Erythrasma
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þ An obese diabetic patient living in Florida comes in at 3 a.m. complaining
  of an irritating rash in his groin and axilla that has been present for
  months.  It fluoresces a characteristic coral-red under the Wood's lamp.
  What is the diagnosis, and what is the causative bacteria? This patient has
  erythrasma, a chronic bacterial infection caused by Corynebacterium
  minutissimum.  It causes sharply marginated finely scaling macules in
  intertriginous areas. Risk factors include: diabetes, obesity, warm and
  humid climates, middle age, and an African-American background.
  Reference: Color Atlas and Synopsis of Clinical Dermatology, 2nd ed.,
  Fitzpatrick TB, et al., p94.