From the perspective of the hydrangea plant, the appearance of brown spots on its leaves is a visible distress signal, indicating an interruption to its normal physiological functions. These spots are often the result of fungal or bacterial pathogens, environmental stress, or cultural issues that compromise the leaf's cellular structure and its ability to photosynthesize effectively. Understanding these causes is key to restoring the plant's health.
Fungi are among the most common assailants. They thrive in conditions that are unfavorable for the hydrangea, primarily excessive moisture on the leaf surface. From the plant's viewpoint, when its foliage remains wet for prolonged periods due to overhead watering, poor air circulation, or dense planting, fungal spores can germinate and penetrate the leaf's epidermis.
Two primary fungal diseases cause brown spots: Cercospora Leaf Spot: This typically begins as small, circular purple or brown spots on the lower leaves. As the infection progresses, the spots enlarge and the centers may turn tan or gray. The plant responds by sacrificing these heavily infected leaves, causing them to yellow and drop prematurely. This severely reduces the leaf surface area available for capturing sunlight. Anthracnose: This fungus causes larger, more irregular brown spots or blotches. In severe cases, the spots may have a sunken appearance. The damage can be so extensive that it kills entire leaves and even young stems, directly impacting the plant's energy production and structural integrity.
Bacterial diseases, such as those caused by Xanthomonas campestris, present a different challenge. Bacteria enter the plant through natural openings or tiny wounds, often spread by splashing water. Once inside, they multiply within the spaces between the plant's cells. The plant's own defense mechanisms can contribute to the visible symptom; it often walls off the infected area, causing the tissue to die and turn brown or black. These spots may appear water-soaked initially and are frequently angular because they are confined by the major veins of the leaf. This internal invasion disrupts the flow of water and nutrients and can lead to significant leaf loss.
Not all brown spots are caused by living pathogens. Often, the issue is cultural, meaning the plant's environment is not meeting its needs, leaving it stressed and vulnerable. Water Stress: Both overwatering and underwatering can cause browning. Soggy soil suffocates roots, preventing them from absorbing water and nutrients, which manifests as brown spots and wilting leaves—a paradox where the plant drowns in water yet dies of thirst. Underwatering simply causes the leaf edges and tips to dry out and turn brown due to direct dehydration. Sun Scorch: Hydrangeas, particularly the bigleaf varieties (Hydrangea macrophylla), prefer bright but indirect light. Intense, direct afternoon sun can literally scorch the leaves, causing large, dry, brown patches, especially on the side facing the sun. The plant loses its ability to photosynthesize in these damaged areas. Nutrient Deficiency: While less common, a lack of essential nutrients can also cause leaf discoloration.
The treatment strategy must align with the cause, always aiming to reduce stress on the plant. For Fungal and Bacterial Issues: Immediately remove and destroy all infected leaves, both on the plant and on the ground, to eliminate the source of spores or bacteria. Improve air circulation by pruning neighboring plants or thinning the hydrangea's own interior. Always water at the base of the plant to keep leaves dry. For persistent fungal problems, a registered fungicide containing chlorothalonil or copper-based products can be applied as a protective measure. For Environmental Stress: Adjust watering habits to keep the soil consistently moist but not waterlogged. Apply a layer of mulch around the base to help retain moisture and regulate soil temperature. If sun scorch is the issue, consider transplanting the hydrangea to a location with morning sun and afternoon shade or providing temporary shade during the hottest part of the day. A balanced, slow-release fertilizer formulated for hydrangeas can address nutrient needs.
