This Iris foetidissima leaf is infected by the Iris Rust, Puccinia iridis. It is the sort of life one overlooks, even as someone with an expressed interest in natural history. The Rusts are all in the genus Puccinia. They are all obligate parasites of other plants and part of the Fungus kingdom, in the Basidiomycota phylum. There are over 4000 species worldwide, many of the them significant economic pests.