What's That Black Gunk On My Plant?
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.
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.