Aphids on Gardenias/Sooty Mold
Your article explains aphids and their residue that results in sooty mold very well, but didn't tell how to be rid of the pesty things. My gardenias are almost black with the stuff.
Certified GKH Gardening Expert
Neem oil has been known to be quite effective for treating both sooty mold and the aphids that can cause this fungus. Here is more info: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm