I have a small hibiscus in a pot on the patio I've noticed the are curling and then turning yellow with some black on them, this is happening all over the plant. It has been fertilized and the temperature is not hot enough to bother it I don't believe, the highs are in the upper 70's to the lower 80's.
Certified GKH Gardening Expert
This could be a fungal disease or watering issue. This article may help: