Black Sooty Mold

Black Sooty Mold

The black powdery substance you’re seeing on plants and other surfaces could be something called black sooty mold.  If it was preceded by a sticky clear liquid and it can be rubbed away, then it probably is.  Learn more about black sooty mold in today’s TrueTreeTalk post.

What Causes Black Sooty Mold?

Black sooty mold is caused by fungi and molds.  The fungi that cause black sooty mold come from different genera including Fumago, Scorias, and Capnodium.  The black sooty mold you see is actually the mycelium of the fungi, which is dark in color.  The black sooty mold doesn’t grow directly on the plant and other surfaces, rather it grows on honeydew.

What is Honeydew?

Honeydew is a sticky, shiny liquid that is excreted by insects that suck sap.  Insects such as aphids and soft scales suck sap from plants.  They do this to get sugars and nutrients.  The issue they face is that the sap is high in sugar relative to the proteins and vitamins they need.  To deal with this, they absorb the nutrients they need from the sap and excrete the excess sugar in the form of a liquid.  This liquid is honeydew.  It is deposited on plants and other surfaces, and this is what the fungi grow on.  We touched on this idea a bit in our post about crepe myrtle bark scale and our post about ants and aphids.

Why Do Fungi Grow on Honeydew?

The fungi that grow on honeydew and cause black sooty mold are airborne and floating in the air outside.  They can also be transferred by water such as when rain hits surfaces they are growing on.

Honeydew is a great medium for certain fungi to grow on.  It contains vitamins, minerals, proteins, and amino acids, in addition to sugars.  It serves as a nutrient source for the fungi.  It’s also sticky, and this helps spores and fragments of hyphae to stay where they land.

Also, black sooty mold can grow on sap that has leaked from a plant, for example from a wound, in addition to honeydew excreted by insects.

Does Black Sooty Mold Hurt Plants?

The answer here is a little complicated.  The fungi that cause black sooty mold are not parasitic.  They don’t penetrate plants nor do they get nutrients from plants.  This is different from other fungi that might, for example, cause different types of rot to occur in a plant.

The fungi though don’t benefit the plant (as an organism would in a mutualistic relationship) nor do they have no significant impact on the plant, as would happen in a commensal relationship (like we mentioned in our post about barklice).  Instead, the harm they cause is indirect.  

Black sooty mold, if it is widespread enough on a plant’s leaves, can block light from reaching the leaves and this can reduce the plant’s ability to produce food through photosynthesis.  In this way, black sooty mold can harm a plant.  Unlike as would be in a parasitic relationship, black sooty mold doesn’t benefit directly from this harm.

Parts of a plant that have excessive black sooty mold can die, but this is more likely caused by insects rather than the black sooty mold.

What Kinds of Plants Does Black Sooty Mold Grow On?

As mentioned, black sooty mold grows on honeydew, rather than directly on plant surfaces.  Because of this, the fungi that cause the sooty mold aren’t selective about which plants they indirectly grow on.  Black sooty mold can appear on both evergreen and deciduous plants.  Some plants you might be likely to see it on include crepe myrtles, gardenias, laurels, camellias, and azaleas. 

What Seasons is Black Soot Mold Most Likely to be Prevalent and What Weather Favors it? 

You might see black sooty mold on plants most commonly spring through fall.  It can be less common in winter.

Black sooty mold can be present in a variety of weather conditions, but the condition can get worse if the weather is fair and hot.  Rain can sometimes wash some black sooty mold away, leaving leaves that look spotted with it.

How can Black Sooty Mold be Reduced?

Black sooty mold isn’t something that is treated directly.  To reduce black sooty mold, the insect population excreting the honeydew that it is growing on must be reduced.  Once the insect population is reduced, the honeydew will be reduced, there will be less new black sooty mold, and the older black sooty mold over time will be removed by nature.

One thing to keep in mind is that the insect population that is producing the honeydew for the black sooty mold might be present because of other conditions, such as weakened plants.  In some cases, a deeper root cause will have to be addressed to correct the issue.

Also, in some cases, depending on the means, reducing the insect population might not be desirable.  In some cases, the population can decline naturally over time from things like predators and changes in weather.

Additionally, insects can be on one tree or plant but excrete honeydew on a tree or plant below them.  The tree or plant with black sooty mold, might not necessarily be the one with the insect issues.

Also, related to a previous post about ants and aphids, sometimes to control the insect population that produces honeydew, the ants that tend and protect them have to be controlled.

One indirect method to reduce black sooty mold is to improve the overall health of the plants in your landscape.  Healthy plants are less attractive to insect pests, and so by having healthier plants, the prevalence of future black sooty mold can be reduced.

Generally, removing black sooty mold from plant surfaces is difficult to do and some methods of removal can be harmful to plants.  Black sooty mold will flake away over time though once the insect population has been reduced.

If you found learning about black sooty mold helpful, check out our other posts on our TrueTreeTalk blog.  Follow us on Facebook to keep up with these and other posts.

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