Has anyone seen this yellow goopy material on new leaf growth on Populus deltoides?

We are growing some P deltoides in our greenhouse for an experiment. About 2 or 3 weeks ago we chopped the trees back to make some props from them, and new growth started up shortly after. Now we're noticing the tips of the newest shoots are covered in a yellow goop. It's a little sticky to the touch, and doesn't seem to be oozing down the plant, just sort of covering the tips. It doesn't seem to be affecting the growth of the plant.

Is this something normal with P deltoides? I've never grown them in our GH or looked closely enough at saplings in the wild.