Sooty Mold, Scale and Whitefly: The Black Film on Your Tree

The complaint usually starts with the car. It is parked under the tree and it keeps getting covered in a sticky film, and then the film turns black. Then the homeowner looks up and the leaves are black too, and the patio furniture, and the pool cage.

That is sooty mold, and the sticky stuff is honeydew. Neither one is the actual problem. The problem is the insect population upstairs producing the honeydew, and treating the mold without dealing with the insects gets you nowhere.

How the chain works

Sap-feeding insects (scale, whitefly, aphids, mealybugs, and on some trees, psyllids) pierce leaves and twigs and feed on sap. Sap is mostly sugar water, and they excrete the excess as honeydew. Honeydew coats everything below. Sooty mold is a fungus that grows on the honeydew. It does not infect the leaf; it just sits on the sugar.

So the black film is three steps removed from the cause. The cause is the insect.

Which insects, on which trees

In Volusia County the common combinations are:

• Whitefly on gumbo limbo, ficus, and a range of ornamentals. Clouds of tiny white insects fly up when you shake a branch. Rugose spiraling whitefly and ficus whitefly have both moved through our area.
• Scale on magnolia, oak, holly, crepe myrtle (crepe myrtle bark scale is now common), citrus and palms. Scale insects look like small bumps on twigs and the undersides of leaves; they do not look like insects at all.
• Aphids on new growth of crepe myrtle, citrus and many others, usually a spring problem.
• Mealybugs on palms, ornamentals and citrus, white and cottony in leaf axils.

Identifying the insect matters because treatment differs. Aphids are easy to knock back. Armored scale is not.

Is it hurting the tree?

Usually less than it looks. A heavy sooty mold coating reduces light reaching the leaf and can weaken a tree over time, and a heavy scale infestation drains real resources. But on a large healthy oak or magnolia, a season of honeydew is mostly a nuisance to the people underneath.

The exception is a tree that was already stressed. Sap-feeding insects find and multiply on stressed trees. If the tree is thinning, has a buried flare or a compacted root zone, or has just come through construction, the insects are a symptom. Fix the tree and the insect pressure usually falls. Our post on feeding trees in sandy soil explains why we are cautious about high-nitrogen fertilizer, which tends to make honeydew problems worse by pushing soft growth.

What works

On small trees and shrubs

Horticultural oil sprays smother scale, whitefly and aphids on contact. They are low toxicity, but they need complete coverage, including leaf undersides, and they should not be applied in the heat of a Florida afternoon or the leaves burn. Repeat applications are usually needed to catch the crawler stage of scale.

On large trees

You cannot spray a sixty-foot oak to coverage from the ground. For large trees, systemic insecticides applied to the soil or injected into the trunk are what actually work, and they need to be applied by a licensed pest control operator. Timing matters, and some systemics have restrictions around flowering trees because of pollinators.

We are arborists, not a pest control company, and we do not apply pesticides. We will identify the insect on a tree health visit, tell you honestly whether it warrants treatment, and refer you to a licensed applicator when it does.

Doing nothing

On many trees, natural predators (ladybeetles, lacewings, parasitic wasps) catch up with the population within a season or two, especially if nobody has been spraying broad-spectrum insecticides that kill the predators along with the pests. On a big, healthy tree, patience is often the right answer.

Getting rid of the sooty mold itself

Once the honeydew stops, sooty mold weathers off with rain over a few months. On patio surfaces and cars, soap and water take it off. On leaves, a strong hose spray helps but is not necessary; the tree will drop those leaves in its next cycle.

When it is more than sooty mold

Black film on leaves along with rapid dieback, sudden wilt or bark cankers is a different problem. Black spots within the leaf tissue rather than on the surface are a leaf disease, not sooty mold. And a palm with black film plus yellowing lower fronds may have a nutrient problem rather than an insect one; see Palm Nutrient Deficiencies.

Frequently asked questions

Will the sooty mold kill my tree?

Almost never on its own. It reduces photosynthesis on coated leaves, which matters on a small or already stressed tree. On a large healthy tree it is cosmetic.

The bumps on the twigs will not scrape off. Is that scale?

Probably. Armored scale forms a hard cover fused to the bark. If you crush one and it is wet inside, it is alive. Dry and hollow means an old shell from a previous generation.

Can you spray my big oak?

Not effectively from the ground, and we do not apply pesticides. For large trees the realistic options are a licensed applicator using a systemic, or improving the tree's vigor and letting predators do the work.

Why is it worse this year?

Insect populations swing with weather, with predator numbers and with the tree's condition. A mild winter, a stressed tree, or a neighbor's broad-spectrum spraying that wiped out the beneficials can all cause a spike.

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ArboristRX assesses tree health across Edgewater, New Smyrna Beach, Port Orange, Oak Hill and Volusia County. Call (386) 444-5959 to have a declining tree looked at properly.