Scab around mouth

I have a female terrier mix about 5 years old. she is about 20 pounds. i have noticed, probably about 4 months ago, that she has black scabs on both sides of her bottom jaw and under her mouth at the front. when the scabs come off her skin/flesh is pink and moist but there doesn't seem to be any bleeding. she doesnt seemed bothered by it unless of course if someone touches her there. her diet consists mostly of dry dogfood and she eats out of both a plastic and a metal bowl. ive read it might be an allergy to plastic, could this be the case?

[Aleda M Cheng, D.V.M., C.V.A]

Plastic allergy is very common.  Eliminate the plastic bowl and if the mouth lesions resolve in a month or so you will know that she had a plastic allergy.

If the lesions don't resolve after you have eliminated the plastic than the lesions should be biopsied.