Discoid Lupus Erythematosus (DLE) in an autoimmune disease that attacks the nasal planum (moist part of the nose) in dogs.
DLE is often not this severe; clinical signs can range from simple depigmentation (such as we are seeing just below the nose in the image above) to full thickness ulcers, bleeding and crusting (as we see here — ulcers in the middle and crusts on top). This disease is worsened by exposure to sunlight, so your veterinarian may recommend that you keep your dog out of the sun (especially in peak sun hours), or that you use a pet-safe sunscreen.
This disease has a fairly unique clinical appearance, but other diseases that can look like it include cancers such as squamous cell carcinoma, and even more rare autoimmune diseases such as nasal arteritis (usually seen in specific breeds such as the St. Bernard). A biopsy is needed to obtain a definitive diagnosis, but your veterinarian may choose to treat empirically (without a biopsy) depending on the severity of the disease and response (or lack thereof) to treatment.