Patch of fur missing at base of dogs tail.?

This could be cause by flea bites. Is the dog rubbing it's back against trees and the like? It's a common condition, your vet will be able to take care of it. After that use a good flea treatment like Frontline.

Yup, bring her in. Your vet will be able to prescribe something like frontline or advantage. It sounds like the beginnings of flea allergies. If that doesn't fix it, you may well be looking at a food allergy. My dog is allergic to pretty much any food found at a department store. Probably the corn in it. She has to eat a low allergen food and get the protein source rotated out every few months. For low allergen food, check out www.naturapet.com

I agree with halsca. Flea allergy dermatitis often causes hair loss in this area. Your vet can recommend treatment, flea treatment would be a start, but something else may be needed depending on severity of bald patch.

Take it to the vet!