This plant disease is caused by any of several bacteria (Erwinia species) that enter the cactus through a wound under wet conditions. Once established, the bacteria produce materials that dissolve the layers between the cells. As the cells die, liquids are released from them, producing a soft, mushy rot. At high temperatures, the bacteria move through the tissue rapidly. The dead tissue dries, producing sunken cavities. The bacteria may invade the whole stem and kill the plant.

Diaspis echinocacti This scale is common on cactus both outdoors and indoors. The infestation may be so heavy that the entire surface appears gray and crusty. The scale lays eggs in the spring. In late spring, the young scales, called crawlers, settle on other parts of the plant. The tiny (1/30 inch), soft-bodied young feed by inserting their mouthparts and sucking sap from the plant. The legs atrophy, and a hard, crusty, gray shell develops over the body of the female. Males are white and remain soft. Mature female scales lay their eggs underneath their shells. An uncontrolled infestation may kill the cactus after two or three seasons. Several other species of scales may also infest cactus.

Mealybugs are the most common pests of cultivated cacti. Cottony cushion scales are also common on cacti. The visual similarities between these insects make separate identification difficult. They are conspicuous in late spring and summer because the females are covered with a white, cottony egg sac, containing up to eight hundred eggs. Females lay their egg masses on leaves or stems. The young insects, called crawlers, that hatch from these eggs are yellowish brown and inconspicuous. They feed throughout the summer on the stems and leaves. Damage is caused by the withdrawal of plant sap from the plant. Some species of scales and mealybugs that infest cacti are unable to digest fully all the sugar in the plant sap, and they excrete the excess in a fluid called honeydew. A sooty mold fungus may develop on the substance, causing the plant to appear black and dirty.

Several species of this common insect feed on cacti. Mealybugs damage plants by sucking sap, causing plant distortion and death. The adult female mealybug may produce live young or may lay eggs in a white, fluffy mass of wax. The immature mealybugs, called nymphs, crawl all over the plant and onto nearby plants. Soon after they begin to feed, they produce white, waxy filaments that cover their bodies, giving them a cottony appearance. As they mature, they become less mobile. Mealybugs cannot digest all the sugar in the sap, and they excrete the excess in a fluid called honeydew, which coats the stems and may drop onto surfaces below the plant.

Most cacti are native to areas that are hot and dry with very sandy soils. Such soils have large spaces between the particles and therefore have little ability to retain moisture. Because cacti need a lot of air around the roots, they do well in sandy soils. If deprived of air because the soil retains too much water, the roots decay and the plants fail to grow, or they die.

Cacti thrive in well-drained, sandy soil. They are very susceptible to root rot, a plant disease caused by any of several soil-dwelling fungi, also known as water molds. These fungi thrive in waterlogged, heavy soil. Some of them attack plant stems at the soil level, while others attack the roots. Infection causes the roots and stems to decay and eventually causes the plant to die.