A: After looking at your photos and examining the samples of your leaves, I believe the spots are caused by a fungus called Cercospora lythracearum as it is the most common fungal leaf spot on Lagerstroemia indica (crape myrtles). In Florida the leaf spot disease was severe in 1976, especially on the pink-flowering variety ‘Near East’. Of the 3 flowering varieties utilized in landscape plantings in Gainesville, it was noted that ‘Near East’ was the most susceptible in terms of greatest leaf infection and defoliation. The red or lavender variety was less susceptible, and the white flowering variety or ‘Far East’ least susceptible with only a very few Cercospora lesions on the leaves.

Heavy infections with Cercospora can cause severe defoliation with a debilitating effect on the plant. The leaf spots caused by this fungus are dark brown, irregular, with no yellow margin. Spotted leaves can become distorted, losing their flat, smooth appearance, particularly as the spots become larger. Leaf spots on the lower surface of the leaves are initiated as tiny brown flecks with no visible spotting on the upper surface of the leaves. As leaf spots enlarge, they appear on both sides of the leaf causing the leaf to turn yellow. The brown spots remain with a green border of leaf tissue, which stands out in sharp contrast to the yellowed leaf. At this stage heavy defoliation occurs.

Sanitation is perhaps the most important tool in disease management. Be sure to remove and destroy these leaves to help prevent future infections and disease outbreaks. Another important cultural practice includes surface watering. Because moisture on the leaves allow disease spores to germinate, avoid getting the leaves wet with overhead irrigation. Also be sure to apply enough nitrogen to maintain a moderate growth rate but too much can cause other issues.

It is also helpful if your plants are not crowded. Good air circulation permits the leaves to dry quickly after a rain, which helps prevent leaf spot diseases. Use a fungicide made specifically for ornamental plants and alter the type used to avoid building up a resistance. For effective control of Cercospora leaf spot with a fungicide, begin applications when leaves begin to appear in the spring and continue applying a fungicide as needed. Be sure to follow label directions – the label is the law!

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