Chrysanthemums, or “mums,” are a vibrant presence in many gardens, but brown foliage signals underlying plant distress. Browning is the plant’s final mechanism for shutting down tissue that is no longer viable due to lack of resources, disease, or injury. Understanding the cause requires a careful look at the pattern and location of the dead tissue. This discoloration is a physical manifestation of cellular collapse, indicating the mum is under significant stress and requires intervention.
The Critical Role of Water and Root Health
Improper watering practices are arguably the most frequent cause of browning in chrysanthemums, leading to two opposite yet visually similar problems. When the soil remains constantly waterlogged, the roots become deprived of the oxygen necessary for respiration, leading to a condition known as root rot. This causes the normally creamy-white, firm roots to turn brown and mushy, effectively cutting off the plant’s ability to absorb moisture and nutrients. The foliage then wilts and turns brown, often starting with the lower leaves, mimicking the look of a plant that is severely underwatered.
Conversely, allowing the soil to dry out completely between waterings results in desiccation stress. In this scenario, the plant actively pulls moisture from the leaf tips and margins to preserve the growing points, causing those areas to turn crispy and brown. This extreme drought condition can also lead to stunted, woody stem growth over time. To accurately determine a mum’s need for water, insert a finger about two inches into the soil. If the soil at that depth feels dry, water the plant deeply, but if it feels moist, no additional water is needed.
Identifying Biological Threats: Pests and Fungal Diseases
Living organisms, including fungi, are common culprits that induce browning by actively destroying plant tissue. Fungal diseases like Septoria leaf spot begin as small, yellowish spots that quickly develop into distinct dark brown or black lesions, sometimes bordered by a purple halo. These spots typically appear first on the oldest, lowest foliage and are encouraged by prolonged periods of moisture on the leaf surface. Another fungal issue, rust, produces pale yellow or green spots on the upper leaf surface, with the more noticeable symptom being powdery, dark brown or orange pustules on the leaf undersides.
Tiny sap-sucking pests like spider mites and aphids feed by piercing plant cells and extracting fluid contents. Spider mite damage initially presents as fine yellow or bronze stippling, progressing into a bronzed or browned appearance as cells die. Heavy infestations are confirmed by fine, silky webbing between the leaves and stems. Aphids, often brown or black, cluster on new growth, sucking sap and causing leaves to curl, distort, and eventually turn brown.
Environmental Shock and Chemical Burn
Browning can also result from a sudden, non-biological shock from the immediate surroundings or improper chemical application. Exposure to a hard frost causes water within the plant cells to freeze and rupture, resulting in tissue that appears blackened and water-soaked before turning completely brown. At the other extreme, intense, direct afternoon sunlight can cause sun scorch, where the leaves facing the highest intensity turn dry, bleached, and brown.
Chemical stress, such as over-fertilization, causes a phenomenon known as salt burn. When too much fertilizer is applied, the high concentration of soluble salts in the soil draws moisture out of the plant roots and leaf tissue. These salts then accumulate at the leaf edges and tips, leading to a distinct brown, dried-out border on the foliage. Similarly, misuse of chemical sprays, including certain pesticides or organic treatments like Neem oil, can cause a phytotoxic burn on the leaves and flowers, resulting in immediate browning of the contacted tissue. Browning of the entire plant after blooming is often natural senescence as the mum prepares for winter dormancy.