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.

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