Native the the southeastern United States, oakleaf hydrangeas have unique characteristics not found in other hydrangea varieties. Their serrated oak leaf-shaped leaves turn a rich burgundy red in the fall, making oakleaf hydrangeas an attractive addition to landscapes. The flower headsare of the oakleaf hydrangea are more cone-shaped, with white flower clusters that turn to pink as they age. Several types of fungus can infect oakleaf hydrangea; control and treatment depends on the causative agent.
Cercospora leaf spot, botrytis blight, mushroom root rot and rust are the primary fungi that attack an oakleaf hydrangea. Cercospora leaf spot and rust fungus, left untreated, will leave a bush void of most foliage, while botrytis blight attacks buds and flowers. Oakleaf hydrangeas are particularly susceptible to mushroom root rot, which can quickly kill an infected bush. Interestingly, powdery mildew, a particularly bothersome fungus for other hydrangea varieties, does not normally infect oakleaf hydrangea, according to research done at the University of Alabama.
Cercospora leaf spot is identifiable by the small dark brown or purple angular shaped spots that first appear on lower leaves of an oakleaf hydrangea. Botrytis blight, also referred to as gray mold, attacks the buds and flowers of an oakleaf hydrangea. Small spots first appear on the petals and look water-soaked, but quickly enlarge to reddish brown, irregular blotches. Gray colored mold may also cover the flower head as the fungus grows. Rust fungus can easily be identified by small rust/orange powdery looking spots that appear on the underside of an oakleaf hydrangea leaf. The rust spores rapidly spread to neighboring leaves. Mushroom root rot is particularly bothersome to oakleaf hydrangeas, more than other hydrangea varieties. New, healthy shoots that suddenly wilt and don't revive with adequate watering is a good indication that your oakleaf hydrangea is infected with mushroom root rot.
Cercospora leaf spot fungus and rust fungus commonly begin infecting the base leaves of an oakleaf hydrangea, then gradually spreads throughout the bush, infecting the entire foliage. Leaves will turn yellow and drop as the infection intensifies, resulting in a scraggly, unattractive bush. Once your oakleaf hydrangea is infected with botrytis blight, it develops quickly on the flower petals and buds, turning large, colorful flower clusters into a dry, brown mass often coated with fuzzy gray mold. Unlike the foregoing fungi, which may be unsightly and slow growth but rarely kill an oakleaf hydrangea, the mushroom root rot fungus can and will kill your hydrangea bush.
Cercospora leaf spot fungus and the rust fungus spores live and overwinter in infected debris and on the wood of an oakleaf hydrangea. The spores are usually splashed onto the lower, base leaves of the bush, where they attach, feed and spread throughout the foliage. Botrytis blight can develop on petals and buds after several consecutive days of humid, cloudy and rainy weather. The spores of any of these fungi can also be airborne and transported to a plant under the right conditions. Mushroom root rot fungus can be present in the soil and be harmless until conditions change to weaken your oakleaf hydrangea, such as a drought condition. Oakleaf hydrangeas can also become infected with mushroom root rot fungus if grown near oak trees or other infected plants.
Good housekeeping is the best prevention of cercospora leaf spot fungus and rust fungus. Remove and destroy all infected debris from under and around your oakleaf hydrangea. Cut out any branches and stems that show infection. Weekly spraying of fungicide until there is no evidence of cercospora leaf spot fungus or rust fungus should eliminate either one. Botrytis blight-infected flower heads should be immediately removed and destroyed. Creating good air circulation and avoiding overhead watering decreases botrytis blight susceptibility. Good site selection and proper care are the best defense against mushroom root rot.