Hi there, Hmmm. Honestly for any aggression issues you need to see a professional. Your dog is displaying a normal reaction (resource guarding - especially for a working breed) but it is an over-reaction. And you are very right to see big potential problems coming up. I actually and respectfully disagree with some of the advice given here as resource aggression won't be abated with getting the dog into trouble over it. You will have more success (and better safety) if you promote the dog to feeling that it doesn't need to defend its resources. I won't post any more advice here because you need to see someone good and in person to give you personalised advice.
Look up the delta society for people in your area. If you can afford to go to one of the vet behaviourists then do it, If you can't then look up a delta dog trainer and call them for some advice.
Good luck.