What kinds of diseases can my crepe myrtles get?
Crepe myrtles are pretty tough and low maintence so the list of diseases is relatively short. The two most common diseases found among crepe myrtles are powdery mildew and leaf spot disease. (If your crepe myrtle has sooty black mildew on it, that’s actually caused by aphids which you can find information on below.)
Powdery Mildew (Erysiphe lagerstroemiae)
The symptoms: White powdery film that develops first on new shoots and then spreads to stems, flowers, and leaves which may show stunted or distorted growth.
The low down: This whitish powdery mildew is almost exclusive to crepe myrtles and most often only in older breeds of the plant. So don’t worry about it spreading to non-crepe myrtles. Most newer breeds have a strong resistance to powdery mildew. You can see what level of resistence a breed of crepe myrtle has by looking at information about the varieties of crepe myrtle. This mildew typically shows up in early June and gets worse as summer progresses. Stagnant air, high humidity, and a lack of rain tend to make it worse.
The solution: The best solution is to get crepe myrtles that have a natural resistance to powdery mildew, but if you are stuck with an older breed there are some things you can do to help your ailing plant. Make sure your crepe myrtle is in a place that has a good amount of air circulation and sun. If you believe fungicide is warranted, try a preemptive strike with a fungicide labeled for powdery mildew. You should start dosing your tree when the leaves are fully expanded and continue dosing your tree once a month until the flowers start to bloom.
Leaf Spot Disease (Cercospora lythracearum)
The symptoms: Brown spots begin to appear on lower leaves around June or July and begin spreading to higher leaves. By mid summer your crepe myrtle’s leaves will begin turning yellow to red and drop from the tree. By late summer most, if not all, of your crepe myrtles leaves will be gone.
The low down: This nasty little fungus invades the leaves of your crepe myrtle and sets up shop producing spores that travel to higher leaves. It thrives in warm wet weather and stagnant air.
The solution: Once again, the best solution is to get a crape myrtle that is resistant to this type of fungus (Catawba, Tonto, Tuscarora or Natchez etc.) but if you are stuck with the disease, you can still combat it with your crepe myrtle’s location and fungicide. Just like the powdery mildew, you can prevent this fungus by making sure your crepe myrtle is in a place with a lot of air circulation so it’s leaves can dry quickly. Fungicides should be used every one to two weeks starting in June or July when it is warm and wet.
What kind of pests can attack my crepe myrtle?
For the most part crepe myrtles don’t have many problems with pests. Japanese beetles (Popillia japonica) or metallic flea beetles (Alticini tribe) may cause some issues depending on where you live, but these normally aren’t too much of a problem. The real problem pest is an aphid that feeds specifically on the crepe myrtle.
Crepe Myrtle Aphid (Tinocallis kahawaluokalani)
The signs: Pale yellow or green aphids that are found on the underside of the leaves. Black or sooty colored mold (Capnodium) that grows on the sugary droppings of the aphids which cover the leaves.
The low down: These little critters are a pain for crepe myrtles but they don’t feed on any other plants that are commonly grown, so at least your other plants are safe. They drink the plant sap from the soft tissues of the crepe myrtle and they excrete sugary droppings called “honeydew”. The sooty mold then grows on the “honeydew”. The sooty mold usually doesn’t cause any problems for the crepe myrtle aside from blocking some of the sun. Dealing with the aphids will eventually get rid of the mold. They reproduce very quickly and population peaks generally occur mid summer but can vary greatly.
The solution: The local insect predators will sometimes keep these aphids at bay but often they can’t keep up, especially if insecticides are being used in the area. You can blast off the aphids with a strong stream of water but the best method to get rid of them is probably to just use an insecticidal soap on the crepe myrtle.