Sucking insects ingest more plant sap than they can digest and secrete the excess as honeydew. The sooty mold fungi grow on the sugary honeydew, but don’t infect the plant. The problem arises when the sooty mold covers the leaves preventing the plant from capturing sunlight for photosynthesis.
If nature is not controlling these pests and sooty mold is a problem, you may need to lend a hand. Dislodge small populations of aphids and mites with a strong blast of water. Use insecticidal soap to kill soft-bodied insects it contacts and wash off the sooty mold. A lightweight horticulture oil can control these as well as scale, mealybugs and other insects. As always, read and follow label directions.
A bit more information: You may notice ants feeding on the honeydew. They help protect the honeydew producing insects, their food source, from natural predators. Controlling the sucking insects will help manage the ants. As always, read and follow label directions when using any pesticide, organic or synthetic.