The two basic rules to follow are to let the soil dry out between waterings, and to refrain from watering during the winter when the plants are dormant. In their natural habitat most cacti will have a long growing season and a short dormant period. As the roots continue to die in the stagnant soil, a point is reached at which they are unable to supply sufficient water and the plant appears to be suffering from lack of water. If more water is supplied, the situation gets worse and the rot may spread upwards into the basal stems or plant body. Eventually the plant body is observed to be soft and discoloured, perhaps yellow or grayish, by which time it is usually too late to save it. The moral is, that if a plant appears to be failing to take up water, knock it out of its pot and examine the condition of the roots before supplying more water.Other reasons for loss of roots include pest damage and dormancy. Watering a plant at the wrong time of year when it is dormant can cause rotting as effectively as can also happen if the roots have been eaten by insect pests.