Crape Myrtle Diseases

The Crape Myrtle group of trees and shrubs are typically very low maintenance and resistant to most pests and diseases, especially when all of its requirements for full sun, well-draining soil, and moderately fertile soil are met. The most common diseases afflicting Crape Myrtle are fungal infections that are very dependent on atmospheric conditions.

Powdery Mildew thrives in cool dry weather whereas the Cercospora Leaf Spot is active during warm and humid temperatures. Both diseases are overwintered in leaf litter or the woody stems of the Crape Myrtle and can be managed with basic garden tasks like removing early leaf drops or spent foliage at the end of the growing season and careful removal of spent flowerheads or affected branches. 

Powdery Mildew

Erysiphe lagerstroemiae is the name of the particular fungus responsible for causing Powdery Mildew on Crape Myrtle trees and shrubs. Even though this is a species-specific fungus, it is carried by wind to other parts of the garden and can lay dormant on host weeds or in the soil. Cool temperatures and low humidity in the spring or early fall are when Powdery Mildew is most active. This fungus does not tolerate damp conditions or temperatures over 90F.

Growing Crape Myrtle in too much shade is the number one way to increase the chances of Powdery Mildew occurring. Crape Myrtle requires at least 6 hours of full sun to bloom and grow healthy clean and healthy foliage. Powdery Mildew will reoccur seasonally on susceptible plants but will not greatly affect their overall growth, other than causing cosmetic mostly cosmetic damage. 

Identifying Powdery Mildew

White or greyish spots on the top or bottom of foliage. Stems and flower buds can also become infected. New and old growth is affected in woody plants with new shoots becoming distorted and smaller in size. Early leaf drop is common. 

Treating Powdery Mildew

Growing Crape Myrtle varieties that have some mildew resistance is the best way to avoid Powdery Mildew. Resistant cultivars are always clearly labeled at the garden center or nursery. Plant the tree or shrub in full to part sun, making sure the soil is well draining.

Avoid heavy mulching, especially near the base of the trunk. Good garden hygiene at the end of the growing season will keep dead plant material from overwintering any mildew spores. Remove affected foliage and stems if possible.

Application of horticultural oils late in the winter and early spring as the new growth appears may help to control mildew but can be difficult to use on large trees and shrubs. Ensure that any fungicide is specifically labeled for use on Crape Myrtle to protect the plant from chemical damage. Do not treat Crape Myrtle with fungicides when temperatures are over 90F or if it is stressed from underwatering.

Cercospora Leaf Spot

Fungal Leaf Spot is common on many plants, mainly occurring as a result of poor garden hygiene practices over many seasons. The fungus can survive in dead top growth over the winter, accumulating in the soil until conditions are right for it to become active and spread to surrounding plants. Cercospora Leaf Spot thrives in moist situations and easily moves between leaves and plants during rain storms or by the wind.

The fungus will reoccur year after year until the life cycle is broken. Woody trees and shrubs like Crape Myrtle may be able to continue normal growth for quite a while but over time the health of the plant will decline dramatically as the foliage is repeatedly damaged. 

Identifying Cercospora Leaf Spot

Brown spots that occur on any portion of the leaves start out as yellow or black dots which quickly spread forming large irregular spots. The spots will become crispy and dry over time. The foliage often drops earlier than normal. Activity and spreading happen during periods of high humidity.

Treating Cercospora Leaf Spot

Proper garden hygiene makes all of the difference in controlling outbreaks of Leaf Spot. The spores can survive in the soil until conditions are right for an infection. Prune the bottom branches to be clear of the ground and improve air circulation. Mulching the soil well helps to keep spores from splashing on plants during watering or natural rainstorms.

Horticultural oils such as NEEM may be useful to protect new growth early in the spring. Along with copper-based fungicides, any chemical treatment needs to be specifically listed for use on Crape Myrtle plants. 

Common Crape Myrtle Disease Chart 

Sources:

“Trees, Shrubs, and Woody Ornamentals Diseases”, Home and Landscape Plants, University of California IPM program. www.ipm.ucanr.edu