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

Gable Rhoads has an AD in radiography. She is passionate about her family, animals, gardening, and the odd and unusual.

Gardenias are just one of the plants affected by black soot.

Gardenias are just one of the plants affected by black soot.

Why Did 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.

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.

Plants That Are 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.

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.

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.

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.

Aphids

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

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.

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.

How to Kill 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.

  1. I have had success making a solution of soapy water and spraying my plants every few days.
  2. I use two big squirts of dish soap to one gallon of water in a one-gallon sprayer.
  3. 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.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Comments

Gable Rhoads (author) from North Dakota on May 07, 2013:

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

Sheri Dusseault from Chemainus. BC, Canada on May 07, 2013:

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!