On a rocky slope overlooking Ash Creek (Lassen County CA), Leonard and I found a strange plant. The top leaves were bright yellow and sticky with green leaves lower on the stem. The yellow part of the plant did not seem to be a flower, yet I had no idea what this plant could be.
We puzzled over the unusual plant for several days and about lost hope of identification. Then serendipity!! I was reading about plant rusts and there was a picture of the strange plant. A lucky accidental discovery!
The plant in question was a rockcress (Arabis species) infected by the rust fungus Puccinia monoica.
Puccinia monoica basidiospores (reproductive spores) are borne by the wind and infect plants in the mustard family, usually members of the Arabis genus. The rust fungus basidiospores germinate, and the fungal hyphae penetrate the stem of the host. The hyphae inhibit their host from flowering – virtually sterilizing the plant – and form reproductive structures bearing spermatia. In order to complete sexual reproduction the rust fungus spermatia must outcross with another infected plant. To accomplish this, P. monoica creates a “pseudoflower”. The fungal hyphae produce a yellow color, sticky sugary substance and scent mimicking buttercups and yellow bells. The yellow color is indistinguishable in the visible and ultraviolet spectra from the color of buttercups and yellow bells. Insects visit the rust fungus infected “flowers” and transfer the spermatia – in the same way bees and other insects transfer pollen between plants. Fascinating adaptation for sexual reproduction in this rust fungus!
Yet the story is not over. Puccinia monoica, in addition to infecting a mustard species, needs a second host. Once the spermatia from two infected pseudoflowers fuse, another reproductive structure, an aecia, forms on the receptive Arabis plant and produces aeciospores. These aeciospores infest an alternate host, usually a grass in the stipa, junegrass or trisetum groups. The aeciospores germinate, penetrate the grass and form a “rust” on the grass. Eventually basidiospores form on the grass rust and are released to once again infect Arabis species and complete the rust fungus life cycle.
Host reproduction and survival is greatly reduced by P. monoica.
This rust fungus life cycle is complicated. To my mind, the way it takes over an Arabis host, creating a pseudoflower to mimic yellow spring flowers and trick insects into spreading its spermatia sounds like something from science fiction.