Crusty skin lesions on a cat's ears, may not be just an ear problem. Immune-mediated Pemphigus, allergies to environmental allergens and food allergies, and Ringworm Fungus can all look the same on cat's ears. Sometimes Ringworm Fungus can be present in addition to the primary disease.
Diagnosis include skin parasite tests, DTM Ringworm fungal cultures, and skin biopsies in a diagnostic work up. Treatments are available for these diseases which are actually systemic (whole body) diseases rather than ear only diseases.