The Crape Myrtle (Lagerstroemia indica) is a highly valued ornamental plant known for its striking summer blossoms and hardiness. A common concern for many gardeners is the appearance of yellowing leaves, a condition known as chlorosis. This discoloration signals stress, indicating that the tree’s ability to produce chlorophyll has been compromised. While seeing yellow leaves can be alarming, the cause is rarely fatal and can often be corrected with careful diagnosis and management.
Yellowing Caused by Watering and Environment Stress
Watering issues are among the most frequent reasons a Crape Myrtle shows signs of stress. Overwatering is common and detrimental because saturated soil displaces the oxygen needed by the root system. When roots cannot access oxygen, they effectively suffocate, preventing them from absorbing water and nutrients, which quickly leads to root rot. Leaves affected by overwatering often exhibit a widespread, uniform yellowing and may feel soft or mushy before prematurely dropping.
Conversely, a Crape Myrtle suffering from drought or underwatering may also display yellow leaves. The tree attempts to conserve moisture by shedding older foliage, typically resulting in yellowing starting on the lower parts of the plant. These leaves tend to feel dry and brittle, sometimes showing crispy, brown edges. Environmental shock, such as sudden, intense heat or the stress of a recent transplant, can also trigger temporary leaf yellowing as the tree adjusts.
Nutrient Deficiencies and Soil pH
If proper watering practices are in place, yellowing often points toward a chemical imbalance within the soil related to nutrient availability. Crape Myrtles thrive in slightly acidic soil, ideally with a pH range between 5.0 and 6.5. When the soil becomes too alkaline (high pH), it chemically binds certain micronutrients, making them inaccessible to the roots even if they are present in the soil.
The most common nutritional problem is iron chlorosis, which produces a distinct visual pattern on the foliage. Leaves affected by a lack of available iron show yellowing between the main veins while the veins themselves remain dark green. This pattern, known as interveinal chlorosis, is most obvious on the newer growth at the tips of branches. Other micronutrients, such as zinc, can also cause this interveinal pattern.
A more generalized yellowing, especially on the oldest leaves, often indicates a deficiency in nitrogen. Nitrogen is a mobile nutrient, meaning the tree pulls it from older leaves to support new growth, causing the older foliage to turn a uniform pale green or yellow. While nitrogen deficiency can be corrected with fertilizer, the soil pH ultimately governs the availability of many micronutrients. Therefore, a soil test is the most reliable diagnostic tool.
Insect Pests and Fungal Infections
Yellowing can also result from biological invaders, specifically sap-sucking insects and common fungal pathogens. The Crape Myrtle Aphid (Tinocallis kahawaluokalani) is a host-specific pest that feeds on the underside of leaves to extract sap. This feeding activity can cause distortion and small yellow spots to develop on the leaf surface.
A more visible sign of an aphid infestation is honeydew, a sticky, sugary liquid excreted by the pests. This residue acts as a food source for sooty mold, a black fungus (Capnodium sp.) that grows across the leaf surface. Although the mold does not directly attack plant tissue, a heavy coating blocks sunlight, hindering photosynthesis and contributing to the tree’s yellowing and stress.
Another frequent cause of discoloration is Cercospora Leaf Spot, a fungal disease prevalent during periods of warm, moist weather. This infection typically begins as small yellow spots on the upper surface of the foliage. As the disease progresses, the spots may turn brown or gray, causing the entire leaf to yellow and eventually leading to premature defoliation in late summer.
Actionable Steps for Recovery and Future Health
Addressing watering issues begins with checking the soil moisture level before applying water. Use a finger or a moisture meter to ensure the soil is dry a few inches below the surface before watering deeply and slowly. Ensuring the planting site has adequate drainage will prevent the root asphyxiation that causes overwatering symptoms.
If nutrient deficiency is suspected, especially the interveinal yellowing of iron chlorosis, a soil test should confirm the pH level. To correct alkaline soil, granular elemental sulfur can be applied to gradually lower the pH over time, increasing micronutrient availability. For a faster response, a chelated iron product can be applied as a soil drench.
For aphid control, initial treatment involves a strong blast of water to physically dislodge the pests from the undersides of the leaves. If the infestation is severe, horticultural oils or insecticidal soaps can be applied. Controlling the aphid population is the only way to stop the production of honeydew and prevent the growth of sooty mold.
Managing Cercospora Leaf Spot requires cultural practices that reduce moisture on the foliage and improve air circulation through careful pruning. Rake and remove all fallen, infected leaves to minimize fungal spores that can overwinter in the debris. Planting Crape Myrtle varieties known for their resistance to fungal diseases is the best long-term preventative measure.