News Feature
“Unfortunately, our climate is very suitable for the spread of ash dieback, so it is likely that a large percentage of our ash trees will become infected over the coming years”
a large branch, or stem from a twig, it often causes a diamond-shaped lesion, although this will become more irregular in shape as it enlarges. Below the affected bark the wood is discoloured brown.
As the infection progresses, twig and branch dieback develops. The tree may respond by producing lots of new shoots directly from the branches and/or trunk (these are known as epicormic shoots). Young trees can be killed within a year or two, but older, mature trees may take several years to succumb.
How does C. fraxinea spread?
The fungus releases infectious airborne spores from small, mushroom-like structures that develop on fallen, decaying leaves shed the previous year - these spores infect the current crop of leaves during wet weather. It is thought that the spores are only released from June to October, so there is currently a period of grace where new infections should not be occurring. Indications are that the wind-blown spores can easily spread the disease over a distance of about 20-30km per year, although they may sometimes be dispersed much further. Long-distance spread of the disease can also occur very easily through the movement of infected plants in the nursery and forestry trades, which is why
import restrictions have been imposed. What action is being taken?
The severity of the threat posed by ash dieback led to a meeting of the Government’s Cobra committee, which is convened in response to periods of national crisis. All imports of ash plants and seeds (the fungus can also be seed- borne) have now been banned. There has been much talk of stable doors and bolting horses, given that the problem had been known in Europe for several years before this action was taken. Unfortunately, there was some initial confusion over the identity of the fungus, until DNA-based molecular techniques confirmed that C. fraxinea was, indeed, a new pathogen to the UK (a very closely- related fungus, which looks virtually identical, is common on dead ash leaves in the UK, but is a saprophyte and does not cause disease). Where C. fraxinea is found on recently planted ash, or on plants at nurseries or garden centres, statutory Plant Health Notices are issued requiring the plants to be destroyed (by burning or deep burial). Unfortunately, under the Plant Health Order there is no compensation paid to the owners of the plants. In certain cases, steps may be taken to contain the problem on site. Where ash dieback affects established woodland or
hedgerows, movement restrictions and biosecurity measures are imposed. The most effective course of action for this type of outbreak is being considered (as mentioned previously, new infections should not occur during the winter, giving a limited time period for a detailed risk assessment and the formulation of an action plan). However, it is highly unlikely that the disease will be eradicated.
What does the future hold?
Unfortunately, our climate is very suitable for the spread of ash dieback, so it is likely that a large percentage of our ash trees will become infected over the coming years. Fraxinus excelsior is the third most common native broadleaved tree in Great Britain (around 80 million trees, or 130,000 hectares of predominantly ash woodland), so the results of the disease will be there for all to see.
Of course, in cases like this, there will be much research undertaken into ways of controlling or overcoming the problem - this has already been happening in mainland Europe where the disease has been present for longer. Quarantine measures, sanitation (e.g. treatment of equipment with disinfectants) and the use of fungicides may all have a role to play.
Lesion on main stem
Large lesion on main stem
Leaf death from ash dieback DECEMBER/JANUARY 2013 PC 13
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