Ash dieback disease. Ash tree affected by ash dieback disease. This disease is caused by the fungus Chalara fraxinea, also known as Hymenoscyphus pseudoalbidus in its fruiting stage. It causes necrosis (tissue death, dark areas) in the tree's stems, causing dieback of the leaves and shoots. The disease can be chronic and can eventually be lethal, especially in young trees. This fungus was first found in the UK in 2012, and it is feared that the spread of the disease may affect millions of ash trees in the country's woodlands. Photographed in the UK by the Food and Environment Research Agency.