Is it territorial behavior?

Hi all,
I have a very submissive female dog who is generally mindful and well-mannered, but when I take her out on leash she growls and barks at other dogs. When we're at the dog park, she plays well, so I think it has to do with the leash. Does anyone know why she's doing this? Any strategies to make her quit? Right now I try to make her sit or lay down when a new dog comes into view but that's not terribly practical. I've had her for about 3 months and she's about a year old.
Thanks,
Robin

It sounds like she's protecting you - and is a bit jealous too, and you're right about it being connected to the leash. You're doing the right thing. Keep having her sit and praise her like crazy and reassure her when she does it, even though it's not completely practical all the time. At some point she'll suddenly get the idea and will be composed on her own. She's reaching an age where things will suddenly start to gel in her mind.