If your crape myrtle's dark green leaves have turned white, your tree might be infected with mildew. Crape myrtles are prized where they can be grown for their beautiful flowers, long blooming season, and attractive, interesting bark. Unfortunately, some crape myrtle cultivars are susceptible to powdery mildew. Mildew is a fungus; the white powder you see is made up of spores. Mildew thrives in warm, moist conditions. It is best to prevent the growth of mildew, but if your tree is already infected, following these steps will help you restore it to its former beauty.
Confirm the problem. Examine your crape myrtle's leaves. Are they coated with a white, powdery-looking substance? Does the powder extend down to the stem, and are there any discolored or dead areas underneath the powder? If so, your tree has mildew.
Apply fungicide. Find a fungicide specifically formulated for mildew on crape myrtles or use a generic fungicide that contains one of these ingredients: propiconazole, tebuconazole, thiophanate-methyl, myclobutanil or triadimefon. Apply as directed; keep applying until the mildew has retreated.
Consider an organic alternative. If you prefer to use a nonchemical fungicide, choose one that contains neem oil, sulfur or potassium bicarbonate. According to the National Arboretum, horticultural oil can also control powdery mildew. Apply as directed.
Prune the tree. According to Mississippi State University, pruning will increase air circulation, decreasing humidity and making it less likely that mildew will spread. Prune any dead branches, but also prune any small, twiggy undergrowth near larger branches. After each cut, dip your pruners in white vinegar to kill any spores.
Take preventive measures. To avoid mildew in the future, water only in the mornings and keep trees pruned. Applying fungicide to new growth in the spring can also help prevent mildew.
Research new crape myrtles. When planting new crape myrtles, consider mildew-resistant cultivars. Plant the trees in full sun, and do not crowd them with other plants.