News Feature
Common ash (Fraxinus excelsior) in a woodland setting
Ashes to ashes?
The UK’s trees seem, at present, to be under constant threat from new pest and disease problems. Leaf-mining moth and bleeding canker disease of horse chestnut, oak
processionary moth, Asian longhorn beetle, acute oak decline, sweet chestnut blight and Phytophthora ramorum are just a few examples. Yet, the latest disease to hit the headlines may well prove to be one of the most devastating - ash dieback.
Report by John Scrace,
Freelance Plant Pathologist 12 PC DECEMBER/JANUARY 2013
What causes ash dieback and where did it come from?
Ash dieback is caused by a fungus called Chalara fraxinea. You may also see the name Hymenoscyphus pseudoalbidus in some of the literature on the disease - this refers to a different stage in the life-cycle of the same fungus.
If you have been watching the news
reports about the disease, you may have got the impression that it has been discovered only recently and, as far as cases in the UK are concerned, this is true. However, it was actually first reported (in Poland) in 1992, although it took some time before Chalara fraxinea was confirmed as the cause of the disease. The problem spread rapidly across mainland Europe in the 2000s, and many countries are now affected. Denmark is the country mentioned most, often due to the severity of the problem there (up to 90% of ash trees affected), but ash dieback is also present in France, Germany, Italy, Belgium, the Netherlands and several other countries. There are, at present, no records outside of Europe.
The first cases of the disease in the UK
were found in early 2012. Ash plants infected with C. fraxinea were imported from the Netherlands by a nursery in southern England, and then sold
throughout the UK. Action to trace and destroy the affected batches of plants was still ongoing when there was a further, even more serious development. In October and November, the first cases were found in the natural environment, and these couldn’t be linked to the arrival of the fungus on new planting material. Since these cases were all in eastern counties (notably East Anglia, Essex and Kent), it is suspected that the fungus may have arrived at the sites by wind-blown spores from the continent.
What are the symptoms?
C. fraxinea is confined to ash trees, with common ash (Fraxinus excelsior) and narrow-leaved ash (F. angustifolia) appearing to be most susceptible. Rowan or mountain ash (Sorbus aucuparia), the leaves of which can look similar to those of ash trees, is not affected.
Infection usually starts in the leaves, which develop dark blotches, often beginning at the base of leaflets or around the central vein. Affected leaves eventually turn black and wilt. The fungus then spreads from the leaves into the twigs, and from the twigs into the branches and main stems. Lesions on the bark are brown in colour, sometimes with associated bark cracking. When the infection spreads into
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