Received these photos of Cercospora leaf spot on hydrangea from Gerald Roberts, county agent in Lafayette Parish. This disease is caused by the fungus Cercospora hydrangeae, which reproduces on infected leaves that have fallen to the ground. The spores produced on these fallen leaves are either splashed onto the lower foliage or are spread by air movement. The pathogen requires moisture on the leaves to grow and cause disease, so avoiding overhead watering is one means of reducing the chance of disease. Many of the standard fungicides available to home gardeners are also effective in controlling this disease, but they must be applied to prevent the disease from becoming established.
Donald M. Ferrin, Ph.D., Louisiana State University Agricultural Center, Baton Rouge, LA