Taphrina johansonii Sadebeck
Aspen tongue
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.
Online References:
4/21/2010 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 6 × 4″ (15 × 10 cm) 
4/21/2010 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 7 × 4½″ (17 × 11 cm) 
This catkin is normal, mostly unaffected by the Taphrina. · 4/21/2010 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 9 × 6″ (22 × 14 cm) 
4/21/2010 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 6 × 4½″ (14 × 11 cm) 
4/21/2010 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 5 × 8″ (13 × 19 cm) 
| Parents | Unknown | Genus is not in the current taxonomy |
| Genus | Taphrina | |
| Species | johansonii |
Taphrina johansonii description by Thomas H. Kent, not updated.
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