Sooty Mold - Ask Extension
I have what appears to be sooty mold on two adjacent Crepe Myrtle bushes/ trees.
Both plants are about 15 feet tall and both are slender. The previo...
Knowledgebase
Sooty Mold #879176
Asked July 29, 2024, 8:52 AM EDT
I have what appears to be sooty mold on two adjacent Crepe Myrtle bushes/ trees.
Both plants are about 15 feet tall and both are slender. The previous owner had someone prune this plants several years ago, but they are now both unshapely.
Both are heavily infested. What is the best way to treat these plants?
Should I just cut down the plants to its base or should I prune the plants and then attempt to spray them? Seeing as how they are both tall, this might present a little difficulty in spraying the tops.
Finally, should I place the plant’s cuttings in a yard waste disposal or bag them somehow?
Thank you.
Sussex County Delaware
Expert Response
Thank you for reaching out to Ask Extension. You’ve called out two issues - pruning of crape myrtles and sooty mold. Let’s look at the latter first.
Sooty Mold is indicative of an insect infestation from piercing/sucking type of insects. When the insects excrete honey dew after feeding, the product is sweet and these sugars are what invites the fungus that causes black sooty mold. The mold itself is very unsightly, but does not have an adverse effect on the tree, however it can slow photosynthesis. Knowing what type of insect infestation is crucial to how both prevent and treat the problem going forward. Aphids are typically the most common.
A new issue Crape Myrtle Bark Scale (CPBS) has emerged beginning in the southern states and has been detected in Delaware. Scale insects are more difficult to control due to their waxy “scale” protection. It is important to distinguish scale from aphids so that the life cycle can be determined and treatment options as spraying occurs when the pesticide can make direct contact with the crawlers as they emerge from their protective covering. Because CMBS is relatively new, experts are hoping biological controls will evolve and develop — chemical controls are still being evaluated. Mississippi State Extension, in writing about CMBS specifically, do address the removal of the trees as a control in this fact sheet.
If you have to deal with sooty mold on Crape Myrtles, aphids, and other sucking insects as the source are easier to control and are the better problem to have. Aphids can be controlled by pesticides and horticulture oils. Louisiana State University has useful information on how to control. LSU also indicates that over fertilizing may be a contributing factor to attracting insects. Spraying with horticulture oil is a favored treatment as explained well in this NC Extension fact sheet.
Successfully controlling the insect will not remove the sooty mold that is already there. It is a bit impractical to hand wash leaves, but if the sooty mold is on the trunk and main branches this can be washed off with a mild detergent and water. Since the trees are deciduous, the problem will fall off the tree in autumn. Generally, it is not recommended to compost any affected leaves. To prevent this next year, refer to the LSU fact sheet on treating the tree in late spring (May) with a preventative, systemic treatment.
Pruning
Pruning a crape myrtle for sooty mold is not effective treatment.
How Crape Myrtles are pruned is often a personal aesthetic choice and is a practice that is much debated among owners, tree experts and extension staff. You may have heard the term “crape murder” which some ascribe to the practice of removing the entire canopy of a tree in the spring, resulting in new growth that is unnatural, often causing growth from “knuckles” at the top of the tree. The new growth will bloom in summer but the stems will be weaker than if allowed to grow naturally. Once that has been done to a tree, it is nearly impossible to restore the tree to its original form. But “corrective pruning” of the tree is often necessary. Crossing limbs that touch each other will eventually fuse together, creating a large clump in the tree’s profile. Many owners (myself being one of them) constantly remove new growth suckers from the ground as they emerge, and small twigs that emerge from established trunks/branches. Crape Myrtles allowed to grow naturally without ground pruning will resemble large shrubs rather than trees. I prefer a tree shape that has an odd number of main trunks or branches emerging from the ground, and are clean to reveal their beautiful exfoliating bark. I raise the canopy high enough that I can walk under my more mature trees - and this helps me also inspect the bark to look for scale. Removing a branch that is causing an imbalance in the overall shape should be done after they bloom - so anytime in September or October would be appropriate or in the early spring when you can stand back and look at the shape. Any branch removed that has active insect activity on it should be destroyed.
