Scanning electron micrograph of pustules on a leaf of groundsel, Senecio vulgaris, infected with the rust fungus, Coleosporium tussilaginis. The pustules have broken through the surface of the leaf in order to release uredospores (brown). An early stage of eruption is visible, upper right. Rust fungi are plant pathogens. Their complex life cycle involves the production of different spore types, and sometimes two hosts. The spores here will go on to infect the alternate host, pine trees, producing winter hardy teleospores. The most economically important rust is Puccinia graminis, that infects cereal crops worldwide. It causes loss of vigour and catastrophic crop failures. It is controlled by continual development of resistant cereal varieties, since it may take only a few seasons for the fungus to produce a new race that can evade the efforts of plant breeders. Mag x 100 at 10x8