It may spread to normal skin later in the disease. There are several forms of phemphigus. If there is nasal depigmentation with this disease it is likely that there will be ulceration of the nose. Sunlight makes discoid lupus lesions much worse and dogs with this may have terrible problems in the summer and very little problem in the winter vitiligo causes white pigmentation of haired areas and pink discoloration of darker skin, like the nose. I think of this as a problem mostly in rottweilers but other breeds may be affected.
Some skin cancers can cause depigmentation of the nose.
There is no need to treat this condition and am not aware of an effective treatment. However, if he eats from plastic or rubber food bowls or has a rubber or plastic water bowl, I think I'd change to something else. If he is young and there is scaling and crusting, I think I'd worry about discoid lupus, too. If Bobby is older and there are no signs of other problems, this still might be idiopathic nasal depigmentation but it would be a good idea to keep a very close eye out for anything that might indicate skin cancer, such as scaling, erosions of the nasal planum, nose bleeds or decreased appetite. Phemphigus tends to show up in middle aged dogs.