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.

Do you have a wooden fence or a chainlink one that she can see through?

My dogs bark at the neighbors dogs as well however, it's something that was learned after their dogs started doing it when we moved in unfortunately. We have a pair of large dogs on both our left and right sides that bark when they come out and mine bark back. Because it is loud and I'm sure annoying for others, I have just come to manage the situation. Since my dogs aren't out when I am not home I don't have to worry about it then and when I am home and hear it, I just bring my dogs inside. It has worked for us.

I bring mine inside when they bark at the neighbor dogs too.