What’s That Black Gunk On My Plant?

September 10, 2015 | By | Comments (5)
Sooty Mold

Sooty mold on crepe myrtle. Photo: lsuagcenter.com

Just when it’s nice to go out and explore the garden again, you discover that your gardenia, crepe myrtle, and other plants have this ugly, black gunk plastered all over the leaves and stems. Did some maniac get loose with the roofing tar?

Nope. What you are looking at is a fungus called sooty mold. It grows on the sticky honeydew secreted by insects that suck plant sap, such as aphids, scales, and white flies. Without the honeydew, there wouldn’t be any sooty mold.

Sooty mold grows on lots of plants — crepe myrtle, gardenia, azalea, camellia, citrus, holly, and magnolia to name a few. It’s more common to see it towards the end of the growing season than the beginning. And that’s good, because while it doesn’t attack these plants directly, when it covers the tops of their leaves, it interferes with photosynthesis.

Sooty mold

Sooty mold on gardenia. Photo: gardenweb.com

How To Get Rid Of Sooty Mold
Well, if you’re the energetic type, you can use a rag and some soapy water to remove the mold by hand. You have nothing to do this weekend, right? Of course, if the insects are still around, the mold will grow back. So you have to get rid of the bugs.

To do so, spray your plant with insecticidal soap, neem oil, or horticultural oil according to label directions. These are good products for organic gardeners. Be sure to wet all leaf and stems surfaces. If “chemicals” don’t bother you, apply a systemic insecticide such as Bayer Advanced Tree & Shrub Insect Control. It’s absorbed by the plant and works up to 12 months. Just don’t use it on anything you plan to eat.

Of course, control assumes you can reach the source of the honeydew. Yesterday, I was visiting Aldridge Gardens in Hoover, Alabama when I discovered an entire corner of a garden bed covered in sooty mold. Victims included sasanqua camellias, beautyberry, and oakleaf hydrangea. When I examined their leaves, I found no trace of sucking insects. Where was the sticky honeydew coming from?

Look up, Grumpy. The branches of a nasty, old hackberry tree were hanging directly above the sooty shrubs. Aphids love hackberries, so honeydew was dripping on everything below. In a case like this, you have but two options — live with the sooty mold or cut down the nasty, old tree.

Grumpy votes for the latter.


  1. Nycitypestcontrol

    There’s a lot of black gunk over my leaves and stem. Thought it was some sort of dirt and I was just watering them over and over again in vain. Gosh, I didn’t know my plants were infected. Glad that I found your blog. Thank you very much for the advice. Will implement the methods today. By the way, do you have a natural alternative for insecticidal soap?

    March 22, 2016 at 3:42 am
  2. Ellie

    I have been fighting to find out what that black stuff was on my gardenia bush, no one seemed to know! Wala! And along you came. I have sprayed the plant with dawn and water, fungus insecticide to no avail. It is in very close proximity to a camphor tree, it’s limbs hang over it. Now after reading your information, cutting the tree down is not an option. So I’ve thought of two different ways maybe trim the tree back OR cut the bush way back and transplant it in another area free of trees. Please help, this gardenia plant was given to me when my husband passed away(my favorite flower). And he planted the tree. What do you suggest ???

    March 14, 2016 at 9:36 am
  3. januaryfarmer

    Thanks Grumpy. I am new to the Deep South and I got some black leaf crepe myrtles needing some treatment. No, I will not saw them to the ground or, worse yet, about a foot or two off the ground.

    September 11, 2015 at 11:06 am
  4. vonnie

    I have been spraying my bushes with this stuff with dawn dishwashing detergent and water and it works pretty good although it seems to come back the next year.

    September 10, 2015 at 4:45 pm
  5. Debbie Nobles

    how to get rid of bindweed that has taken over my garden big time.I had surgery on my foot and couldnt get out to do anything.It has taken over.Don’t want to move my garden as the area is fenced,with drip irragation.The area was covered in thick black plastic in the beginning.I cut the plastic to plant.I had…a nice start on herbs.I don’t want to use roundup as my 2 mini horses are near by.I also have chickens close by.I have called my county master gardener and she said to “Roundup the hell out of it”.Do you have any other ideas.

    September 10, 2015 at 2:02 pm

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