Scanning electron micrograph of a leaf of groundsel, Senecio vulgaris, infected with the rust fungus, Coleosporium tussilaginis. The picture shows the initial appearance of a uredinia (pustule) as the fungus breaks through the leaf epidermis. Rust fungi are obligate plant pathogens; their complex life cycle involves the production of different spore types, and sometimes two hosts. Uredinia produce uredospores (pink); they are able to produce new infections on the same host species. Spores produced later will go on to infect the alternate host, 2-needled pine trees. The following Spring, winter-hardy teleospores then reinfect new groundsel plants. The most economically important rust is Puccinia graminis, that can cause loss of vigour and catastrophic crop failures in cereals. It too uses a woody host - barberry - to survive the winter when its main host is absent. Mag x 350 at 10x8