Crape myrtle trees are susceptible to many diseases. You can identify many diseases through their initial symptoms and choose a treatment based upon the type of infection.
The Cercospora lythracearum fungus causes cercospora leaf spot, while the Erysiphe lagerstroemia fungus causes powdery mildew infections in crape myrtles. Crape myrtle aphids also indirectly cause a disease called sooty mold.
If your tree is infected with cercospora leaf spot, it will have irregular brown spots on the leaves that gradually grow until the leaf turns yellow and falls from the tree. Identify powdery mildew by the cottony white patches that form on leaves and shoots; the leaves may turn brown, curl or prematurely fall. Some diseases are also caused by insects. If you see small yellowish-green insects on your tree or see a sticky residue or black fungus on the leaves, crape myrtle aphids are probably feeding on plant sap and exuding a sticky-sweet liquid that acts as a growing medium for sooty mold.
Treat cercospora leaf spot and powdery mildew infections with timed applications of fungicides. Kill the insects to treat sooty mold by spraying the tree with a steady stream of water to dislodge the bugs or spraying the bugs with an insecticide. When the insects are gone, wash the mold from the plant with a mild solution of water and dish soap.
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