The crepe myrtle (Lagerstroemia indica) is a popular ornamental tree known for its vibrant summer blooms and attractive exfoliating bark. Although generally hardy, it is often affected by fungal diseases that detract from its appearance and health. Effective management requires accurately identifying the specific fungus and applying targeted treatments, from cultural adjustments to chemical intervention for severe cases.
Identifying the Specific Fungal Problems
Accurate diagnosis requires recognizing the distinct visual symptoms caused by the three most common fungal problems. Powdery mildew is the most recognized issue, appearing as a dusty, white-to-gray coating on the leaves, flower buds, and new shoots. This fungus thrives in high humidity, especially when nighttime temperatures are cool and daytime conditions are dry. Heavy infections can cause buds to fail to open and new growth to appear stunted or distorted.
Sooty mold presents as a black, velvety, or crusty coating on the leaves and branches. This mold is not parasitic; it grows on honeydew, a sticky, sugary substance excreted by sap-sucking insects like aphids and scale. While the mold is harmless to the plant tissue, a severe coating blocks sunlight, reducing the tree’s ability to photosynthesize.
Cercospora leaf spot is caused by Cercospora lythracearum. Symptoms typically begin in mid-to-late summer on mature leaves in the lower canopy. They appear as small, circular or irregular reddish-brown spots, often with a yellow halo. These spots can cause leaves to turn yellow, orange, or red before dropping prematurely. The fungus overwinters in fallen leaves and spreads easily via splashing water during warm, moist weather.
Non-Chemical Management and Long-Term Prevention
Cultural controls are the most sustainable defense against crepe myrtle fungi, focusing on reducing environmental conditions that allow fungi to thrive. Proper pruning during the dormant season is important, as thinning the canopy enhances air circulation. Increased airflow reduces humidity within the foliage, preventing the spread of powdery mildew spores.
Sanitation practices are a fundamental preventative measure, especially for leaf spot and powdery mildew. Fungal spores survive winter in fallen leaves, acting as a source of infection the following spring. Removing and destroying infected leaves and spent flowers during the growing season, and cleaning up fallen debris, breaks the disease cycle.
Watering habits must be adjusted to minimize fungal risk. Always water the soil directly using a soaker hose or drip system, avoiding overhead irrigation. If overhead watering is necessary, allow foliage to dry completely before nightfall. This limits the time leaf surfaces remain wet, which is necessary for fungal spores to germinate.
Addressing sooty mold requires focusing on eliminating the sap-sucking insects, such as aphids or scale, that produce the honeydew. Horticultural oils or insecticidal soaps can be applied directly to the pests, or a strong jet of water can be used to physically dislodge them. Once the insect infestation is controlled, the honeydew source disappears, and the sooty mold will eventually weather away or be shed with the exfoliating bark.
Long-term prevention is aided by planting crepe myrtles in full sun, as eight hours of direct sunlight daily keeps the foliage dry and discourages fungal growth. Adequate spacing between trees promotes air circulation and reduces disease transmission. Selecting resistant cultivars, such as ‘Tuscarora’ or ‘Natchez,’ offers the best defense against recurring infections.
Applying Fungicides for Severe Infections
Chemical treatment is reserved for severe or persistent infections. Fungicides function primarily as a preventative measure. They are most effective when applied before the fungus takes hold or at the first sign of disease, working by creating a protective barrier on the plant tissue to prevent spore germination.
Homeowners have access to several classes of fungicides. Horticultural oils, such as Neem oil, offer a dual benefit: suppressing mild powdery mildew and acting as an insecticide against pests that cause sooty mold. Copper-based fungicides are a broad-spectrum option effective against both powdery mildew and leaf spot.
For more aggressive control, synthetic fungicides containing active ingredients like myclobutanil, propiconazole, or chlorothalonil are effective. These systemic or contact products provide a stronger defense against established infections. Application timing is important: treatment typically begins shortly after leaves emerge in spring for powdery mildew, or when spots first appear on lower leaves in mid-summer for leaf spot.
Fungicides often require multiple applications, usually at 7 to 14-day intervals, especially during periods of high disease pressure. Complete coverage of all susceptible plant parts, including the undersides of leaves, is necessary for successful treatment. Follow all label instructions regarding mixing ratios and application frequency to ensure efficacy and user safety.