From a plant's perspective, the appearance of brown spots is often a direct signal of a microbial attack. My leaves are covered in a protective waxy cuticle, but it can be compromised. Overly moist conditions on my leaf surface, often from overhead watering or high humidity, create an ideal environment for fungal spores (like those of Colletotrichum or Fusarium) or bacteria (e.g., Pseudomonas) to germinate and invade. These pathogens penetrate the epidermis, killing the cells inside and consuming their contents. The resulting lesion appears as a soft, mushy, or sometimes crusty brown spot as the plant tissue necrotizes. This is my attempt to compartmentalize the infection and prevent it from spreading to the stem and vital parts, a process known as the hypersensitive response.
Sometimes, the cause is not an external invader but an internal imbalance. My roots are very efficient at absorbing water, especially when the soil is warm and moist. If the surrounding air is cool and humid, however, the stomata on my leaves close to prevent water loss. This creates a scenario where water pressure builds up inside my cells faster than it can be transpired. The excess pressure can cause individual cells to rupture, particularly in the leaf mesophyll. These dead, collapsed cells initially appear as small, watery, brown spots that later become corky and scab-like. This physiological disorder, called edema, is a clear sign that my root and shoot environments are out of sync.
My leaves are adapted to specific light levels. A sudden, significant increase in light intensity, such as being moved abruptly from a shaded area to direct, intense sunlight, overwhelms my photosynthetic systems. The excess solar energy damages the chloroplasts and other cellular components within the leaf tissues. This effectively "cooks" the cells, causing them to die and turn a bleached, light brown or tan color. These sunburn spots are typically flat and papery and appear most frequently on the top surfaces of leaves that are most exposed to the light source. It is a form of necrosis resulting from severe photoinhibition and oxidative stress.
My succulent leaves are turgid and fragile. Physical damage from handling, abrasion from nearby objects, or even pests nibbling on the tissue can break the skin and underlying cells. This injury creates a direct opening, causing the affected cells to die and oxidize. As I initiate my healing processes, the damaged area is sealed off with suberin and lignin, forming a permanent, corky brown scar. This scar tissue does not photosynthesize but serves the crucial function of protecting my internal systems from water loss and further microbial entry through the wound site.
