Taphrina johansonii Sadebeck
I was long perplexed by these miniature bunches of stunted bananas. The leaves of the tree are quaking aspens Populus tremuloides, but aspen catkins are green and narrow, not swollen and bright yellow. Was this some weird cultivar?
Turns out that Taphrina johansonii, a fungus, sometimes infects the green catkins, causing the individual flowers to swell and turn yellow. (Yes, those are flowers, though they lack petals.) These fungi are ascomycetes, or sac fungi. I was not able to determine whether they pose a significant threat to the tree.
Taphrina johansonii on Forestry Images
Taphrina johansonii description by Thomas H. Kent, last updated 16 Aug 2013.