Rust fungus. Coloured scanning electron micrograph (SEM) of a rust fungus (order Pucciniales) on a greater periwinkle (Vinca major) plant. Rust fungi are parasitic on both wild and cultivated plants, often causing severe disease. It is most commonly seen as coloured powder, composed of tiny aeciospores (brown). Magnification: x150 when printed at 10 centimetres wide.