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What is the Black Soot on My Plant's Leaves?

Updated on November 25, 2016
Gardenias are just one of the plants affected by black soot.
Gardenias are just one of the plants affected by black soot. | Source

My Gardenia's Leaves Turn Black

Gardenias are my favorite flowering shrub. They make an excellent accent plant for your home. I always plant my gardenias near a window or door so I can enjoy the fragrant flowers even while I am inside.

Gardenia flowers exude an aroma fit for the gods - in my opinion their scent is sweeter than the most fragrant rose.

Gardenias have glossy, dark evergreen leaves which make a beautiful backdrop to their flowers. So the first time I saw my gardenia's leaves covered with a sooty film, I panicked. I didn't want to lose my beautiful gardenia plants!

With a little research, I discovered the "soot" wasn't lethal, just unattractive.

If you have a white powdery mold growing on your plants read: Organic Control of Powdery Mildew.

Sooty mold on a gardenia leaf.
Sooty mold on a gardenia leaf. | Source

What Was the Black Soot on My Gardenias?

The sooty substance is actually called sooty mold, and it is a fungus that is spread by insects. It is an unattractive nuisance, but in itself it is rarely lethal.

According to the Cooperative Extension Service of Hawaii, the most common sooty molds are Cladosporium, Aureobasidium, Antennariella, Limacinula, Scorias, andCapnodium.

Sooty mold is easily differentiated from other diseases by the ease with which it wipes off. It does not actually penetrate the leaf's surface, so no damage is done to the structure of the plant. The fungus will interfere with the plant's photosynthesis process, so a severe infestation will weaken a plant considerably.

White Flies and Aphids

Sooty mold feeds upon the "leavings" of insects, mostly whiteflies, aphids, scales and mealybugs, which feed on the sap of plants. While feeding on a plants sap, the insects will excrete a substance called honeydew. A heavy insect infestation will cause all of a plant's leaves to be coated with honeydew. A heavily infested tree will drop the sticky substance down onto lower plants.

The fungi start feeding on the honeydew and it quickly spreads to surrounding leaves, stems and plants. Like most fungi, sooty mold spores are always present in the environment, and they are easily spread by wind, water and contact. The best solution to the mold is prevention and insect control.


Plants susceptible to sooty mold

 
 
 
Crape myrtle
Apple and Crabapple
Viburnum
Pear
Fir
Citrus trees
Hawthorn
Buckthorn
Oak
Pecan
Privet
Linden
Tulip tree
Ligustrum
Arborvitae
Dogwood
Juniper
Mountain Ash
Magnolia
Maple
Walnut
Gardenia
Holly
Sassafras
Beech
Azalea
Rhododendron
Closeup of aphids on the stem of a flower.
Closeup of aphids on the stem of a flower. | Source

Which insects spread sooty mold?

Sooty mold feeds upon the "leavings" of insects, mostly whiteflies, aphids, scales and mealybugs, which feed on the sap of plants. While feeding on a plants sap, the insects will excrete a substance called honeydew. A heavy insect infestation will cause all of a plant's leaves to be coated with honeydew. A heavily infested tree will drop the sticky substance down onto lower plants.

The fungi start feeding on the honeydew and it quickly spreads to surrounding leaves, stems and plants. Like most fungi, sooty mold spores are always present in the environment, and they are easily spread by wind, water and contact. The best solution to the mold is prevention and insect control.


Did you know sooty mold was caused by insects leaving honey dew on plants?

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Identifying the Pests

To rid your plants of sooty mold, you need to control the insects which are producing the sugary honeydew that the fungus feeds on.

Insecticides are useful for controlling insect populations, but first you must identify which insects are feeding on your plants.

The whitefly causes sooty mold.
The whitefly causes sooty mold. | Source

Whiteflies

The adult whitefly is 1/32" long with white wings. It feeds on the underside of leaves. The eggs will be laid on the underside of leaves, usually in small groups.


Picture showing aphid colors. Aphids produce honeydew which feeds sooty mold.
Picture showing aphid colors. Aphids produce honeydew which feeds sooty mold. | Source

Aphids

Aphids are approximately one-tenth of an inch long and have two long antenna. They may be grey, brown, green or black.




Scale insects also feed sooty mold. (Coccoidea)
Scale insects also feed sooty mold. (Coccoidea) | Source

Scales

Scales are small insects that have a protective waxy coating covering their bodies. They are approximately 5 mm long, and are usually brown or tan in coloring.

Mealybug - Pseudococcus calceolariae - are a contributor to sooty mold.
Mealybug - Pseudococcus calceolariae - are a contributor to sooty mold. | Source

Mealybugs

The mealybugs you see on plants are the females. They are one-tenth of an inch long, and are surrounded by a cottony substance which they deposit on the leaves and stems of plants.

Killing the Pests Which Cause Sooty Mold

There are quite a variety of insecticides available to control the insects munching on your plants. I prefer to not use a broad-based insecticide; I want to kill the pests not the beneficial insects.

I have had success making a solution of soapy water and spraying my plants every few days. I use two big squirts of dish soap to one gallon of water in a one-gallon sprayer.

It is important to douse every inch of the plant, paying special attention to the underside of leaves.


For whitefly and aphid control, try products containing neem oil or paraffinic oil.

Neem Oil

  • Bonide Safer
  • BioNeem®Green Light Neem®
  • Green Light Rose Defense®
  • Southern Ag Triple Action Neem Oil®

Paraffinic Oil

  • Bonide All Seasons Horticultural and Dormant Spray Oil®
  • SunSpray Horticultural Oil®

For scales and mealybugs try products containing either fish oil or horticultural oil.


Fish oil

  • Organocide®

Horticultural oil

  • Volvk®

What should you do about the sooty mold?

While there are fungicides available to kill the mold, I have never found it necessary to use one. Once the bugs are gone, the food source for the sooty mold is also eliminated. The fungus will slowly starve to death.

As to the residual mold, spraying my plants with the soapy water washes most of the mold off. If there is still quite a bit of sooty mold left, I use a garden hose with an adjustable spray nozzle to finish the job.


An interesting note about ants and sooty mold

Ants love to feed on honeydew! They will protect the honeydew producing pests from their natural predators such as ladybugs.

If you see ants crawling on your plants, you may have honeydew producing pests starting to feed on your plant's leaves. If you rid your plant of the ants, you may allow the natural predators to come in a wipe out your pest population.

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    • Sheri Faye profile image

      Sheri Dusseault 3 years ago from Chemainus. BC, Canada

      I have been an avid gardener for years and did not know that bugs leave henydew that feeds the mold! What do you know! Well done and very informative!

    • Gcrhoads64 profile image
      Author

      Gable Rhoads 3 years ago from North Dakota

      Hi, Sheri Faye. I'm glad I could help. :)

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