The crepe myrtle, sometimes spelled crape myrtle or crapemyrtle, is a large, flowering tree. It is used frequently in southern gardens because the hot summer climate allows it to thrive. Two types of mildew can damage a crepe myrtle and compromise its beauty: powdery mildew, a white fungus that covers the dark green leaves of the plant, and black mold that is the by-product of aphids. Improving air circulation within your crepe myrtle and applying chemical controls can eliminate mildew problems and restore the plant's beauty.
Treat mildew by improving the air circulation in your crepe myrtle's canopy, or the upper- and outer-most areas of the tree's leaves. Powdery mildew thrives in hot, humid environments and will not live as well when the air flows freely through the branches. The Mississippi State University Extension recommends pruning back your crepe myrtle during its winter's dormancy for best results. Remove small twig-like branches that are located inside and below the tree's canopy.
Spray your crepe myrtle with fungicides as soon as you see mildew on the leaves. A variety of chemicals can control powdery mildew, including neem oil, sulfur-based sprays, propiconazole, potassium bicarbonates and tebuconazole. Depending on the severity of the disease, you may need to apply fungicides every week or two. Alternate using formulas with different active ingredients to prevent the crepe myrtle from developing a resistance to any one kind of fungicide.
Treat black mold by getting rid of the pests creating the mold. Aphids are common predators and leave behind a black residue. The North Carolina State University Cooperate Extension reports that chemicals effective against aphids include insecticidal soap, horticultural oil, pyrethroids and organophosphate. Application intervals vary; if the aphids reappear, treat the tree again to prevent another round of black mold.