The Rust Fungi

Abstract The rust fungi are a monophyletic group of approximately 7000 species in the Basidiomycota and are highly specialised obligate parasites of plants. The life cycle of rusts can be complex. Some rusts have up to five spore stages that alternate between haploid and dikaryotic nuclear conditions and that can occur on two taxonomically unrelated host plants. The rusts have evolved specialised structures that allow them to penetrate and obtain nutrients from living host cells. Biologic forms of a single rust species may differ in ability to attack different genera of host plants. Furthermore, within a single rust species they can be highly variable in ability to attack different genotypes of one host species. At the species level, genes that condition avirulence/virulence in rusts interact in a specific relationship with rust resistance genes in plants. Many of the most important plant diseases in the world are caused by rust fungi. Key Concepts The rust fungi are a monophyletic group of approximately 7000 species within the Basidiomycota, the group of fungi that includes mushrooms and shelf fungi. Rust fungi have complex life cycles with up to five spore stages that alternate between haploid and dikaryotic nuclear condition on two unrelated host plants. The rust fungi are obligate parasites of plants and have evolved specialised structures to infect and obtain nutrients from plants. Rusts have a highly specific host range. Genes in rust that express avirulence or virulence interact with rust resistance genes in host plants to condition a compatible or incompatible interaction. Some rust species are highly variable for biologic forms that vary in their ability to attack host genotypes with different rust resistance genes. Many of the most important plant diseases worldwide are caused by rust fungi.