Sooty mold on Plants

Symptoms

The most noticeable sign is a black, sooty coating on the plant, and it spoils the way the plant looks. The coating is the dark, threadlike growth of a fungus, and it looks like a layer of soot. You can find it on leaves, twigs and fruit. The name of the problem comes from that soot-like look. The dark growth is called mycelium, the threadlike body of the fungi.

The growth is a fungus living on honeydew, which is a sweet, sticky liquid left by sap-sucking insects. Fungi in the genera Capnodium, Fumago and Scorias are the ones growing on that honeydew. The mold can harm the plant indirectly. A thick coating stops sunlight from reaching the leaf surface. With less light reaching the leaf, photosynthesis is reduced. Coated leaves may age early, die and drop from the plant. Sooty molds do not infect avocado and generally cause no damage there.

Causes

Sooty mold comes from fungi growing on honeydew. The fungi belong to genera such as Capnodium, Fumago and Scorias. Honeydew is a sweet, sticky liquid that plant-sucking insects excrete while they feed on sap from a plant. The insects, not the fungus, are the source of that sweet liquid.

Several kinds of insect produce honeydew, including aphids, leafhoppers, mealybugs, psyllids, soft scales and whiteflies. Oak leaves infested with aphids can carry sooty mold. Fungi that grow on honeydew leave the dark, threadlike growth you see on the leaves. The coating can then reach the point where sunlight cannot get to the leaf surface.

Control of sooty mold begins with managing the insect that creates the honeydew. Ants also play a part, because they protect honeydew-producing insects from their natural enemies. Keeping ants away is one of the steps listed under treatments.

Diagnosis

Start by looking closely at what the dark layer is made of. Sooty mold is a dark, threadlike growth that looks like a layer of soot. It sits on leaves, twigs and fruit.

What sets sooty mold apart is its link to honeydew, the sweet, sticky liquid that sap-sucking insects excrete. So the next step is to check the plant for aphids, leafhoppers, mealybugs, psyllids, soft scales and whiteflies. Oak leaves infested with aphids show how the insects and the mold appear together. Ants are worth noticing too, because they protect honeydew-producing insects from natural enemies.

The harm from sooty mold is indirect, because the coating blocks sunlight from reaching the leaf surface. Coated leaves may age early, die and drop. The mold itself does not need treatment once the insects are controlled, so finding the insect is the key step in diagnosis. If your plant is an avocado, the coating generally causes no damage, since sooty molds do not infect it.

Treatments

Control of sooty mold begins with managing the insect that creates the honeydew. Once the insects are controlled, the sooty mold itself does not need to be treated. Cultural steps come first, then organic ones.

Prune out and remove the most heavily infested plant parts, since careful pruning can remove most of the infested material. Ants need attention as well, because they protect the insects from their natural enemies. To keep ants away from honeydew-producing insects, apply a sticky barrier compound around the trunk. Sooty mold on oak leaves infested with aphids, for example, points to aphid control as the starting step.

Among organic methods, you can knock insects off the plant with a strong stream of water. You can also wash the sooty mold off plant surfaces with a strong stream of water, or with soap and water. A strong stream of water appears twice in the list, once for the insects and once for the mold. Washing removes the coating from the surface, but the insects still need to be controlled first.

Pruning, ant barriers and water are the steps the facts list, and they are cultural or organic. If you choose a product of any kind, the label is the legal instruction and must be followed for the crop and site being treated.

Prevention

Sooty mold needs honeydew to grow, so keeping honeydew-producing insects in check is the main way to prevent it. Watch for aphids, leafhoppers, mealybugs, psyllids, soft scales and whiteflies, which are the insects named as honeydew producers. Sooty mold on a plant is a sign to look for the insects behind it.

Because the mold grows on honeydew, a plant with no honeydew gives it nothing to grow on. Knocking insects off with a strong stream of water is one organic step you can take. Ants protect honeydew-producing insects from natural enemies, so keeping ants off the plant matters. A sticky barrier compound around the trunk keeps ants away. Removing the most heavily infested parts is another cultural step.

Plants most often affected

The hosts recorded for sooty mold on this page are oak, pomegranate and avocado. Oak leaves infested with aphids can carry the mold. On avocado, sooty molds do not infect the plant and generally cause no damage. Those three plants are the hosts recorded for this problem.

• Barbados Aloe
• Cape Jasmine
• Chinaberrytree
• Deodar Cedar
• Lantana
• Monkeypuzzle Tree
• Norfolk Island Pine
• Southern Magnolia
• Tuliptree