Penn State Department of Plant Pathology &
Environmental Microbiology Archives,
Penn State University, Bugwood.org

Cercospora leaf spot is an infectious disease that affects smooth, panicle, oakleaf and bigleaf types of hydrangea in both landscapes and nurseries. This disease is caused by the fungus Cercospora hydrangea and is perhaps the most common disease seen on this ornamental during the months of July through October. The disease rarely kills the plant, but if it is severe, it can reduce overall plant vigor by repeated defoliation. For bigleaf-type hydrangeas in the landscape, Cercospora leaf spot tends to be less severe under shady conditions, but in nursery environments under shady conditions, frequent overhead irrigation can intensify disease activity and subsequent defoliation and loss of vigor.