Crepe myrtle (Lagerstroemia) is a very popular shrub, with spectacular blooms that last for an extended period. This drought-resistant and heat-tolerant plant comes in many sizes and can be grown in most climates. If you want to grow crepe myrtles as trees, low shrubs, or container plants, you should know about all the common diseases and pests that can plague these plants.
In this article, we will focus on the main crepe myrtle diseases including prevention and treatment so you can grow healthy and happy plants in your garden.
Crepe Myrtle Pests and Diseases
Most of the diseases of crepe myrtles are caused by pests. These include mold and aphids. These common pests must be washed off the plant using a garden hose and an environmentally-friendly insecticide mixed with water.
There are several other predators that can feed on crepe myrtles, including ladybugs and their immature larvae, hoverfly maggots, wasps, ladybird beetles, and insect fungi. These predators can be removed from the plant by spraying them with a strong stream of water. Repeat the spraying regularly, if necessary.
Japanese beetles are another common pest that will eat the tree. Their larvae can produce enough beetles to destroy the entire tree. To prevent these pests from damaging your crepe myrtle, use traps and insecticides.
Some common diseases of crepe myrtles include.
This is one of the most common diseases of crepe myrtles, caused by a fungus that leaves patches of white powdery growth on the surfaces of leaves, new shoots or flowers. If the flower is heavily infected, its buds won’t open and the entire plant will usually stunt or distort. Powdery mildew most often occurs in damp, shady locations, where the crepe myrtle doesn’t receive adequate air circulation. Fungus develops at high humidity, usually during spring and fall.
Treatment and prevention: The most effective control measures include planting the crepe myrtle in full sun and removing the sprouts from the plant’s base. Removing infected branches will also help prevent future occurrences. If only a couple of shoots are infected, just remove those in order to stop the spread of powdery mildew. If left neglected, the infected sprouts will spread the fungus to the upper parts of the plant.
Cercospora Leaf Spot
Leaf spots are caused by a fungus species and typically occur during warm, moist weather. The symptoms include yellow spots that appear on the upper leaf surface with white or grey fungus sporulation appearing on the lower leaves. If left neglected, the disease can completely defoliate the plant in late summer or fall
Treatment and prevention: To avoid leaf spot, the easiest prevention measure is to select fungus-resistant varieties when planting in the garden. These hardy varieties include Tuskegee, Tonto, Fantasy, and Tuscarora, to name some of the few. However, the amount of fungus resistance of these crepe myrtle varieties can vary from location to location, depending on the specific environmental conditions in which they are grown.
As a rule of thumb, always provide your crepe myrtle adequate air circulation and avoid overcrowding the plant. If the leaf spot disease has severely infected the plant, use chemical fungicides, such as myclobutanil, and apply it according to the directions on the product’s label.
Black sooty mold affects the leaf and stem surfaces of the plant, causing them to go black. If your crepe myrtle has sooty mold, this is an indication of an insect problem. These molds are usually caused by fungi that grow a sticky substance (honeydew) on the plant. The sticky substance is produced by various sap-sucking insects. The most common insects that cause this disease are whiteflies, scales, mealybugs and aphids.
Treatment and prevention: Although sooty mold is an unsightly disease, it is quite harmless and doesn’t directly attack the plant. However, if you notice excessive amounts of sooty mold, you can control this pest problem by allowing beneficial insects, such as ladybugs to inhabit the plant. A strong spray of water will also remove the pests from the plant.
Other treatment options include washing the leaves by spraying a dish soap solution mixed with water. Leave the soap solution on the plant for 5 minutes and then rinse the foliage with a strong spray of water.
This unusual fungus is composed of algae and often appears as a green-greyish crust on the branches or trunks of crepe myrtles. In case of infestation, the plant’s health will quickly decline - but this rarely happens, unless the crepe myrtle wasn’t getting enough sunlight in the first place.
Treatment and prevention: Controlling lichens isn’t necessary as they are usually harmless. Most often they will disappear gradually once the plant’s health is restored.
Improper pruning is one of the biggest problems that can make crepe myrtle susceptible to diseases. To keep your plant healthy, remove the dead branches, crossing limbs, and any dense twigs that overcrowd the plant. This will help increase air circulation and prevent mold or fungal growth. It is not a good idea to top off all the branch ends as this will disfigure the plant and weaken its growth. This practice will also increase the tree’s susceptibility to disease and insect infestation.
Severely topped crepe myrtles can’t get their energy from their branches and leaves, thus resorting to growing more limbs rather than flowers. It is, therefore, best not to prune them heavily, especially not from the top. To improve the crepe myrtles surroundings, add a layer of mulch or compost every spring, and remember to water the plant more often during summer. For a full guide on how to prune a crepe myrtle, you can read this article.
Keep in mind that none of the diseases we’ve mentioned can kill your crepe myrtle. But they are unsightly and cause gradual leaf loss, which means you won’t get the most out of your plant. To prevent these diseases, always prune your crepe myrtle before the growing season, and ensure the plant receives adequate sunlight. By following our treatment and prevention tips, you’ll have thriving crepe myrtles from spring right through winter.