The same thing has been happening to my black lab's nose! This is now the third time -- the other 2 times it healed in time. I had assumed it was related to using plastic bowls (I had heard of this in cats a few years ago) and replaced them with stainless bowls.
My dog is outside a lot so it makes sense that it's sun-related. Getting him to allow me to apply sunscreen or neosporin on his nose (and not licking it off) is going to be a challenge. But it's good to know his nose is NOT rotting out.