Crepe myrtles are a fixture in many summer landscapes, celebrated for their long-lasting blooms and attractive bark. When homeowners notice white spots appearing on their trees, it signals a problem that requires prompt action. These white spots, whether fuzzy or waxy, are often the visible sign of an underlying pest or disease issue that needs to be addressed quickly to restore the plant’s health. Identifying the specific cause is the first step toward effective treatment and long-term management.
Pinpointing the White Spot Cause
The appearance of white spots on a crepe myrtle tree is typically caused by one of two problems: Crepe Myrtle Bark Scale (CMBS) or Powdery Mildew. These two issues are very different, requiring distinct treatment strategies, so correct identification is crucial before any removal efforts begin.
Crepe Myrtle Bark Scale is an insect infestation that manifests as small, felt-like white or gray bumps, frequently found clustered in branch crotches and on the bark of trunks and twigs. The insects excrete a sugary waste product known as honeydew, which often leads to the secondary development of black sooty mold. If you crush one of the white insects, it may exude a pink liquid, confirming the presence of this specific pest.
In contrast, Powdery Mildew is a fungal disease that appears as a dusty, flour-like white or grayish coating, primarily covering the leaves, new shoots, and flower buds. This mildew coating can cause new growth to become distorted or stunted, and it may prevent flower buds from opening properly. The fungus thrives in conditions of high humidity, particularly when warm days are followed by cool nights. It does not produce the honeydew or sooty mold.
Cultural and Physical Removal Methods
Cultural and physical removal methods offer an effective first line of defense, especially for smaller infestations. Heavily infested limbs and twigs can be selectively pruned and removed from the tree during the dormant season to reduce the overall population of the pest or disease. This physical reduction is particularly helpful in areas where the infestation is concentrated.
The white CMBS insects and the accompanying black sooty mold can be physically scrubbed from the bark using a soft-bristled brush. Use a solution of water mixed with a small amount of common dish soap to help dislodge the insects and dissolve the sticky honeydew residue. After scrubbing, rinse the trunk and branches thoroughly with a strong jet of water to remove the soap and debris.
Horticultural oils and insecticidal soaps are contact treatments that work by suffocating scale insects, including the CMBS crawlers. Apply these products directly to the affected surfaces, ensuring thorough coverage of the bark and all crevices. To prevent potential plant damage, always apply horticultural oils when temperatures are moderate, ideally between 45°F and 90°F. Repeated applications may be necessary to target newly hatched scale crawlers, so follow the product’s label for weekly re-application guidelines during the growing season.
Choosing the Right Chemical Treatment
When infestations are severe or persistent, a targeted chemical treatment is necessary, though the approach differs significantly for insect scale and fungal mildew. For Crepe Myrtle Bark Scale, the most effective long-term control comes from using a systemic insecticide, which is absorbed by the plant’s root system and distributed throughout the tissue. Active ingredients like imidacloprid, dinotefuran, or thiomethoxam are commonly used in a soil drench application poured around the base of the tree.
The optimal time to apply this systemic soil drench is in the late winter or early spring, just before the tree begins to leaf out. This timing allows the chemical to be fully absorbed and circulated within the tree before the scale populations begin to multiply rapidly in the summer. It is important to note that when using systemic products, you must follow all label instructions to protect pollinators, since these chemicals can be present in the nectar and pollen of flowering plants.
Targeted fungicides are the appropriate chemical treatment for Powdery Mildew when cultural methods fail. Active ingredients such as myclobutanil or propiconazole can be applied as a foliar spray to protect the new, susceptible growth from infection. Fungicides are most effective when applied preventively or at the very first sign of the white, powdery growth. To prevent the fungus from developing resistance, it is often recommended to alternate between products containing different active ingredients throughout the season.
Preventing Future Outbreaks
Long-term management focuses on optimizing the plant’s environment and selecting resistant varieties. Proper planting location is important for minimizing the risk of Powdery Mildew infection. Ensure that your crepe myrtle receives full sun and has adequate spacing from other structures or plants to promote good air circulation around the foliage.
Selecting varieties known for their resistance to Powdery Mildew is the most proactive measure. Cultivars such as ‘Apalachee,’ ‘Muskogee,’ ‘Tonto,’ and ‘Zuni’ have demonstrated high resistance to this fungal issue. Currently, no crepe myrtle cultivars have proven to be fully resistant to the Crepe Myrtle Bark Scale pest.
An application of dormant oil in late fall or late winter before bud break can help prevent scale outbreaks. This pre-emptive treatment smothers overwintering scale eggs and nymphs hidden in the bark crevices and on the branches. Avoid the excessive use of high-nitrogen fertilizers, which promote a flush of tender, soft new growth that is highly susceptible to both Powdery Mildew and insect pests like the bark scale.