Scanning electron micrograph of a leaf of groundsel, Senecio vulgaris, infected with the rust fungus, Coleosporium tussilaginis. The picture shows a uredinia (pustule) that has broken through the leaf surface to release uredospores. Rust fungi are obligate plant pathogens. Their complex life cycle involves the production of different spore types, and sometimes two hosts. The spores produced on groundsel will go on to infect the alternate host, pine trees; winter hardy teleospores then reinfect new groundsel plants in spring. 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 150 at 10x8