It could be the automimmune and immune mediated skin disease called Discoid Lupus Erythematosus. This condition affects the flat area of the nose causing ulceration and depigmentation.
See the attached website, does the photo of the dog remind you of your dog's nose:
http://www.naturalcanine.com/Merchant2/merchant.mv?Screen=CTGY&Store_Code=TNC&Category_Code=DL
Please let me know if I can be of further help.
~Christina
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Peace and happiness to you and your pet! ~ Christina
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