Issue 8, August 4, 2022
Cercospora Leaf Spot of Hydrangea
Cercospora leaf spot of hydrangea is a fungal disease that affects bigleaf hydrangea, Hydrangea macrophylla; smooth hydrangea, H. arborescens; and oakleaf hydrangea, H. quercifolia. This disease does not kill the hydrangeas, but the spots can cause unsightly foliage. Severely infected plants may defoliate and often have reduced plant vigor and fewer flowers.
The Cercospora leaf spot pathogen overwinters on leaf debris. The pathogen’s spores are disseminated to foliage via splashing rain or overhead irrigation. The first leafspots appear on older leaves near the plant’s base, then progressively spread upward through the plant. Initial spots are purple and small with a circular shape. As spots enlarge, they often become irregular or angular in shape and develop a tan or gray center surrounded by a purple or brown border. Leaves that are severely spotted often become a yellow-green color.
Cercospora leaf spot on bigleaf hydrangea, Travis Cleveland, University of Illinois
Sanitation is an essential tool in disease management. Remove and destroy fallen leaf debris to help reduce overwintering spores and future infections. Avoid overhead irrigation that wets the foliage. When possible, use drip irrigation or soaker hoses. Adequately space plants to allow air movement and quick drying of the foliage.
Chemical control options are available for severely affected plants. Products containing chlorothalonil, myclobutanil, or thiophanate-methyl are highly effective if applied at the onset of the disease. Homeowners have access to products containing chlorothalonil (Bonide Fung-onil, Ortho MAX Disease Garden Control or Daconil), or myclobutanil (Spectracide Immunox Multi-Purpose Fungicide)
Author:
Travis Cleveland