Answer: If you can wash the substance off, I think it is either bug poop (such as fly spots) or the excretement of sucking insect pests such as scale or mealybugs. When pests such as these feed, the excrete a sticky substance which eventually turns black or brown. Carefully inspect your plant for colonies of feeding insects. Sometimes you can pick or peel them off, or wash them off with a strong stream of water. In the case of mealybugs or scale insects, you can dab them with a cotton swab dipped in rubbing alcohol. Good luck with your corn plant.
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