Not all crape myrtle cultivars are alike. Some are more susceptible to diseases and insect pests more than others. Overall, the U.S. National Arboretum (USNA) hybrid crapemyrtles are more disease and insect resistant, but not to all problems.

Some cultivars are susceptible to aphids, usually in the spring. Their sugary excretions over leaves and stems blacken them with a sooty mold. Eventually, leaves yellow and the shrub may bloom poorly. Spray leaves with malathion, diazinon, or an ultra-fine horticultural oil when you spot aphids or the sooty mold symptom.

In late spring and summer, metallic dark green flea beetles may chew the leaf margins in an irregular pattern. Damage is usually slight and does not warrant pesticide spraying.

In late July Japanese beetles chew on the flowers and skeletonize the leaves. Many pesticides are effective to reduce Japanese beetle numbers. Hand removal works but may becomes a daily chore. Pheromone traps are largely ineffective because they attract many more Japanese beetles than they trap.

A strong stream of water from your garden hose can blasted many injurious insects off the foliage. Insecticidal soap sprays are also effective.

Powdery mildew is a fungus disease that deposits a white powdery residue over the leaves and flower buds. Symptoms are worse in high humidity, during lack of rain, and in areas with poor air circulation. If left unchecked your crape myrtle can be weakened and eventually die. Most USNA crapemyrtles are resistant to powdery mildew. Spray susceptible cultivars at the first sign of disease with Funginex™ or Immunox™ and repeat sprays as necessary. Horticultural oil has been shown to be quite effective in controlling powdery mildew.

In the deep South crapemyrtles may shed some leaves with brown spots. This is symptom of Cercospora leaf spot fungus disease during periods of warm, wet weather. In severe cases, only the youngest leaves at the ends of branches remain. Repeated annual infections by cercospora may result in loss of plant vigor and reduced flowering.

These U.S. National Arboretum introductions have some resistance to Cercospora disease: Apalachee, Caddo, Catawba, Sioux, Tonto, Tuscarora, Tuskegee, and Yuma. You can prevent this disease on a susceptible cultivars by spray any fungicide products such as Bayer Advanced Rose and Shrub® or Thiram® when leaf spotting is observed; continue applying the fungicide at weekly intervals through mid-summer.