Semi-desert, foothills, montane. Woodlands, shrublands, openings. Spring, summer.
Canyonlands National Park, March 25, 2001.
Canyons of the Ancients National Monument, April 16, 2004 and April 2, 2005.

In the early spring, several species of Boechera appear to have yellow flowers. 

A close look shows that the seeming flowers are actually abnormally large masses of leaves covered with yellow, tiny, shiny bumps  --  droplets of super-sweetened nectar produced by flask-shaped structures, spermagonia, on the back of the leaves.

These structures are the result of a rust fungus, Puccinia monoica, a pathogen afflicting several Mustard species.  The shiny, sweet secretions attract pollinators which then spread the fungus.

Semi-desert, foothills, montane. Woodlands, shrublands, openings. Spring, summer.
Upper Mule Canyon, Utah, April 8, 2005.

The two plants pictured are actually one plant; they are growing from the same roots.  The plant at the left is healthy and is flowering (top center); the plant on the right is infected with Puccinia monoica.

Semi-desert, foothills, montane. Woodlands, shrublands, openings. Spring, summer.
Canyons of the Ancients National Monument, April 16, 2004.

Of six plants growing within two feet of each other, three were infected with rust and three had lovely, one-third inch flowers.