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Chalara fraxinea is a fungus which is causing a serious disease of ash trees known as ash dieback. The infection causes wilting leaves and crown die back and it usually leads to tree death.

The ash tree is a significant part of the UK landscape and one of Britain’s few native, tree species with important conservation value. There are approximately 80 million ash trees in the UK representing 5% of Britain's woodland cover.

Ash dieback was confirmed in the UK in February 2012 and a ban on ash imports introduced on 29 October 2012 after a voluntary moratorium by the industry. The Horticultural Trades Association asked for a ban back in 2009, having seen the impact of the disease in Denmark. Much of the immediate debate on the issue therefore focussed on how the UK Government reacted to these early warnings and why it did not act sooner to ban ash imports.

Now that the disease has been confirmed in established trees in the UK, the focus has shifted to informing and developing action plans to deal with the disease in the short and longer term. In addition, a Tree Health and Plant Biosecurity Taskforce has been established to review the UK's strategic approach to tree health and biosecurity. Its final report is due in January 2013. The Task Force published an interim report on 6 December 2012 along with Defra’s Interim Chalara Control Plan. This note will be updated to provide more detailed analysis of these in due course.

The current scientific understanding is that the disease only spreads in the summer (the period of spore release) so the Government is seeking to develop its response whilst there is a window of opportunity for action. Overall, the current approach is to try and slow the spread of the disease and to minimise its impact to gain time to find those trees with genetic resistance and to restructure our woodlands to make them more resilient.

POST Note No 394 Invasive Tree Pests and Diseases (October 2011) provides further detail on the UK plant health regime in general.


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