I use Fungiside 3 in 1 on my hibiscus it will control your problem with fungus or black spots. But as a word to the wise use late in evening when the sun is not as hot due to the burning of your leaves and put you plant under even more stress. How often are you watering your hibiscus? This could be a part in the stress of your plant as well as the fungus.

Thank you for your reply. I don't like to spray chemicals in my garden, but may have to in this case. I water about twice a week, now that it is raining. Otherwise, during the drought, I was watering daily or alternate day depending on the temperatures.

The tricky thing with hibicus plants they are fussy love lots of water. I would say that if your plants shows no other signs of stress I would go with the 3in 1 spray I agree with you on not liking tou use chemicals I live in Michigan where the Jap. beetles love hibiscus plants, so far this year I have only had to use spray once on my new hibiscus I purchased this year. I hope that it works out for you and if you ever run into a bug problem there are plenty of homeade concotions that are all natural pesticides..Hope this was helpful - Evie