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This disease is killing many Ash trees and is concerning everyone as it is spreading across Britain.
The disease was first noted in 2012 in Buckinghamshire and is spreading across the country westwards. Young trees seem particularly vulnerable to this disease and evidence from continental Europe suggests that older, mature ash trees can survive infection and continue to provide their landscape and wildlife benefits for some time.
The best hope for the long-term future of Britain's ash trees lies in identifying the genetic factors which enable some ash trees to tolerate or resist infection, and using these to breed new generations of tolerant ash trees for the future.
Chalara or ash dieback is a disease of ash trees caused by a fungus called Hymenoscyphus fraxinea. Chalara causes leaf loss, crown dieback and bark lesions in affected trees. Once a tree is infected the disease is usually fatal, either directly, or indirectly by weakening the tree to the point where it succumbs more readily to attacks by other pests or pathogens, especially Armillaria fungi, or honey fungus.
Diseased saplings typically display dead tops and/or side shoots. At the base of the dead shoots lesions can often be found on the subtending branch or stem. Lesions which girdle the branch or stem can cause wilting of the foliage above.
Mature trees affected by the disease initially show dieback of the shoots or twigs at edge of the crown. Dense clumps of foliage may be seen further back on the branches where recovery shoots are produced. In late summer and early autumn fruiting bodies of Hymenoscyphus can be found on the blackened leave stalks in the damp leaf litter beneath the trees.