Have you been having problems with black or gray film on your houseplant leaves, stems, and flowers?
Sooty mold is a fungal growth that can weaken plants by blocking sunlight.
Pests or environmental factors can cause it.
The best way to get rid of it is to figure out what is causing it and treat that as well.
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What is Sooty Mold?
Sooty mold is a type of fungus that grows on plant leaves, stems, and flowers.
Honeydew, a sticky substance created by insects that feed on plant sap, is a food source for sooty mold.
Having honeydew on your plants makes it much more likely you will develop sooty mold problems.
Symptoms of Sooty Mold
The main symptom of sooty mold growth is the presence of a dark gray or black coating on your plant’s leaves.
It can also sometimes be on the stems and flowers of the plant.
It looks just like black soot on your plants, hence the name, sooty mold.
How Does Sooty Mold Damage Plants?
Sooty mold damages plants by blocking the sunlight and preventing photosynthesis.
The leaf surface of affected plants becomes covered in a black velvety coating that prevents that leaf from getting sunlight.
As more leaves get covered, the plant’s ability to photosynthesize is reduced.
Severe cases of sooty mold can eventually cause the death of the plant.
How Does Sooty Mold Spread?
Sooty mold spreads when sap-feeding insects excrete honeydew.
The honeydew secretions fall on leaves, stems, and flowers, and the sooty mold fungus grows on it.
Is Sooty Mold Harmful to Humans?
No, sooty mold is not harmful to humans.
How to Get Rid of Sooty Mold
If you have sooty mold on your plants, there are a few steps to get rid of it.
While you are treating the infected plants, you should quarantine them away from your other plants to prevent further spread.
Control the Pests that Produce Honeydew
The most effective way of getting rid of sooty mold is to get rid of the source of the problem.
The first step is finding the insect populations that are attracting the sooty mold.
Check all parts of the plant with sooty mold looking for signs of other pests.
Look at the entire plant, but pay special attention to the areas that have sticky liquid on them, the pests should be near where the honeydew is.
Once you know what is causing the honeydew, you can treat the plant for those pests.
Which Pests Produce Honeydew?
The most common pests that produce honeydew on houseplants are aphids, soft scale insects, and mealybugs.
These pests are all large enough to see when checking your plants and look different enough, it should be easy to identify which one you have once you have found them.
The links above have pictures, as well as recommended ways of dealing with these pests.
Wash Sooty Mold Off the Plants
Once you have controlled the pests, the next step is to wash the sooty mold off of your plants.
To do this, you can either mix your own soap solution or use insecticidal soap.
If you use insecticidal soap, follow the directions on the bottle.
To make your own soap solution, mix 1 TBSP of mild dish soap, like the regular Dawn dish soap, with 1 gallon of warm water.
Then spray the soapy water mixture on the affected leaves and let it sit for 15 minutes.
Finally, rinse the soapy water off by spraying it with a strong stream of water.
This may need to be repeated once a week for a few weeks until the sooty mold is gone.
A word of caution: dish soap can damage plant leaves by breaking down their natural waxy coating, so this option should be avoided on sensitive plants.
Apply Neem Oil
Another way to get rid of sooty mold is to apply neem oil.
Neem oil is one of several horticultural oils that will kill the fungus without harming your plants.
To use neem oil, mix according to the directions on the bottle and spray it on the leaves of the plant.
Make sure to get all of the leaves, even the underside, and any parts of the stem that have signs of sooty mold or other pests.
The advantage of using neem oil is that it helps with the major pests that produce honeydew as well as helping to get rid of sooty mold.
When using neem oil to get rid of your insect problem, multiple applications are necessary.
Reapply weekly for 3 to 4 weeks to get rid of the insects.
How do I Prevent Sooty Mold?
The best way to prevent sooty mold is to have a routine of checking your plants for pests and treating them accordingly.
If you find any pests, be sure to treat the plant right away.
The sooner you can get rid of the pests, the less honeydew they will produce and the less chance there is of sooty mold.
Even if you get control of the pests quickly, make sure to clean the leaves to get rid of any honeydew that was left by the pests.
You can also try using systemic insecticides as a preventative.
Just make sure you get one that works on aphids, mealybugs, and soft scales as they are the biggest honeydew producers.
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