What Causes Black Soot on Gardenia Bushes?

Honeydew

• Sooty mold makes its home on a sticky substance called honeydew, which is produced by sap-sucking insects. These insects, such as aphids, whiteflies and soft scales, suck and ingest the sap from leaves. However, they can't digest all of it; therefore, the excess is secreted as honeydew.

Types of Fungi

• Usually, sooty mold develops on the honeydewed parts of the bushes due to the Fumago, Capnodium or Scorias fungi, although less common fungi sometimes lead to the black substance as well. The type of fungus developed depends on the plant, environment and which type of insect produces the honeydew. Some fungi occur due to several honeydew-producing insects, while others are particular to certain plants or insects.

Warning Signs

• If you see a swarm of ants around your plants with no anthill nearby, then your gardenia bushes may be infected with honeydew. Ants eat it, and they attack natural predators and parasites of the insects that produce it. Watch for honeydew-making insects. Aphids are small, pear-shaped pests that range in color from yellow to black and feed as a group on the foliage of gardenia bushes. Tiny, white insects with wings are whiteflies that live on the underside of leaves. Elevated areas or circular discoloration on the leaves indicate soft scales, which are immobile and don't appear to possess a head or limbs. Other sap-sucking, honeydew-making insects include leafhoppers, psyllids and mealybugs.

Control Options

• Get rid of ant by using ant baits or stakes under the bushes. Trim any branches touching the ground, buildings or other plants that can provide a path for ants to make their way into the shrubs. Inspect any plants above your gardenia bushes. One of them might be the source of an insect infestation, causing honeydew to drip onto anything below. Check the gardenia shrubs for insects. Sometimes you can wash them and their honeydew off with a forceful spray of water. If that doesn't work, introduce their natural predators, such as lady beetles and lacewings. In the absence of natural enemies, another option includes the application of horticultural or neem oil or insecticidal soap all over the foliage. Once you eliminate the insects, there are several ways to get rid of the black soot. Cut off as many of the infested plant parts that you can, wash it off with a strong stream of plain water or soapy water, wipe it off with a moist paper towel, peel it off to uncover the undamaged parts of the plants or allow it to gradually wear away. Wash off any remaining honeydew early in the day to allow the bushes to dry off quickly and reduce the chances of attracting fungal diseases.