Chalara dieback of ash – (Chalara fraxinea)

Tree health has been a hot topic in recent weeks following the introduction of a new tree disease into the UK. There has been much press coverage regarding the potentially serious threat to one of our most valuable native trees – Ash.

Chalara dieback of ash is a serious disease of ash trees caused by a fungus called Chalara Fraxinea. The disease causes leaf loss and crown dieback in affected trees, and usually leads to tree death.

The disease appears to have been introduced on imported nursery stock and is now being treated as a quarantine pest under national emergency measures. However, now that the disease has been found in older trees (East Anglia, Kent and Essex) with no apparent connection with plants supplied by nurseries, government scientists are investigating the possibility that it might have entered the UK by natural means; such as wind borne spores or birds coming across the North Sea.

It is still very early to judge what the position is and the Forestry Commission and DEFRA are still gathering information so the full impact that the disease will have in England is not yet known.

If we look to the other countries that have suffered with the disease it looks very likely that 60-90% of UK ash trees will be infected in the foreseeable future. Ash trees make up 5% of the UK tree population so this will have a significant impact on our towns and countryside.

The disease is a fungus that readily infects young trees and saplings but affects older trees more slowly. The main spread of the disease is through the movement of young plants and this is now being controlled. The majority of the reported outbreaks relate to young planting with a few cases of mature trees being affected.

The disease spreads slowly on its own (12-20 miles per year) and mainly through spores on leaf litter and twigs from June to September. This means that we will get a much better idea of the extent of the spread by the end of next summer. Our current knowledge of the spread is based on sample plots inspected by DEFRA.

There are no reasonable controls (at the moment) and it looks very likely that the policy will be to allow the disease to run its course rather than undertake sanitation felling. The problem with sanitation felling (removing all the trees in an area around an outbreak) is that it is expensive for landowners and you lose trees that may be resistant to the disease. So in the long term it is better to identify the resistant trees so that we can re-establish the ash tree population.

The current advice is that mature trees may live for some time after infection so felling trees hastily may be too harsh a measure when some benefit can still be gained from keeping the tree.

For more information go to: http://www.forestry.gov.uk/chalara

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