Steve Wolfe wrote:
It's a gall, maybe Petiole Gall of Poplar.
It's unsightly but not dangerous.

Thanks for the tip on it being a gall. To my untrained eye, it looks like
it may be a crown gall:

http://soilplantlab.missouri.edu/pla.../crowngall.htm

Thoughts?

steve

Split one open, if there are no chambers inside then
it's probably caused by bacteria (crown); if there are
chambers present then probably caused by insect
(aphid, mite, moth, etc.)
If you can't ID it, play it safe, prune off and burn them.