How can I help my dog-aggressive dog?

Tailbiter of an expression, I know, but that is the term. Specifically, her aggression is caused by being afraid of the other dog. They will try to play or sniff and her tail will drop and she’ll start snarling. This is new; she was well socialized as a pup, loved other dogs and never showed signs of aggression until about four months ago (she is now 3). She is trained, engaged mentally on a daily basis, and well-exercised. She is never aggressive to humans. I have been told that a slow process of desensitization with the help of a trainer is my only option – is this the case; has anyone here had experience with this issue? She is a Chow/Lab mix.