Cercospora Leaf Spot (CLS) is a common fungal disease of Crepe Myrtles (Lagerstroemia), caused by the fungus Pseudocercospora lythracearum. This pathogen thrives in warm, humid conditions during summer and early fall. While CLS rarely poses a fatal threat to a mature tree, it significantly detracts from the plant’s aesthetic appeal and can reduce its vigor. The primary consequences are cosmetic, leading to noticeable leaf spots and premature defoliation of the lower canopy. This guide provides actionable steps for homeowners to correctly identify the issue and implement effective treatment strategies.
Confirming the Diagnosis on Crepe Myrtles
Proper identification is essential for managing Cercospora Leaf Spot and distinguishing it from issues like powdery mildew or aphid damage. Initial signs are small, dark, circular to irregular spots on the leaves, typically measuring 1/8 to 1/4 inch in diameter. These spots often have a purplish-brown or gray center with a dark border.
The infection appears first and most densely on the older, lower leaves. As the spots mature, the surrounding leaf tissue begins to turn yellow, a process known as chlorosis. Severe infections cause premature fall color—leaves turn yellow, orange, or red—before dropping entirely in mid-to-late summer. This early defoliation is the most noticeable effect and makes the tree look sparse long before the natural autumn leaf drop.
Non-Fungicidal Management Strategies
Cultural practices are the most effective long-term defense against CLS, creating an environment less favorable to fungal growth. Sanitation is crucial because the fungus overwinters in fallen leaves and plant debris. Homeowners should diligently rake and remove all infected fallen leaves throughout the season and before new growth begins in the spring.
Altering watering habits reduces the spread of fungal spores. Since spores are dispersed by water splash and require moisture to germinate, avoid overhead irrigation. Use drip irrigation or soaker hoses to deliver water directly to the soil and root zone. Watering early in the morning is advised so that any moisture reaching the foliage can dry completely in the morning sun.
Improving air circulation helps leaves dry faster, suppressing fungal development. Achieve this through selective pruning, ideally performed during the dormant season. Pruning thins inner branches and removes suckers, allowing better airflow and sunlight penetration into the lower tree parts. Additionally, avoid excessive nitrogen fertilization, which promotes soft, susceptible new growth; apply lower rates more frequently instead of a single high dose.
When and How to Apply Fungicides
Chemical treatment is reserved for trees with a history of severe infection or when cultural controls are insufficient. Fungicides act primarily as a preventative barrier, stopping new infections rather than curing existing leaf spots. Therefore, timing the application is critical for successful control.
The first application should occur at the beginning of the growing season, shortly after the leaves have fully emerged. Repeat applications are necessary, spaced according to the product label, typically every seven to twenty-eight days, especially during warm, wet periods when the fungus is most active. Common active ingredients available to homeowners include myclobutanil, thiophanate-methyl, or copper-based fungicides.
To prevent the fungus from developing resistance, rotate between different classes of fungicides, identified by the FRAC (Fungicide Resistance Action Committee) code on the product label. When spraying, ensure thorough coverage of all foliage, paying special attention to the underside of the leaves where spores may be sporulating. Always follow the manufacturer’s instructions precisely regarding mixing ratios and safety precautions, as chemical applications supplement but do not replace good sanitation and cultural practices.