I would get to the vet asap and get that area cultured - it will take about 10-14 days to grow the culture. This could be ringworm (a fungus, not a worm) and would need to be treated asap; and, btw, in dogs it will not always show the typical "ring" as it does in human skin. Very common in dogs to start on the head.
It could be something else too, but either way I'd get to the vet soon.
