For long- term management of such situations, you must improve drainage, lighten the soil with a mixture of organic matter and sand, and avoid too much additional water. Keep in mind that improving drainage includes draining away from the planting site. A well-prepared planting hole with plenty of organic matter still holds water like a bucket if it is in a clay soil. If you are not certain if water is the problem, dig up some of the soil around the suspect plant. In a typical situation with too much water, the soil is saturated and standing water may be evident.
Roots are black or brown internally instead of white, as with healthy, new roots.
The water problem must be alleviated for new roots to form. In some cases, wet soils predispose plants to root rots. For instance, Pythium and Phytophthora are common water-mold fungi that invade stressed plants in wet soils. If the water problem has been eliminated and root rot is still present, then a root-rot fungus might be involved as well. This is particularly true if not all plants in a bed are affected.