Need help with 1 year old GSD's territorial barking
So I have a 1 year old GSD female who is friendly towards everybody (people and dogs) when she meets them. When she was 3-5 months old I had both of my next-door neighbors babysit her whenever I was out of the house and since they also have dogs, she got used to them.
However, she never met my neighbor that lives in the house right behind mine nor her dog so she always barks at her through the fence. She's been doing this for the past few months but it's escalated quickly in the past two weeks. My neighbor came to speak to me about the issue and I told her I would try to lessen the barking. I've tried treats and everything but to no avail and I would prefer not to use a bark collar.
Does anyone have any ideas on how to lessen her barking?
By the way, I mentioned the thing about my other neighbors because my dog never barks at them or their dogs when they're in their backyards, only the neighbor that lives behind me.