Why do cactus plants turn brown?

A cactus plant may turn brown, because of natural aging. Not all cactus plants are green in color, or stay that color in the course of a lifetime. Or a cactus plant may turn brown, because of environmental stress or stressors. One example of stress related browning is scorching from too much direct sunlight at too high temperatures. Other examples include viral, fungal or bacterial diseases; and pest attacks by predatory bugs and insects. Or a cactus plant may turn brown, because of drought. Way too little water dries out the fleshy tissues on the inside of the cactus. Or it may turn brown, because of death.