Rose rust fungal spores. Scanning Electron Micrograph of reproductive spores, known as uredospores, from the rose rust fungus, Phragmidium sp. . Rust fungi (Uredinales) are parasitic basidiomycetes which are common on both wild and cultivated plants and which often cause severe disease. Many rusts have extremely complex life cycles involving two hosts and five distinct types of spore, although some show modifications in which one or more of these spore stages is absent and the cycle is restricted to a single host. Magnification x5600 at 7x10 inch size.