Here is a problem that is no longer considered an exception to the rule or unusual for our area. Two or three years of abundant rainfall have set the stage for the root problems often referred to as wet feet ("feet" referring to roots). Symptoms are often the same as those resulting from a lack of water and include withering of leaves, little terminal growth, yellowing of foliage, and dieback of shoots and roots. Some woody plant species are particularly sensitive to wet conditions, including yews, rose, white birch, Norway and sugar maples, flowering dogwood, and forsythia, to name only a few. Water tolerance of many plants is discussed in Sinclair, Lyon, and Johnson's book, Diseases of Trees and Shrubs. Most good tree-identification books also list such sensitivities as part of the species description.
Roots need oxygen to grow and to absorb nutrients. In water-saturated soil, the oxygen content is low; without oxygen, roots cannot respire properly and cannot take up water. Even though there is an abundance of water, it cannot be absorbed by the plant. 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. In most cases, fungicides do not help (they help protect healthy plants from root-rot pathogens, but they do not revive dead 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. In such a case, consult a lab or specialist trained to identify root-rot fungi. Root rots will cause roots to be discolored brown, black, or pink. In early stages, the tips of roots are discolored or lesions are present on the roots. In more advanced stages, the entire roots rot, at which point plant decline will be very noticeable. Soil fungicide drenches are available to stop the progress of root rots in herbaceous plants and small shrubs, but there is nothing that can be used on mature trees.
Information on root rots is available in Report on Plant Diseases No. 615 (Damping-Off and Root Rot of House Plants and Garden Flowers), No. 602 (Armillaria Root Rot of Trees and Shrubs), and No. 664 (Phytophthora Root Rot and Dieback of Rhododendrons and Azaleas).