeply buried.  And what about trees nearby that might be infected but not yet showing symptoms.  Would the National Trust, for example, be expected to dig up and burn or bury  all the ash trees in Dovedale should Chalara strike there?

Sadly I suspect the FC and other government advisers know perfectly well that they are unable to stop spores crossing the Channel and that the current red flag waving and whistle blowing is just to try and show a public that loves its trees that something is being done.

With Dutch elm disease, the various afflictions of oak, phytophthera on sweet chestnut and other species I sometime wonder if these diseases might be the way nature operates, killing whole swathes of trees from time to time and opening up forests for other flora and fauna. Often a few individuals of the species attacked seem to survive, like post-myxomatosis rabbits, to repopulate the old habitat which, by then, may be relatively free of their pests and diseases since these have had little or nothing on which to subsist.  Indeed in a Danish study (which might have given rise to the frequently heard comment that Denmark is particularly badly affected by ash die back) it has been shown that different strains of ash have different levels of resistance.

Unless the disease mysteriously dies out naturally in nearby mainland Europe and where it has appeared in the UK, which seems unlikely, might it not be better to let things run their course?  In good natural selection style, the fittest would survive and repopulate our countryside with resistant strains.