Question of the Week 2: February 23, 2017
“Neil, how can I get rid of the black mold on my crape myrtle stems? Will it harm the plants?”
(I posted this on my Facebook page a few days ago. The longer I worked on the post for Facebook, the more I realized that I wanted to share it with my e-gardens friends as well.)
This is a fungus known as sooty mold, and here is what Texas A&M has in their Plant Disease Handbook.
Sooty mold develops in the sticky residue that is exuded by aphids, scale insects and other pests. The fungus is not particularly harmful to the crape myrtles or to other plant species on which it may appear. In fact, you’ll see it on concrete, patio furniture and even old cars that are parked for periods of time beneath large shade trees.
The photo is of a crape myrtle stem that is unusually heavily coated in sooty mold. Most of this mold will slough off as the plant sheds its old bark this spring. It could also be washed off with a hard stream of soapy water.
In this particular case, this stem shows evidence of last year’s crape myrtle scale insects (the white specks). They exuded the honeydew which in turn gave rise to the sooty mold. There are no active scales on the stem at this point, but they could return this year.
It’s important to note that this is a comparatively new insect pest in the United States, showing up first in Richardson, Texas, in 2004. The one year that was especially bad was 2007 when a very rainy and cool June encouraged scales of all types on many species of plants. Since then a very special ladybug called the twice-stabbed ladybug has kept crape myrtle scale populations in check. See her in the photos on the page to which I’ll link you in the next paragraph.
I’ve been watching both the scale and sooty mold for 14 years and I have never seen a crape myrtle that was killed by either. However, if you want to avoid the messiness, this page from our Crape Myrtle Trails of McKinney should help. http://crapemyrtletrails.org/pest-control/. Scroll down to the TAMU information.