Sooty black mold is a fungus that can appear on the surface of plant leaves, stems, twigs, branches and even fruits. The fungus does not harm or feed off the plants, but instead gets its nourishment from honeydew. Honeydew is a sticky, sweet substance that certain insects secrete. When sooty mold spores are carried by the wind and land in the honeydew, the fungus feeds on the honeydew and grows, covering the plants and causing a charcoal-like sooty coating of mold. When your plants have black sooty mold, your real problem is caused by the insects, such as aphids, whiteflies, mealybugs and scale. You can apply all the fungicide you want to your infected plants, but if you don't get rid of these insects, you can't eradicate the sooty mold.
Identify the bugs involved in your sooty mold infestation. Aphids, mealybugs and whiteflies all cause black sooty mold. Aphids are tiny round bugs that can be black, brown, green, yellow or red. Mealybugs are also tiny, but the females are covered in a cottony wax and the males often look like gnats.
Determine whether your sooty black mold is caused by scale. Look for tiny insects that look like small, whitish, wax-covered bumps on your plant. Use a magnifying glass to help in identifying these tiny insects.
Apply an insecticide to the affected plants if you detect aphids, mealybugs, scale or whiteflies. Select an insecticide that specifies use for these exact bugs, and follow the directions closely for application. You'll likely need to apply the insecticide more than once.
Wash the affected plants every day with a soft cloth and water, paying special attention to the underside of leaves. Whiteflies are especially resistant to insecticides, so you may also need to use an initial application of insecticide, followed by a spray application of Safer-Soap.
Recheck the infected plants 1 week after the initial application of insecticide. If you still see active, live insects, apply another dose of spray.