Just like people, plaque and bacteria cause tooth decay and smelly dog breath.  

Gingivitis and infected gums from untreated dental diseases smell rotten. Seek treatment.

Stuck objects like sticks trap bacteria and smell bad. Inspect mouth.  

Digestion issues from poor diet or food allergies may contribute to bad odors.

Improper hydration concentrates bad mouth smell. Ensure ample clean water.   

Internal issues like kidney or liver disease can cause breath odor. See your vet.

Unmanaged diabetes causes fruity-scented breath called ketones. Requires medication.