Cutting the trees down to the base will not prevent your sooty mold issue. Crape Myrtles are almost impossible to eradicate, and where you had one trunk, many more trunks and growth emerging from the roots will emerge. I recommend evaluating the shape of your trees, remove branches that are growing outside your desired shape, and treating the trees in May to prevent your sooty mold issue. You do however, want to eliminate “Crape Myrtle Bark Scale” as the cause/agent.
Sooty Mold is indicative of an insect infestation from piercing/sucking type of insects. When the insects excrete honey dew after feeding, the product is sweet and these sugars are what invites the fungus that causes black sooty mold. The mold itself is very unsightly, but does not have an adverse effect on the tree, however it can slow photosynthesis. Knowing what type of insect infestation is crucial to how both prevent and treat the problem going forward. Aphids are typically the most common.
A new issue Crape Myrtle Bark Scale (CPBS) has emerged beginning in the southern states and has been detected in Delaware. Scale insects are more difficult to control due to their waxy “scale” protection. It is important to distinguish scale from aphids so that the life cycle can be determined and treatment options as spraying occurs when the pesticide can make direct contact with the crawlers as they emerge from their protective covering. Because CMBS is relatively new, experts are hoping biological controls will evolve and develop — chemical controls are still being evaluated. Mississippi State Extension, in writing about CMBS specifically, do address the removal of the trees as a control in this fact sheet.
If you have to deal with sooty mold on Crape Myrtles, aphids, and other sucking insects as the source are easier to control and are the better problem to have. Aphids can be controlled by pesticides and horticulture oils. Louisiana State University has useful information on how to control. LSU also indicates that over fertilizing may be a contributing factor to attracting insects. Spraying with horticulture oil is a favored treatment as explained well in this NC Extension fact sheet.
Successfully controlling the insect will not remove the sooty mold that is already there. It is a bit impractical to hand wash leaves, but if the sooty mold is on the trunk and main branches this can be washed off with a mild detergent and water. Since the trees are deciduous, the problem will fall off the tree in autumn. Generally, it is not recommended to compost any affected leaves. To prevent this next year, refer to the LSU fact sheet on treating the tree in late spring (May) with a preventative, systemic treatment.
Pruning
Pruning a crape myrtle for sooty mold is not effective treatment.
How Crape Myrtles are pruned is often a personal aesthetic choice and is a practice that is much debated among owners, tree experts and extension staff. You may have heard the term “crape murder” which some ascribe to the practice of removing the entire canopy of a tree in the spring, resulting in new growth that is unnatural, often causing growth from “knuckles” at the top of the tree. The new growth will bloom in summer but the stems will be weaker than if allowed to grow naturally. Once that has been done to a tree, it is nearly impossible to restore the tree to its original form. But “corrective pruning” of the tree is often necessary. Crossing limbs that touch each other will eventually fuse together, creating a large clump in the tree’s profile. Many owners (myself being one of them) constantly remove new growth suckers from the ground as they emerge, and small twigs that emerge from established trunks/branches. Crape Myrtles allowed to grow naturally without ground pruning will resemble large shrubs rather than trees. I prefer a tree shape that has an odd number of main trunks or branches emerging from the ground, and are clean to reveal their beautiful exfoliating bark. I raise the canopy high enough that I can walk under my more mature trees - and this helps me also inspect the bark to look for scale. Removing a branch that is causing an imbalance in the overall shape should be done after they bloom - so anytime in September or October would be appropriate or in the early spring when you can stand back and look at the shape. Any branch removed that has active insect activity on it should be destroyed.
Cutting the trees down to the base will not prevent your sooty mold issue. Crape Myrtles are almost impossible to eradicate, and where you had one trunk, many more trunks and growth emerging from the roots will emerge. I recommend evaluating the shape of your trees, remove branches that are growing outside your desired shape, and treating the trees in May to prevent your sooty mold issue. You do however, want to eliminate “Crape Myrtle Bark Scale” as the cause/agent.
