Ash dieback disease fungus. Coloured scanning electron micrograph (SEM) of fruiting bodies of the fungus Chalara fraxinea, also known as Hymenoscyphus pseudoalbidus in its fruiting stage. This fungus causes disease in ash trees, leading to necrosis (tissue death) in the tree's stem and 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.