Polarised light cross section of a leaf bud of European ash (Fraxinus excelsior). To the outside four large scale leaves wrap around the developing primordial foliage leaves. These leaves are arranged in rings with the midrib to the outside and with the lamellae lying in parallel pairs. Developing vascular tissue shows up as bright areas under polarized light. Ash dieback disease, also known as chalara, is caused by infection by the fungus Hymenoscyphus fraxinus (formerly Hymenoscyphus pseudoalbidus or Chalara fraxinea). This serious disease of European ash was first detected in about 1995 and was first reported in the UK in 2012. It is feared that the spread of the disease may affect millions of ash trees in the country's woodlands. Efforts are bring made to identify resistant trees. Magnification: x28 at 10cm wide.