I have this problem every year its starts around end of May through October. The gardenias bloom then quickly die leaves turn yellow then brown then curl up and fall off. The have holes in them and a layer of a dusty brown powder like that youcan wipe with your finger. I have lost all my leaves on one side if the house and now it's starting in the other side. I was told spider mites but looks like fungus. I live in Florida so the possibilities or endless.
