Root rot is a disease of the plant's roots that prevents the plant from obtaining the water and nutrients it needs. It is a soil-borne fungus caused by too much moisture. Root rot is difficult to recognize quickly because it attacks only the roots of a plant.
A plant with root rot will have stunted growth, show "necrotic spots," (small black spots of decayed tissue) and brown soft roots. The plant will turn yellow, wilt, try to recover in the evening and eventually die.
One cause of root rot is over-watering, which can starve the roots of oxygen. Another cause is a fungus within the soil. That fungus can lay dormant for a prolonged period and become active after the area is over-watered a couple of times.
Roots that suffer from this disease are easily recognizable: They turn black and mushy. As the disease progresses, the roots may also fall off the plant.
Remove the plant from the soil and gently wash the roots under running water while trying to remove the infected soil and roots. Use scissors to cut infected roots. Dip the remaining roots in a fungicide solution. Re-pot the plant in new soil. If removing a large percentage of the plant's roots, trim the top of the plant to reduce the number of branches and leaves to reduce the stress and nutritional needs of the plant. Do not fertilize the plant in the weeks immediately after treating for root rot. The fragile, growing roots can not handle it.
When planting, choose pots that drain well. Don't water every day. Instead, test the top inch of the soil with your finger and water only when the soil is dry.