Mustard Flower Rust

Infection of host plants (including Boechera and several other members of the mustard family) occurs via wind-borne basidiospores in late summer. Upon germination of the spores, fungal hyphae penetrate the stem of the mustard plant and siphon off nutrients. However, in order to reproduce sexually, the fungus must facilitate the transfer of spermatia from the spermatogonia on this plant to receptive hyphae borne in the spermatogonia on another infected mustard plant. To accomplish this, the fungus sterilizes the host plant, preventing it from producing true flowers. Instead, it forces the infected plant to grow clusters of leaves into brilliant yellow pseudoflowers bearing the fungal spermatogonia. Insects visiting the pseudoflowers transfer spermatia from one host plant to another, in the same way that pollinators transfer pollen between the true flowers of uninfected plants. Spermatia transferred in this way fuse with receptive spermatogonial hyphae on the recipient plant. The resulting hyphae subsequently form aecia. At this time, the pseudoflowers lose their green colour and stop producing nectar. Spores produced in the aecia, referred to as aeciospores, are responsible for infecting P. monoica's alternate host plant (a grass species of Koeleria, Trisetum, or Stipa). Upon germination hyphae produced by the aeciospores penetrate the grass, leading to the production of uredia. …

It was originally described and published by American mycologist Charles Horton Peck (1833–1917) as Aecidium monoicum in 1879. It was found on the leaves of Arabis retrofracta in Colorado, USA. Then in 1912, mycologist Joseph C. Arthur transferred it to the Puccinia genus, as Puccinia monoica.