Semi-desert, foothills, montane. Woodlands, shrublands, openings. Spring, summer.
Canyons of the Ancients National Monument, April 21, 2009.

It is fairly clear that the greenish-yellow coloring at left belongs to the leaves of some small plant, but from the height of a hiker passing by, this color seems to be from minute yellow flowers.  This plant is typical in height at five inches. 

What seems to be flowers turns out to be, upon closer inspection, abnormal clusters of leaves covered with tiny, shiny bumps (droplets of super-sweetened nectar) produced by flask-shaped structures, spermagonia.

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  --  which can infect plants at various altitudes.  The top photo of P. monoica is from a plant found at 6,000 feet and the bottom is from 11,500 feet.

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

The two plants pictured at left are actually one plant; they are growing from the same roots.  The plant at the left is healthy and is flowering; the plant on the right is infected with Puccinia monoica and it will not flower.

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.