Sooty mold is removed by controlling the honeydew-producing insects that cause it, such as aphids, whiteflies, scale, and mealybugs, and then clearing the black residue with a copper fungicide spray.

What Causes Sooty Mold on Tree Leaves?

Sooty mold is a fungus that blackens leaves by growing on the honeydew secreted by insects such as whiteflies, aphids, and mealybugs, rather than by infecting the leaf tissue directly, which is why controlling the insects is the most effective way to prevent it.

How Should the Honeydew-Producing Insects Be Controlled?

Controlling the insect population behind sooty mold calls for spraying the tops and undersides of the leaves, since these pests often colonize the underside, with a second treatment about 10 to 14 days later if the infestation is heavy.

An insecticidal soap is one option for the soft-bodied pests responsible; Safer Brand Insect Killing Soap uses potassium salts of fatty acids to dehydrate aphids, whiteflies, mealybugs, and soft scale on contact, is OMRI listed for organic gardening, and can be used up until the day of harvest.

A horticultural oil offers a similar smothering approach with broader coverage of scale insects; Horticultural Oil Insect Spray blocks the pores of mites, scale, mealybugs, whiteflies, and aphids, among other pests, though it is not recommended for beech, butternut, Colorado blue spruce, Douglas fir, hickory, holly, sugar maple, Japanese maple, walnut, or ferns.

How Is Existing Sooty Mold Removed From Leaves Once the Insects Are Controlled?

Existing sooty mold is cleared by spraying the affected foliage with a copper fungicide once the insect population is under control, and treehelp.com's own guidance notes that one application is usually enough, with a second about 14 days later only needed for major outbreaks.

A copper fungicide is the product category used for this step; Liquid Copper Fungicide Spray is sprayed to thoroughly wet the foliage, with one pint treating at least 5 to 6 large trees, and its general label allows reapplication every 7 to 14 days under heavier disease pressure.

How Can Sooty Mold Be Prevented From Returning?

Sooty mold is less likely to return when the insects that cause it are caught and treated early, supported by a few consistent habits:

• Inspect both the tops and undersides of leaves regularly, since honeydew-producing insects often colonize the undersides where they are easy to miss early on.
• Treat light infestations promptly, since a heavier buildup requires a second insecticide treatment 10 to 14 days later.
• Reapply an insecticidal soap or horticultural oil at the first sign of returning pests, following label instructions.
• Recheck treated trees for sooty mold about two weeks after clearing it, since major outbreaks can require a second copper fungicide application.

Frequently Asked Questions

Does sooty mold damage a tree's leaves directly?

Sooty mold is a fungus that grows on insect honeydew rather than on the leaf tissue itself, which is why treehelp.com's guidance focuses on controlling the insects producing the honeydew rather than treating the leaf for a direct fungal infection.

Which insects most commonly cause sooty mold?

Whiteflies, aphids, mealybugs, and scale are the honeydew-producing insects identified as the source of sooty mold.

How soon can sooty mold be expected to clear after treatment?

One copper fungicide application usually clears sooty mold, though major outbreaks may need a second application about 14 days later, and the underlying insect population may need a follow-up spray 10 to 14 days after the first treatment if the infestation was heavy.

Can sooty mold return after it has been treated?

Sooty mold can return if the honeydew-producing insects were not fully controlled or come back, since new honeydew gives the fungus fresh material to grow on, which is why rechecking treated trees is worthwhile.