Karen Cambre, Sharpe, Kenneth W.
News article for April 23, 2018
Crape myrtle trees are very popular in landscapes. They fill the bill of being a small, summer-flowering, deciduous tree. They are usually one of the last trees to put on leaves in the spring, but earlier than pecans.
As summer approaches, it is not unusual for crape myrtles' lush green growth to turn black as they are covered in sooty mold. Sooty mold is the black, crusty material that encompasses the leaves during the growing season. It is very unattractive and a source of anxiety for many gardeners.
It is easier to prevent sooty mold than to control it, and that means starting early. Sooty mold is really an insect problem, and if you control the insects, you will not have sooty mold. You have heard that “an ounce of prevention is worth a pound of cure.” It applies here.
Sucking insects, such as aphids and scales, are attracted to crape myrtles and will suck the leaves for plant juices. As they digest the plant nutrients, they excrete a very sugary substance that we call honeydew. The first sign that you have a sucking insect problem will be very shiny or oily looking leaves. These leaves will feel sticky from the sugary honeydew. You will usually notice ants trailing up the tree to get the sugar, and you might notice a lot of wasps and bees flying around the tree also. If you park your car under the tree, you will see the honeydew dripping on your hood.
The sugar not only attracts other insects, it also attracts sooty mold, which can live and grow on the honeydew. You might see a white fence under the tree turn black, as well as patio furniture or even your car if honeydew is falling on them.
There are several options to prevent sooty mold. Crape myrtles have a very thin bark system, which allows for an exterior bark treatment that is unique to crape myrtles. I like to make the first application of insecticide about May 1 to prevent sooty mold.
You have two easy options.One is to use an acephate product, such as Orthene 75S or Acephate 75S. This is a soluble powder that mixes readily with water. Take 4 tablespoons (or 4 parts) of acephate and mix with 1 tablespoon (or 1 part) of water in a disposable container to make a paste. If you have a loose layer of exfoliating bark peeling off near the base of the tree, gently remove it. Then, paint the paste around the trunks of the tree near the base of the tree. The width of the band should be twice the diameter of the trunk. So, if you have a 2-inch trunk, paint a 4-inch band around the trunk about 3 or 4 inches above the soil or mulch line. Every part of the tree that is attached to that trunk will be protected. If you have multiple trunks, you will have to apply a band to each trunk.
This insecticide is systemic and will move to the leaves and twigs. If an insect comes along and pierces the leaves to feed, the insect will be killed, and you will not get sooty mold. This treatment will last through July, and then you will need to reapply one more time to take you through the end of this growing season.
I have found this to be a very easy and inexpensive way to control sooty mold.
Your other option is to use a drench around the tree roots that is designed to give one full growing season of control. These drenches or granular products are applied at the base of the tree and use an insecticide with the active ingredient Imidocloprid. This product may take a month to be absorbed by the roots and then travel into the tree leaves, so do not delay treatments.