A cactus can wilt for several reasons, usually over-watering and poor air circulation, but a wilting cactus may also indicate that there may be other pest or disease problems. The concerned cactus owner should examine his plant for signs of root rot, mites and mealybugs.
Spots and Patches
An injury or bruise to a cactus may cause corking, in which round or angular brown spots that increase in number appear. Tend the cactus carefully, by increasing the air circulation around the plant, and decreasing water and fertilizer because overfeeding contributes to this problem. If the cactus is pale, with tiny yellow specks, check for spider mites. Isolate the plant and clean its body by spraying with water and then insecticidal soap. Brownish to gray patches on the leaves indicate sunburn. Move the cactus to a shadier spot and gradually reintroduce it into sunlight.
Mealybugs on a cactus look like white, woolly deposits. Isolate the plant and remove the bugs with a toothpick, and spray with an insecticidal soap containing neem. Brown disk-shaped insects are scale. Remove these also with a toothpick every few days until the plant is insect-free.
A cactus that is shrunken and withered has been under-watered. Lightly water the plant and apply a fine mist of water. Water again the next day, wait three days and then water again. Repeat this routine if necessary. If the plant is wilting with crooked or uneven new growth, reduce the amount of light and fertilizer it receives. Cut off the misshapen parts with scissors or a sharp knife. Many cactus flowers are short-lived, so flowers that wilt and fade within one day are usual and not a problem. Extend the flower's life by moving the cactus to a cool location when it starts to bloom, and not moving it when it is in bloom.
Root rot turns the soil near the base of the cactus black. The rot starts in the root but spreads to the body of the plant. Over-watering and poor air circulation contribute to this problem. Scrape away the black portions of the cactus with a serrated knife, and allow the wound to heal before watering. If the root rot is widespread, take a healthy cutting from the plant to propagate a new one, and discard the infected cactus.