An order of fungi known as plant rusts that belong to the division (phylum) Basidiomycota. In nature, all 7000 species are obligate parasites of many vascular plant species. They cause diseases known as rust on numerous cultivated crops such as asparagus, onions, beans, snapdragons, and sunflowers. Ferns, fern allies, pines and their relatives, and many other trees are also attacked. Each rust species infects one or just a few closely related plant host species.