Hydrangeas are popular shrubs, but their broad leaves are susceptible to various blemishes, including brown spots. Finding brown spots on the leaves can be alarming, yet this common issue is not a single disease but a symptom of several possible causes. These spots act as visual clues, requiring a careful diagnosis to determine if the problem is a biological infection or a reaction to the growing environment. Understanding the appearance and location of the spots is the first step toward effective management and ensuring the plant remains healthy.
Fungal Diseases Causing Spotting
The appearance of distinct spots on hydrangea leaves often signals the presence of a fungal pathogen, which typically thrives in warm, humid conditions and areas with poor air circulation. Cercospora leaf spot, caused by the fungus Cercospora hydrangeae, is the most frequent culprit in landscape settings. This disease starts as small, circular spots, typically 1/8 to 1/4 inch in diameter, appearing on the lower, older leaves first and progressing upward.
On bigleaf hydrangeas (Hydrangea macrophylla), these lesions often develop a distinctive “frog-eye” look: a light tan or gray center encircled by a dark purple or reddish-brown border. Though rarely fatal, a severe infection can cause premature leaf yellowing and drop, reducing the plant’s vigor.
Another common fungal issue is Anthracnose, caused by the Colletotrichum gloeosporioides fungus, which attacks both leaves and flower petals. Anthracnose spots are typically larger and more irregular in shape than Cercospora spots and may appear anywhere on the plant canopy. The lesions often have a sunken or target-like appearance with alternating rings of darker and lighter brown tissue. In severe cases, the fungus can cause sunken cankers on the stems. Fungal spores from both diseases are spread primarily by splashing water from rain or overhead irrigation, moving from infected fallen debris onto healthy foliage.
Environmental and Cultural Stress Factors
Brown spots that are not caused by infection often result from environmental stress or improper cultural practices. Sun scorch, a physiological problem, occurs when the leaves lose water faster than the roots can supply it, a situation common during heat waves or in plants receiving intense afternoon sun. Symptoms of scorch include irregular, dry, brown blotches that commonly appear on the leaf tips and margins, especially on the side of the plant most exposed to the sun.
Water stress also manifests as brown discoloration, with symptoms varying based on the issue. Chronic underwatering leads to crispy, dry browning along the leaf edges and tips as the plant conserves moisture. Conversely, overwatering or poor soil drainage causes root stress. Saturated soil limits oxygen uptake and root function, causing leaves to brown and wilt due to the inability to absorb water and nutrients.
Another non-infectious cause is fertilizer burn, which occurs when too much fertilizer is applied, leading to a buildup of soluble salts in the soil. High salt concentrations draw moisture out of the root tissue, causing brown, necrotic tissue, particularly along the leaf margins and tips. This damage is often concentrated in container-grown plants or those receiving heavy, concentrated applications of synthetic fertilizer.
Immediate Steps for Managing Brown Spots
Once the cause of the brown spots is diagnosed, immediate action is needed to stop the spread and minimize damage to the plant. Sanitation is the most important first step, especially if a fungal infection is suspected, as pathogens overwinter in dead plant material. All affected leaves, whether attached or fallen, must be promptly removed and destroyed by bagging them for disposal, not adding them to a compost pile.
If canes are heavily infected with lesions or cankers, prune them back to healthy wood to eliminate a source of future infection. After pruning, clean the tools with a disinfectant, such as a 10% bleach solution or rubbing alcohol, to prevent transferring spores or bacteria to other plants.
If a fungal disease like Cercospora or Anthracnose is confirmed and severe, a fungicide application may be warranted to protect new growth. Fungicides containing chlorothalonil or copper are often recommended for leaf spot diseases. They act primarily as a protective barrier on healthy tissue, not as a cure for existing spots. These products must be applied strictly according to label directions, ensuring thorough coverage as new growth emerges.
Preventing Future Spotting Through Better Care
Preventing the recurrence of brown spots involves adjusting cultural practices to create an environment less favorable for disease and stress. Changing the method of watering is a significant adjustment; always apply water directly to the soil at the base of the plant, avoiding overhead irrigation that wets the foliage. Watering early in the day allows moisture on the leaves to dry quickly, reducing the duration of leaf wetness that fungi require to thrive.
Improving air circulation around the shrub helps the leaves dry faster and reduces humidity that encourages fungal growth. This is achieved by ensuring hydrangeas are properly spaced from other plants and by performing light, selective pruning to thin the dense canopy. Avoid over-fertilizing, as the resulting flush of tender, new growth is susceptible to disease, and excess salt buildup can cause burn symptoms.
For hydrangeas prone to sun scorch, optimizing the plant’s location is the best solution, often by transplanting it to a spot that receives morning sun and afternoon shade. Applying an organic mulch layer around the base helps maintain consistent soil moisture and keeps the root zone cooler, reducing the risk of water stress and leaf scorch during hot periods.