Many of you know that I work as a chess coach for the Mountain Lake Chess Camp, led by the legendary International Master Larry D. Evans. Last Sunday before one of our big scholastic chess tournaments, Larry’s wife Sharon and I were standing outside looking at the landscaping and she asked “What is this black, sooty-looking stuff on this tree?” (a Crepe Myrtle or Lagerstroemia).
The black sooty-looking stuff commonly occurs in San Diego gardens and landscapes; you’ve likely seen it on plants like Bird of Paradise, Lemons, Limes, any citrus, Gardenia, any type of Ficus, palms, Star Jasmine, Podocarpus, Pittosporum, Ivy (Hedera), Rhaphiolepis, Eugenia, and others.
This black stuff is actually a fungus called “sooty mold” that grows on the sugary secretions of sucking insects like aphids, mealybugs, and scale. You’ll notice that the black stuff rubs off pretty easily. Luckily for this Crepe Myrtle, all the leaves fall off at the end of the year. But for citrus and palms, for example, the leaves remain on the plant. So what does one do?
The presence of sooty mold indicates that there is a sucking insect on your plant or on a plant above the one with the sooty mold. Sooty mold frequently occurs on plants because there are bugs in a plant above it. For example, I commonly see sooty mold on plants growing under native oak trees. In the oak tree, the bugs secrete sticky stuff that falls down on plants under the oak. Sooty mold then grows on the plants under the oak.
In either case, if you get rid of the bugs, the sooty mold dies. There is no reason to “control” the sooty mold. The sooty mold doesn’t hurt the plant, but leaves the plant with the feeling of dirty glasses. That is, the plant’s ability to photosynthesize is diminshed, because the sooty mold covering the leaves.
First you need to get rid of the bugs in the plant. Then, you will need to clean or remove the sooty mold from the plant.
First, the presence of sooty mold indicates a sucking insect is present. An organic, Neem Oil-based product will work well here. See my post from June 5, 2019
Then, once the bugs are gone, you will still need to remove or clean the sooty mold on the leaves. I have found two products that help remove sooty mold: Simple Green (ready-to-use) and Dawn dish soap mixed as 2 Tablespoons per gallon of water.
Thoroughly spray the soap all over the plant, letting the soap soak into the sooty mold. Wait a minute or two, the blast the plant with water, preferable with a high pressure nozzle to remove the sooty mold. Sooty mold that has been present for a few years is thicker, harder, and may require a few soap and water applications to remove it.