A cactus turning brown signals distress, ranging from a harmless natural process to a serious, life-threatening issue. Accurately identifying the type of browning is the first step toward diagnosis and intervention. Observing the texture, location, and speed of the discoloration helps determine if the plant needs a simple adjustment or immediate care.
Identifying Benign Browning (Corking)
Corking, or lignification, is a natural part of the aging process and is not a problem. It occurs when the lower, older stem tissue hardens, developing a brown, bark-like exterior for structural support. This adaptation is common in mature, columnar cacti that need to support significant weight.
This process typically begins at the soil line and progresses slowly and evenly upward around the stem. The affected tissue remains firm, dry, and woody to the touch, distinguishing it from concerning types of browning. Corking is permanent and requires no treatment, as it signifies a healthy, aging plant.
Browning Due to Overwatering and Rot
Rot is the most common and destructive cause of browning, resulting from fungal or bacterial infection due to excessive moisture. Overwatering or poorly draining soil creates an anaerobic environment where pathogens thrive and attack the cactus tissue. This problem often starts hidden in the roots before spreading rapidly up into the main body of the plant.
Rot-induced browning is characterized by a soft, mushy, or squishy texture that appears dark brown or black. Decaying tissue may leak fluid or have a foul odor in advanced stages. To save the plant, immediately stop watering and unpot the cactus to assess root damage. Healthy roots are light-colored and firm, while rotten roots are black, slimy, and easily crumble.
To halt the infection, surgically remove all compromised tissue using a sterile, sharp knife. Cut away all soft, discolored areas until only clean, healthy green tissue remains. The remaining healthy cutting must be left in a dry, shaded area for several days to form a protective, dry callus. Once the cut surface is completely dry and hardened, replant the healthy portion in fresh, sterile, well-draining cactus mix without watering for at least a week.
Environmental Stressors and Pests
Browning that occurs suddenly or in specific patches is often a reaction to environmental stress or pest activity. Sunburn appears as pale yellow, tan, or dry, crusty brown patches, typically on the side exposed to intense, direct light. This damage is permanent but stops spreading if the plant is moved to a location with filtered or less intense afternoon sun. Cold damage causes soft, mushy, dark green, or black spots when water inside the cells freezes and bursts the cell walls. If localized, the affected mushy tissue should be removed to prevent secondary infections.
Certain pests manifest as brown spots that are actually the insects themselves. Scale insects are small, dome-shaped, brown or tan bumps fixed to the surface, sucking out plant sap. Spider mites cause a reddish-brown or rust-colored discoloration, often accompanied by fine webbing on new growth. Both pests can be treated by gently scraping them off or dabbing the affected area with a cotton swab dipped in 70% isopropyl alcohol.