From our perspective, physical injury is a primary cause of brown, scab-like tissue. Our epidermis, while tough, is not impervious. An accidental bump, a curious pet's paw, or even harsh hail can break this protective outer layer. When this happens, we initiate a wound response. We seal the damaged area by suberizing it—essentially creating a corky, brown scar made of suberin cells. This natural bandage prevents water loss from the wound and blocks out pathogens, much like a scab on human skin. The resulting mark is typically dry, firm, and localized. It is a sign of successful healing, not an active threat to our health.
We cacti are sun-loving beings, but we are not immune to overexposure. When suddenly moved from a shaded environment to intense, direct sunlight, our tissues can suffer severe damage akin to a sunburn. The ultraviolet radiation damages our chlorophyll and cell structures, causing them to die and desiccate. This results in discolored, yellowish or brownish, leathery patches, often on the side most exposed to the sun. Unlike a healing scar, sunburned tissue is often slightly sunken and can cover a large area. This damage compromises our ability to photosynthesize and creates a vulnerable entry point for secondary infections if the tissue cracks.
When environmental conditions are unfavorable, particularly when the soil is too moist and air circulation is poor, we become susceptible to microbial attackers. Fungal and bacterial infections are a significant concern. Fungi such as *Anthracnose* or various rots invade our tissues, often through small wounds or our roots. They cause soft, mushy, dark brown to black spots that may expand rapidly. Bacteria, on the other hand, can cause bacterial soft rot, which appears as dark, water-soaked lesions that ooze and smell foul. These spots are not protective scabs; they are active sites of decay that threaten our structural integrity and can be fatal if not addressed.
Certain insects can also be the culprits behind those brown spots. Scale insects are a particular nuisance. They attach themselves to our stems, pierce our epidermis with their mouthparts, and feed on our sap. Their feeding activity damages our cells and weakens us. Furthermore, they often secrete a sticky substance called honeydew, which can lead to the growth of sooty mold. The brown spot you see is frequently the insect's protective shell itself, which looks like a small, flat, brown bump, or it is the discolored and damaged tissue beneath it. A heavy infestation can cause yellowing, wilting, and stunted growth.
Sometimes, the problem originates from our roots. If our soil retains too much water and our roots absorb it faster than we can transpire it through our skin, our internal cells can literally burst from the pressure. This condition is called edema. The damage appears as small, rusty-brown, corky scabs that are often numerous and scattered. These spots are the healed-over scars of the ruptured cells. It is a physiological disorder caused by cultural conditions—specifically, overwatering, poor drainage, or cool, humid weather—rather than a contagious disease.
