Q: I have been plagued by a black, sooty substance that covers the leaves on my gardenia bushes.
In the summer it is accompanied by tiny white flies. They seem to be related: if I kill the flies with insecticide the leaves sort of clean up, but they return.
Can you tell me what to do?
A: You have good observational skills!
Indeed the black stuff and the little white flies are related. The whiteflies suck sap from leaves and excrete what they can’t digest. The honeydew they give off falls onto nearby leaves and a fungus called sooty mold grows on the leaf surface.
The white scale-like things on the backside of the leaf are immature whiteflies.
I’ve gotten great control by applying systemic insecticide to my shrubs in early April each year. This pesticide poisons the gardenia’s sap but doesn’t hurt beneficial insects.
Once the bugs are gone, the fungus will gradually wash off in the rain.
Remember to repeat the application every spring.