I have a cutting of a variety of black raspberry that I want to root- two weeks ago I received it, promptly scratched it a bit, dipped in rooting hormone and put into peat moss that I then watered, and it's been covered by a plastic bag since then to prevent water evaporating.

While the leaves are still green and it has looked good, over the last 2 days the stem has turned black and I'm afraid it's starting to rot. I took the bag off when I saw that, but, is there anything else I can do to prevent it from rotting and dying?