Est is the pathogen turning plants and trees black? - Ask Extension
My pachysandra under trees, a couple of azaleas, camellias, my river birch and a Japanese snowbelk are turning black. Picture This is telling me to sp...
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Est is the pathogen turning plants and trees black? #921366
Asked November 04, 2025, 9:55 AM EST
My pachysandra under trees, a couple of azaleas, camellias, my river birch and a Japanese snowbelk are turning black. Picture This is telling me to spray with a baking soda solution, and I have been trimming the leaves that are completely black. What is this and what else might I do to limit its spread?
Montgomery County Maryland
Expert Response
This is not the work of a pathogen, and no action is needed. The black residue is sooty mold, a type of fungus that grows on honeydew residue but which does not infect plants or cause disease. Honeydew is produced by various sap-sucking insects, and it's a clear sugary liquid that falls below where they are feeding, coating leaves, bark, and other objects. It weathers away on its own eventually, as will the mold on top of it.
The sooty mold is probably appearing due to honeydew from either adult Spotted Lanternfly feeding (our primary suspect; a few are visible on the birch tree trunk), or perhaps a scale insect population somewhere in the branches above that area. If scale are the cause, a certified arborist can help assess the situation, but rarely would this type of scale need to be treated since they don't do much damage to trees. Spotted Lanternfly feeding will end very soon, as overnight temperatures approach freezing and kill the adults.
Do not spray anything homemade on the sooty mold, especially a baking soda solution, which could damage the plants. Some insecticide products containing horticultural oil are labeled for sooty mold suppression (they mostly are used to smother pest insects like aphids), but it probably won't cause the mold to die and fall off quickly; it will probably just weaken it and help it to weather off a bit faster than it otherwise would. Follow all label directions if you try this approach, as there can be temperature restrictions for applying horticultural oil, and it should not be mixed with other materials.
Miri
The sooty mold is probably appearing due to honeydew from either adult Spotted Lanternfly feeding (our primary suspect; a few are visible on the birch tree trunk), or perhaps a scale insect population somewhere in the branches above that area. If scale are the cause, a certified arborist can help assess the situation, but rarely would this type of scale need to be treated since they don't do much damage to trees. Spotted Lanternfly feeding will end very soon, as overnight temperatures approach freezing and kill the adults.
Do not spray anything homemade on the sooty mold, especially a baking soda solution, which could damage the plants. Some insecticide products containing horticultural oil are labeled for sooty mold suppression (they mostly are used to smother pest insects like aphids), but it probably won't cause the mold to die and fall off quickly; it will probably just weaken it and help it to weather off a bit faster than it otherwise would. Follow all label directions if you try this approach, as there can be temperature restrictions for applying horticultural oil, and it should not be mixed with other materials.
Miri
