Answer: The sooty looking patch is not the problem. It is just a sooty mold growing on a patch of sweet honeydew on the plant surface.
Yellowing leaves on hibiscus is usually due to either over or under watering. Even a brief period of soggy soil can result in root damage and yellowing leaves. Low fertility levels in the soil may also result in yellowing of older leaves of hibiscus. When growth resumes in spring, start feeding the plants periodically and keep them moist but not soggy.
Good luck with your hibiscus!
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