If some or all of the leaves on your hydrangea or crape myrtle develop small spots and turning red and prematurely dropping from the plant during summer, this is most likely Cercospora leaf spot. This condition usually occurs during hot summers with more than average rainfall. While some varieties have more resistance to the spot than others, all can be affected during a rainy and humid summer.
Cercospora leaf spots form when water from rain or irrigation sits on leaves during hot and humid conditions. This is especially true in landscapes that use overhead irrigation frequently. Therefore, when using overhead irrigation, it is best to water in early to mid-morning, so the sun can dry the water from foliage.
While Cercospora leaf spot rarely, if ever, kills the target plant, heavy spotting and/or discoloration of the leaves and premature leaf shed is unsightly. It may reduce plant vigor during the current season.
NSH Ornamental Tree & Shrub Program: We offer two programs designed to optimize ornamental plant health and control pests/diseases (The Natural State Ornamental Program & The Ornamental Essentials Program). Our ornamental programs utilize a wide variety of advanced fungicides applied both preventatively and curatively to control diseases in your landscape. While our programs will drastically reduce the proliferation of disease, Cercospora is almost impossible to control with chemical intervention alone.
DIY Management Strategies: If possible, irrigate plants using a soaker hose or drip irrigation. Splashing water from an overhead sprinkler can spread the fungus from leaf to leaf and create an ideal environment for disease activity.
Also, the fungus can easily survive in infected leaves that fall and remain on the ground or on the bush. The removal of these leaves can help prevent future infections and disease outbreaks. Spotted leaves should be removed at any time during the growing season when they are present, especially before new leaves begin to form in the spring.
Fungicide sprays are rarely needed to control this disease. However, treatment with fungicides can be helpful on plants that suffer noticeable damage every year. Begin treatment in late spring or early summer before symptoms appear. Of course, we always recommend leaving chemical disease management to the professionals but there are several viable products available to homeowners that may help supplement professional programs. A systemic fungicide that contains propiconazole (Fertilome Liquid Systemic Fungicide II or Bonide Infuse Systemic Disease Control) or tebuconazole) or (Bayer Advanced Garden Disease Control for Roses, Flowers, and Shrubs).