The genus Hydrangea consists of several deciduous shrubs with showy flowers and green leaves. Some diseases, caused by fungi, result in brown spots developing on the leaves of these plants.
According to the Alabama Cooperative Extension System, infected leaves spread the fungus Cercospora hydrangeae, which cause the disease known as Cercospora leaf spot. Another common condition known as Anthracnose occurs when the fungus Colletotrichum gloeosporioldes attacks the plant.
Cercospora leaf spot begins with small brown or purple spots scattered on the leaves that are nearer to the ground. The spots on the bigleaf hydrangea often turn tan to light gray in the center and brown or purple in the margins. On oakleaf hydrangea, the spots appear angular and are dark brown to purple throughout.
According to Alabama Cooperative Extension System, anthracnose attacks bigleaf hydrangea in the landscape and fields, affecting the leaves and the flowers. The brown spots first appear circular or slightly irregular in shape. They then develop alternating dark and lighter rings and become more angular in shape.