Pear rust fungus. Light micrograph of a fruiting body (reproductive structures of the rust fungus Gymnosporangium sabinae on a pear tree leaf. The fruiting bodies are covered in reproductive cells (spores, brown). Rusts are parasitic fungi that cause some of the most destructive plant diseases. The disease appears as numerous rusty, orange spots that rupture the epidermal layers of the leaves. The fungus rarely causes the tree to die but it greatly reduces its ability to photosynthesize, often leading to severely reduced yields. Magnification: x100 when printed 10 centimetres wide.