What Black Mold Is

• Black mold on hibiscus and other plants is caused by mealybugs. Mealybugs are small, off-white, cottony-looking wingless insects that suck moisture from plants. The bugs produce a sticky, clear liquid called honeydew that drops onto the leaves, which in turn can cause the growth of fungus that's referred to as black or sooty mold.
  
  

Controlling It

• Use limonene to clean the outside of plant pots to get rid of mealybugs.plant pot image by Allyson Ricketts from Fotolia.com
  
  Hidden Valley Hibiscus suggests a two-pronged approach to eliminate mealybugs. First, use a systemic pesticide, which is absorbed into the plant, making the sap toxic to mealybugs. Since mealybugs are difficult to get rid of, you'll also want to clean the plants.
  
  To clean one or two plants, remove mealybugs with a cotton swab dipped in rubbing alcohol. Citrus-based limonene works well for a larger amount of infested plants. Spray it on to kill the mealybugs and dissolve the honeydew and black mold. A mixture of pure limonene and water must be used in the correct amounts so that it works and doesn't cause your hibiscus to drop its leaves.
  
  

Prevention

• While there's no sure way to prevent mealybugs, keeping your plants healthy will help. Natural predators such as ladybugs could stop an infestation before it starts. Check your plants frequently and separate and treat them if you see one that's exhibiting signs of a mealybug attack.