Dark spots on hydrangea leaves are a common garden concern, particularly as summer progresses. While alarming, these spots rarely threaten the plant’s survival, usually causing only cosmetic damage. The discoloration generally stems from two distinct origins: a biological infection or an environmental stressor. Identifying the specific pattern determines the appropriate course of action for managing the problem and maintaining the foliage’s health.
The Primary Culprit: Fungal Leaf Spot
The most frequent cause of black spots on hydrangeas is a fungal infection, typically Cercospora leaf spot. This disease is caused by the fungus Cercospora hydrangea and thrives in high humidity and prolonged leaf wetness. Initial symptoms appear as small, circular spots, often dark purple or brown, on the older, lower leaves.
As the disease progresses, these spots enlarge and develop a characteristic “frogeye” appearance. The center of the lesion becomes pale gray or tan, surrounded by a distinct purple or dark brown border. The fungus overwinters in infected leaf debris, and its spores are easily splashed onto the leaves by rain or irrigation.
The infection is most noticeable from mid-to-late summer. While the spots may cause affected leaves to turn yellow and drop prematurely, reducing the plant’s vigor, the condition rarely results in the death of the shrub.
Other Environmental and Abiotic Causes
Not all dark spots indicate a spreading biological infection; some result from environmental stress or physical damage. Intense, direct sunlight, especially during the hottest part of the day, can cause sun scald. This abiotic damage appears as irregular brown or scorched patches, often affecting leaves on the top or sun-exposed side of the plant.
Darkened patches can also signal injury from cold temperatures or late spring frost. In early spring, this damage may manifest as purple or blackened areas along the leaf edges, but it will not spread to new growth. Water management issues, such as root damage from waterlogged soil, can also cause leaves to wilt and develop dark, necrotic areas that mimic disease.
The key difference is that abiotic damage is self-limiting and does not produce infectious spores that travel to healthy foliage. If the spots lack the distinct concentric rings of a fungal infection and are not progressively spreading, the issue is likely due to the plant’s location or care routine.
Immediate Treatment and Remediation
Once dark spots are observed, the most immediate action involves physical sanitation to lower the concentration of fungal spores. Pruning and removing all heavily spotted leaves eliminates the current source of infection. Rake up and dispose of any fallen leaves or debris from beneath the plant, as the fungus can survive the winter in this material.
Diseased foliage should be discarded in the trash, not added to a home compost pile where spores may survive and reinfect the garden. For severe or recurring fungal infections, a protective fungicide may be warranted to shield new growth. Products containing active ingredients like chlorothalonil, myclobutanil, or thiophanate-methyl are effective.
Fungicides act as a preventive barrier and are not curative, meaning they cannot remove spots from already infected tissue. They must be applied as directed on the label, typically every 10 to 14 days, starting in late spring or early summer before symptoms appear. This proactive treatment is usually reserved for plants that experience significant defoliation each year.
Long-Term Cultural Prevention Strategies
Preventing the conditions that favor fungal development is the most sustainable long-term solution for managing leaf spots. Modifying watering habits is a primary step, involving avoiding overhead irrigation that wets the foliage. Instead, direct water to the soil using a soaker hose or drip system, and water early so moisture on the leaves can dry quickly.
Improving air circulation around the shrub reduces the duration of leaf wetness. This is achieved by ensuring adequate spacing between hydrangeas and surrounding plants, typically three to five feet for many varieties. Strategic pruning to remove dense inner branches allows air to flow through the canopy, helping leaves dry rapidly after rain.
Applying a two- to three-inch layer of organic mulch around the root zone, kept away from the crown, serves a dual purpose. Mulch retains soil moisture and acts as a barrier, preventing fungal spores from splashing onto lower leaves during watering or rainfall. Choosing resistant hydrangea cultivars can significantly reduce the risk of future outbreaks.