Cylindrosporium Leaf Spot
Spirea leaves with both surfaces exhibiting light-yellow lesions that darken over time suffer from Cylindrosporium filipendula leaf spot. Inspecting their undersides reveals yellowish, waxy spore masses. On heavily infected plants, the tiny, round leaf spots merge into large dead patches. Extensive summer leaf drop may follow. Splashing water spreads Cylindrosporium spores from leaf litter and contaminated plants.
Leaf Spot Control
Spireas tolerate mild leaf spot with no serious damage. Removing infected leaves and debris eliminates the spores they harbor. Watering the shrubs from beneath in the morning minimizes foliage moisture, reducing the risk of infection. Cylindrosporium seldom requires fungicidal treatment.