Get it growing
As summer wears on, it’s not uncommon to spy a crape myrtle in the Louisiana landscape whose bark — and maybe even leaves — appears to have grown a powdery, black crust.
Usually, this is not a disease. It’s more likely to be something called sooty mold.
“It’s actually a fungus that is a symptom of the problem,” said LSU AgCenter entomologist Aaron Ashbrook. “It is growing on the sugary excretions from several different pest insects that are on the tree.”
Those pests often include aphids, scales and mealybugs. They favor crape myrtles but also target other trees and shrubs, and they tend to come out in force in the heat of summertime.
“They’re using their piercing-sucking mouthparts to feed upon the xylem and phloem to obtain sugary nutrients from the plant itself,” Ashbrook said.
The insects’ sticky waste, known as honeydew, lands on bark and leaves.
“Honeydew has a little bit of sugary shine to it, and ultimately, that material will grow the sooty mold,” Ashbrook said.
The problem isn’t limited to the infested plant. Honeydew can drip onto anything nearby — other plants, mulch, sidewalks and vehicles, for instance — and create sooty mold breeding ground.
(By the way, if you’ve ever stood under a crape myrtle and felt mysterious water droplets hitting your skin, that probably was honeydew.)
To get rid of sooty mold, you must get rid of the insects that cause it. To do this, Ashbrook suggests applying a systemic insecticide to the plant’s roots.
“But we want to be protecting our pollinators, so be sure to use those products when flowers and blooms are not present,” Ashbrook said.
Alternatively, spray the plant with a solution of 1 tablespoon of natural castile soap mixed into 1 gallon of water to help control both insects and sooty mold. You also can dislodge insects with a strong blast of water.
Ashbrook recommends keeping ant populations in check, too.
“They will farm or tend the scales or aphids,” he said.
Keep in mind that sooty mold won’t vanish immediately, even once the insects are under control.
