From a fellow grower: White Widow plants stop growing and stem looks weird. Pro-mix soil
3 Likes
Looks like stem rot due to being too wet for too long. Called damping off.
Don’t know if you’ll be able to save that one.
It’s pretty far gone.
Edit : Just a lil tip. The stem should never be smaller at soil level than it is above. 
8 Likes
agreed with @Oldguy and that stem rot is bad, its like a beaver chewed around it waiting to say timber.
5 Likes
Wayyyy overwatered
6 Likes
Agree with @Oldguy. Soil should have a wet to dry cycle, this will allow the roots to breath 
5 Likes
2 Likes
Improper drainage
Did you have good air flow on the surface of the pot? Sometimes that’s a overlooked thing it can make a difference.