The appearance of brown spots on hydrangea leaves is a common issue that signals the plant is reacting to one of several potential stressors. This discoloration is a visual symptom, not a diagnosis. Identifying the precise cause requires examining the spot’s appearance, its location on the plant, and the surrounding environmental conditions. Determining if the spots are caused by a pathogen, environmental trauma, or a chemical issue is the first step toward effective treatment.
Spots Caused by Fungal Pathogens
Fungal diseases are the most frequent biological cause of brown spots, thriving in moist conditions and high humidity. Cercospora leaf spot (Cercospora hydrangeae) is the most common fungal culprit seen in home gardens. This infection typically manifests as small, circular spots, measuring about 1/8 to 1/4 inch in diameter, and often starts on the older, lower leaves of the plant. The spots initially appear purple or reddish and later develop a tan or gray center surrounded by a dark border, giving them a distinct “frog-eye” look.
Anthracnose (Colletotrichum gloeosporioides) creates a different pattern of damage. Anthracnose spots tend to be larger and more irregular in shape than Cercospora spots, often developing a light brown or tan center. This fungus can also infect the flowers, causing them to develop brown spots or blotches, and it can appear simultaneously on both the upper and lower parts of the plant canopy. Spores from both diseases are easily spread by splashing water from rain or overhead irrigation, which explains why the diseases are often more severe during periods of extended wet weather.
Damage from Environmental Stress
Many brown spots are not caused by an infectious pathogen but are instead the result of the plant reacting to an unfavorable growing environment. Leaf scorch is a common physiological problem that results when the leaves lose water faster than the roots can supply it, leading to irregular dry, brown blotches. This damage is frequently observed on the leaves most exposed to harsh conditions, such as those on the sunniest side of the plant, or during periods of drought and high temperatures. The symptoms often begin as browning along the leaf edges and tips, but the lack of a defined border helps distinguish it from fungal infections.
Inconsistent watering practices can also cause browning, with leaves turning dry and crispy if the soil is allowed to dry out completely between waterings. Furthermore, hydrangeas that leaf out early in the spring may suffer from winter injury or frost damage. New leaves that emerge too soon can have their tips and edges turn brown and withered from overnight temperatures near or below freezing. This type of cold damage is often seen on the most exposed new growth and is a localized injury to the tissue, not a spreading condition.
Chemical Exposure and Nutrient Imbalances
Chemical inputs and soil composition can also lead to leaf spotting and browning that mimics disease symptoms. Over-fertilization, known as fertilizer burn, causes browning or scorching along the edges and tips of the leaves because excess fertilizer salts accumulate around the roots. This salt buildup draws water out of the plant’s roots, leading to dehydration and leaf tissue death, and is often accompanied by a visible white crust on the soil surface. This type of damage typically presents as a continuous, scorched margin rather than a defined spot within the leaf blade.
Unintentional exposure to herbicides through drift or overspray produces distinct symptoms, especially on new growth. Herbicide damage often results in distorted, cupped, or twisted leaves, stunting, and irregular blotches or yellowing. Unlike a disease, which spreads progressively, herbicide injury appears suddenly and affects the shape of the new foliage. Nutrient deficiencies can also cause discoloration; for instance, a lack of potassium first appears on older leaves as yellowing between the veins, eventually leading to brown scorching and curling of the leaf tips and margins.
Identifying the Cause and Management
Accurate diagnosis begins with a close inspection of the spots and the pattern of damage on the plant. Small, circular spots primarily located on lower, older leaves suggest a fungal disease like Cercospora leaf spot. Conversely, irregular, dry browning concentrated on leaves facing the afternoon sun indicates heat stress or sunscald. Chemical burn from fertilizer shows up as uniform scorching on the leaf margins, while herbicide damage is characterized by deformed new leaves.
Once the cause is determined, cultural adjustments are the most effective line of management. For fungal issues, immediately remove and destroy all fallen leaves and heavily infected foliage to reduce the source of spores. Adjusting watering practices is effective; avoid overhead irrigation to prevent splashing spores, and instead water the soil directly. For environmental stressors, providing afternoon shade or a windbreak and ensuring consistent soil moisture will help the plant recover. If the issue is chemical, flushing the soil with large amounts of water can wash away excess fertilizer salts, helping the plant outgrow any herbicide damage.