Root rot is a condition found in both indoor and outdoor plants, although more common in indoor plants with poor drainage. As the name states, the roots of the plant rot. Usually, this is a result of overwatering. In houseplants, it is a very common problem, and is slightly less common in outdoor plants. In both indoor and outdoor plants, it is usually lethal and there is no effective treatment.
The excess water makes it very difficult for the roots to get the air that they need, causing them to decay. To avoid root rot, it is best to only water plants when the soil becomes dry, and to put the plant in a well-drained pot. Using a heavy soil, such as one dug up from outdoors can also cause root rot.
Many cases of root rot are caused by members of the water mould genus Phytophthora. Perhaps the most aggressive is P. cinnamomi. Spores from root rot causing agents do contaminate other plants, but the rot cannot take hold unless there is adequate moisture. Spores are not only airborne, but are also carried by insects and other arthropods in the soil.
A plant with root rot will not normally survive, but can often be propagated so it will not be lost completely. Plants with root rot should be removed and destroyed.