3yr old Lab protective of 6month old mix!

Question
Scout, our 3 year old lab, was everything BUT aggressive.  She is very submissive, loving, and gentle with all ages of humans including infants, puppies, and other dogs.  She has never shown aggression or her teeth...until...we brought home a new puppy.  Boo, a now 6 month old lab mix, is a energetic and fun little puppy.  Scout now growls and barks angerly at any dog she does not know.  It is scary to see her act this way.  ANY new dog she will show her teeth and bark loud.  THIS IS A 180degree turn.  We can't introduce her now to new dogs, for the fear of her being aggressive!  I hope I have explained this well enough. THANK YOU!!!

Answer
This is a tough one.  At age 3, she is mature and ready to take a major role protecting the rest of the pack.  Certainly better leadership on your part may help.  The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm

Even the above may not be enough.  If you could find a good professional dog trainer, it might help.  From what I have seen on the net, it is tough going finding a decent one.