new dog acts aggressive towards other dogs on walks

Training and socialziation if you want the dog to be around others. 

Has the dog met any other dog in an approprate manner?  How old is she? 

It can be frustrating to the dog if unallowed to meet and greet.  But, I understand your fears too.

Get with folks who can help you work through it.  If you want her to be around other dogs, she needs to be desensitized...and the only way to do that is to get her around other dogs and be taught what is OK and what is not.

I'm sure you will get much better advice from the experts on the board...good luck!

My dog is 3 years old.  She has spent much time in a shelter environment.  She is great with people  I don't have another dog, so I don't care if she can't play with any other dogs.  I just want her to be able to go for a walk with me and walk nicely past a dog.  She seems ok with little dogs. 

Well, yeah, she does need to be able to be sociable with other dogs, if for no other reason than so she CAN go for a walk with you and walk nicely past another dog.

You could start out by stepping off to the side and having her sit when another dog is passing. Make sure to give her good quality high level treats while she is sitting. Keep the treats coming at a fast pace until the other dog has passed.

Reward her every time she looks at another dog without pulling or getting over excited. In a short time she will understand that being around other dogs and acting sane means good things for her.

loretta

My dog is 3 years old.  She has spent much time in a shelter environment.  She is great with people  I don't have another dog, so I don't care if she can't play with any other dogs.  I just want her to be able to go for a walk with me and walk nicely past a dog.  She seems ok with little dogs. 

The first step is to train her so that she can rely on you for information and to let her know what you expect.  You want to do that in a way that does not confuse her or make her anxious, anxiety or frustration being two of the biggest reasons for "leash reactivity".  Most dogs do not like to be forced to greet one another face to face on tight leashes, and you are right that many dogs, when off leash, act differently than they do on.  Find a good positive trainer to help you with introductions at class in a safe, controlled way, and it may give you much more confidence.  Emma Parsons' book, Click to Calm, is a good step by step training protocol for dog aggressive dogs or for dogs that are just reactive.  I suspect that if she's ok with little dogs, then she is anxious or fearful, rather than frustrated or aggressive.  So, the object of your training will be to build her confidence and teach her to look to you when she isn't sure what to do.