Dog nose red and he is constantly rubbing it

For a couple weeks my dog has been rubbing his nose on anything he can get his nose to. It has now started to be really red under his nose itself and looks really raw. What could be causing this

[Cliff Redford, DVM]

There are many possibilities for an itchy nose.
Allergies, viral or bacterial infections, autoimmune disorders, I have even treating a dog with similar symptoms who has inhaled/"snorted" a corn kernel and it was sprouting roots in his sinus cavities.
It needs to be assessed by a vet asap.