I have two gardenia plants that have done well in the garden for many years. I recently noticed many yellowing and browning leaves. Upon inspection, it appears that some of the gardenia buds had turned black and there are spots on the stems. Some stem tips were black. I removed those and all of the discolored parts, but it seems clear that the whole plants are sick.
A couple of years ago, I noticed what appeared to be sooty mold on many of the leaves of one of the plants, and to a lesser degree on the other plant. I tried to clean it off with water and cloth and get rid of the aphids I found with soap and water. After that, the plants seemed to be doing better. Now I'm thinking I didn't do enough.
Is there anything I can do for my gardenias?