Hydrangeas are prized for their abundant, colorful flowers and lush foliage. However, black spots on their leaves are a common problem, often signaling fungal activity. This fungus thrives in humid environments and can rapidly diminish the plant’s vigor and appearance. Understanding the specific cause and taking swift, targeted action is the most effective approach to preserving the plant’s health.
Identifying the Source of Black Spots
The vast majority of black spots on hydrangea leaves are caused by Cercospora leaf spot, an infection caused by the fungus Cercospora hydrangeae. This disease is typically seen in mid to late summer, especially after periods of high humidity or frequent rain. The spots first appear on the lower, older leaves, then progressively spread upward toward new growth.
These initial spots are small, measuring 1/8 to 1/4 inch in diameter, and have a distinct purple or dark reddish-brown color. As the fungus matures, lesions on varieties like the Bigleaf hydrangea (Hydrangea macrophylla) develop a tan or gray center encircled by a dark border. This “frogeye” appearance is a telltale sign of Cercospora infection.
Other fungal issues, such as Anthracnose, may present similar symptoms. Anthracnose spots are typically larger and more irregular, sometimes forming concentric rings, and may also affect stems and flowers. Sun scorch can also cause irregular brown or black patches, but these lack the defined circular shape characteristic of fungal spots. Accurate identification ensures the most effective treatment plan.
Immediate Remedial Treatment
The first step in controlling a fungal outbreak is immediate, thorough sanitation. All spotted leaves, both those still on the plant and any that have fallen, must be carefully removed and disposed of in the trash or burned. Pruning tools used to remove infected foliage must be sterilized between cuts using a solution of 10% bleach or rubbing alcohol to prevent the mechanical transfer of spores.
Fungicide application protects new, healthy growth from being infected. Fungicides are not curative; they will not remove existing spots on damaged foliage. Common chemicals for this purpose include chlorothalonil or copper-based sprays.
Application should begin as soon as symptoms are first noticed, following label instructions for frequency and dilution. Consistent, protective spraying creates a barrier against new spore germination. For plants with severe outbreaks, a preventative spray program starting in late spring or early summer may be necessary.
Preventing Future Outbreaks (Cultural Management)
Since fungal diseases require moisture on the leaf surface to germinate and spread, modifying watering techniques is a primary preventative measure. Avoid overhead watering from sprinklers, which saturates the foliage. Instead, deliver water directly to the base of the plant using drip irrigation or a soaker hose.
Timing the watering is also important. Irrigating in the early morning allows the sun and air currents to quickly dry any splashed leaves. Avoid watering late in the day, as this leaves the foliage wet throughout the cooler night hours, extending the time spores have to infect the tissue.
Proper air circulation around the plant is a physical barrier against disease. Hydrangeas should be planted with adequate spacing to allow for good airflow. Selective pruning to thin out inner branches helps reduce humidity within the canopy and accelerates the drying of internal leaves after rain.
Fungal spores can overwinter on fallen leaf debris, making end-of-season cleanup necessary. In the fall, all leaf litter beneath the plant must be thoroughly raked and removed from the garden area. Maintaining moderate soil fertility also helps, as excessive nitrogen encourages soft new growth highly susceptible to disease.