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 stems on the left are healthy and flowering; the stems on the right are infected with Puccinia monoica and will not flower.

It is not uncommon to find a number of plants growing within several feet of each other, some infected with rust and some with lovely flowers.

Foothills, montane, subalpine. Woodlands, meadows. Summer.
Bear Creek Trail, June 29, 2010.

The Puccinia rust affects a number of native wildflowers and although I do not find many infected plants, the more aware I am, the more I find.  I have not been able to identify this species of Puccinia.