If it is a sooty mold,caused by white flies, a misting of a weak solution of bleach in the late afternoon should clean them.
be sure to spray with water before the sun hits them
Getting rid of the white flies should follow

Hi
I have growing gardenias for about 8 years on the Fl west coast. The first couple years was difficult. I finally figured out that the gardenias couldn't handle full sun in my front yard. Always yellowing, bug problems and bud drop. I transplanted a couple to the back yard in only a half day of full sun. What a turn around! I probably have a couple hundred blossoms right now!
If sooty mold is on the leaves, it should wipe off the leaf fairly easily. If it is sooty mold, you need to first apply a systemic bug killer. The mold is caused by bug droppings. The next step, I would recommend is applying a fungicide like dithane. I use Fertilizer for acid loving plants a few times per year. In between fertilizing application I put down some ironite. This helps keep the leaves nice and green. It is normal for gardenias to periodically go through a leaf drop period. Lots of leaves will turn yellow and drop only to be replaced by new young leaves.

Never sprayed anything on this or gave any fertilizer in 7 years, full all day sun @ AL/FL border. Aphids/sooty mold can come and go, and this past winter made the leaves a little brown around the edges from being encased in ice for 48 hours, but looks great now, and I'm sure this June it will look like the pic below again. Never had a bunch of leaves go yellow, just a very few occasionally, as is the natural way the plant discards older, unneeded leaves. Just let it do its' thing and give it a drink if it doesn't rain for a while. Pruning excess twigs from the middle can help with air flow too, if you're seeing black mold often.

Gardenias thrive in abandoned yards where nobody has lived for years, as long as they don't get sprawly from too much shade.

I have four gardenias near my front door and ONE of them is being attacked by ant type insects and has sooty mold. I have sprayed it with a 3 in one fungal. / mite / insect product twice but ants still come back. Will the systemic product help? They are Miami supremes and bloomed pretty well - this was their first bloom season in this location. Fairly healthy looking so far but I am concerned for the one that seems affected. I have pets snd birds so don't want to put anything on them that is harmful to them. I would rather relocate or get rid if the gardenias than harm the hummingbirds and others that come to nearby plants. Help?

Likely the ants are farming the aphids for their honeydew (which attracts/facilitates sooty mold.) Knock the aphids off of the gardenia with spray of water from hose for a few days in a row. That usually causes the ants to give up farming on that plant. Sooty mold is ephemeral when conditions are right, it should go away when conditions change. Pruning some of the excess growth from the middle/inside can help by increasing air flow also.

Thanks! I pruned, hosed it down, and finally ended up dousing it with a 3 in 1 solution that seemed to kill them. I have seen no more ants or evidence of pests. Odd that the one plant was so affected yet the other three right near it are fine.

Another question: These are Miami Supremes. I understand they grow fast and can get quite large. I did not realize that when I got them. I will have to prune them pretty strongly to keep the size right for where they are. Will that harm them? I can move them and get a slower growing, smaller variety for the front door area if need be (suggestions?) If they can be kept at around 3 feet with pruning and not be harmed that would be ideal.

Thanks so much for the advice!