"My poor gardenias are suffering!" writes faithful reader, Sheri Chamblee. "The leaves are black. I tried rubbing the black off, but the black remains. What else can I do?"
The answer, of course, is to ask Grumpy, your font of gardening wisdom.
The black stuff on the leaves is a fungus called sooty mold. It doesn't attack plants directly. Instead, it grows on the sticky honeydew secreted by sucking insects feeding on the plant. So inspect the foliage carefully, both top and bottom surfaces. Do you see anything like this?
Congrats. Your plant is infested with white flies -- very common, very destructive, and very hard to control once established.
Or do you see little bumps on the leaves and stems that look like this?
Cowabunga! Your plant is infested with scales. There are lots of different kinds, but the little suckers all do the same thing. Suck plant juices and damage and kill plants.
Kill the Suckers, Prevent the Mold If you kill the sucking insects and prevent their return, the mold goes away. I'll give you two ways to do this.
1. Apply Bayer Advanced Tree & Shrub Protect & Feed according to label directions. This product contains a systemic insecticide that's absorbed by the plant. When insects suck the sap, they suck in the systemic and it kills them. This product works for months.
2. If you prefer a natural pesticide, spray all leaf and stem surfaces according to label directions with an all-season horticultural oil. This product kills insects, their larvae, and their eggs on contact, but it doesn't keep them from returning. So you'll have to spray multiple times.
Other Plants to Watch Gardenia isn't the only plant targeted by sucking insects and sooty mold. You'll often find both on crepe myrtle, camellia, citrus, holly, and tulip poplar.
Don't fall asleep at the wheel. Kill the bugs before the mold takes hold!