Black spots on the fleshy leaves of a Kalanchoe, a popular succulent houseplant, often signal a problem requiring attention. Because this succulent stores water in its leaves and stems, it is uniquely susceptible to issues related to moisture, temperature, and poor air movement. These dark blemishes are not a single disease but a symptom with multiple potential origins, ranging from infectious pathogens to non-infectious environmental stress. Diagnosing the specific cause of the spotting is the first step toward effective treatment and restoring the plant’s health.
Fungal and Bacterial Pathogens
Infectious leaf spots are caused by fungal or bacterial pathogens that thrive in high humidity and poor air circulation. These diseases begin as small, distinct lesions where the pathogen has invaded the plant’s tissue. Fungal species like Alternaria or Cercospora cause true leaf spot diseases characterized by necrotic tissue. These spots may appear dry, sunken, and sometimes feature a concentric ring pattern, indicating progressive fungal growth.
Other serious infections involve water molds such as Phytophthora, which often manifest as dark, wet-looking spots starting at the crown or base. This soil-borne organism suggests excessive moisture and leads to root and crown rot before symptoms appear on the leaves. Trapped moisture on the leaf surface can also allow fungi like Diplocarpon rosae (black spot) to cause discoloration and eventual leaf drop.
Environmental Stressors
Black spots not caused by infectious organisms usually trace back to issues with the plant’s cultural conditions, particularly water and temperature management. Edema, a physiological disorder, is a frequent non-infectious cause, resulting from the plant absorbing water faster than it can transpire. This excess pressure causes epidermal cells to rupture and die, forming small, dark, raised, or bumpy spots. Since this is internal damage, the spots are irreversible and result from inconsistent watering combined with high humidity or cool temperatures.
Exposure to low temperatures can also lead to necrotic black spots, especially if the succulent is placed near a cold window or draft. Kalanchoe species are sensitive to chill; temperatures near freezing can cause leaf cells to collapse, creating dark, dead patches on the foliage. Another source of non-infectious spotting is chemical burn, which occurs when excessive fertilizer salts build up in the soil or cleaning products contact the leaves. These dark, irregular patches are a sign of cellular damage from chemical concentration.
Insect Damage and Sooty Mold
A distinct type of dark blemish is caused indirectly by common garden pests that secrete a sugary substance called honeydew. Sap-sucking insects like aphids, scale, mealybugs, and whiteflies feed by piercing the plant tissue and extracting phloem, excreting the excess sugar as a sticky residue. This honeydew provides a nutrient-rich substrate for non-parasitic fungi collectively known as sooty mold. The sooty mold grows superficially on the honeydew layer, creating a dark, smudgy, soot-like coating on the leaf surface.
The appearance of sooty mold indicates a current or past pest infestation. While the mold does not infect the plant tissue, a heavy layer can block sunlight, interfering with photosynthesis and potentially stunting growth. Control relies entirely on eliminating the underlying insect problem, which removes the honeydew food supply. Once the insect population is managed, the sooty mold will eventually dry up and flake off.
Differentiating Causes and Treatment Strategies
Differentiating Causes
Effective management requires careful observation to distinguish between the three primary causes. True fungal or bacterial spots are distinct, defined lesions that may be sunken or have a yellow halo, and they are impossible to wipe off. Edema spots are small, firm, and slightly raised bumps, representing permanent internal damage. In contrast, sooty mold is a superficial, dark film that can be easily rubbed or scraped off the leaf, revealing healthy green tissue underneath.
Treatment Strategies
Treating infectious pathogens involves sanitation, such as pruning and destroying all infected leaves to reduce the source of spores. Improving air circulation and watering the soil directly, rather than the foliage, helps eliminate the moist conditions fungi require; in severe cases, a targeted fungicide application may be necessary. For non-infectious environmental stressors, the solution is cultural adjustment, including improving soil drainage and allowing the soil to dry more thoroughly between waterings to prevent edema. The plant should also be moved away from cold drafts or direct sources of chemical sprays. If sooty mold is present, the focus must be on eliminating the sap-sucking insects, followed by gently washing the mold residue from the leaves with a mild detergent solution.