Chalara dieback of ash is a serious disease of ash trees caused by a fungus called Chalara fraxinea (C. fraxinea). The disease causes leaf loss and crown dieback in affected trees, and it can lead to tree death.
Ash trees suffering from symptoms likely to be caused by C. fraxinea have been found widely across Europe. These have included forest trees, trees in urban areas such as parks and gardens, and also young trees in nurseries. In February 2012 it was found in a consignment of infected trees sent from a nursery in the Netherlands to a nursery in Buckinghamshire, England. In June 2012 it was found in ash trees planted at a car park in Leicestershire which had been supplied by a nursery in Lincolnshire, and the origins of the disease in this case are being investigated. In July 2012 our colleagues in the Food & Environment Research Agency (Fera) confirmed cases in the nursery trade in West and South Yorkshire and Surrey, and by September 2012 it had been reported in a nursery in Cambridgeshire. It has also been found at four recently planted sites - a Forestry Commission Scotland woodland at Knockmountain, near Kilmacolm, west of Glasgow; the car park in Leicester, a college campus in South Yorkshire, and a property in County Durham.