Its Sinaloan, not sinolian.
It might be scale damage or else infection due to improper conditions or injury. Not zonata disease.
Soak the snake (just an inch of water is plently) in room-temp water for about 10 minutes. After removing the snake, apply Neosporin to the affected area. Wash your hands well after handling the snake before you touch other snakes.
Thoroughly clean and dry the snake's cage. Use newspaper substrate for the time being, and keep the cage immaculately clean. If the room has low humidity, you may provide a plastic box filled with slightly damp sphagnum moss, but keep the rest of the cage dry.
Monitor the snake. The injury/whatever may heal or look alot better after successive applications of Neosporin and a few sheds. If not, see the vet.