I would cut out the rotted part, and replant the healthy part above it.

Yes, cut off all branches with rot. Sterilize cutters in between cuts. Otherwise the rot might work it's way down to the roots.

good luck.

I cut the whole stem and remove infected one and now i have 2 long cuttings to replant , it was tall stem sadly

Black dots stuff still in the other plant its just didn't rot yet , anyone knows what it is ? Rust or infection !