Aecidium (Caeoma, Uredo)
Among the rust species that go through a complete life cycle, most species form aecia and telia on different host plants. Many groups of related species then have their aecial stage on one and the same host species, and their telial stage on an array of different hosts. Often in such case the aecial stage is (almost) identical for all species of the complex, making a definite identification impossible. To make them nevertheless discussable, the form-genus Aecidium is available. Aecidium species obviously only have spermogonia and aecia.
The genus Aecidium also is used to acommodate species of which the life cycle is not (yet) known, only spermogonia and aecia having been observed, while the aecium typically is cupulate, generally with a clear recurved margin. When the aecium is flat and lacks a clear margin the genus is Caeoma.
Of course the opposite may occur, when uredinia and telia are known while s complete cycle is to be expected. For that situation another form genus is available: Uredo.