I live in Southeast Texas where it is very hot and humid most of the year, but this winter we had several nights 29 degrees F, a couple even down to 10. My tropical hibiscus made it through its second winter in the house. Now outside again it's blooming like crazy, but huge amounts of leaves are developing yellow spots, some almost looking like a small squash leaf borer (without the borer!!), then turning completely yellow, wilting and falling off. I've looked for any sign of pest and can't find any. Could I have a fungus or some other disease? My potted tomatoes are beginning to do the same thing. Help. This has never happened before.
I would recommend that you water your Hibiscus with Epson salts. The magnesium is found in high concentrations, which the Hibiscus seem to thrive on. You will want to keep the soil moist but never constantly wet. For this issue I fill a milk jug with tap water and proper amount of salts and let sit next to plant on the floor. So that during the course of a week when cleaning or tidying up I feel the soil, and if dry to the touch, I water it a little about every other day. Just make sure you shake up the water well before applying to plant. Good Luck.