There are a number of options for powdery mildew control on crepe myrtles. Among your least toxic options are products that contain either of the following ingredients: neem oil (not just Azadirachtin, but instead "neem oil") or potassium bicarbonate. Potassium bicarbonate is similar to baking soda (which is sodium bicarbonate) and is sold under the brand name Remedy. Read the label for application instructions and frequency. As a general rule, these sprays generally need to be applied on a 10-14 day basis when conditions favor powdery mildew development.
Synthetic fungicides containing bayleton are a very effective and longer lasting control option. Your County Extension Office can offer other options for powdery mildew control.
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