Most of Illinois received intense rainfalls a few weeks ago. Amounts varied by location, but some received 10 inches in 2 weeks, setting the stage for root problems referred to as “wet feet” on trees and shrubs. Diagnosis of water damage can be difficult because symptoms are often the same as from a lack of water or from other root injuries. Symptoms may include withering of leaves, little terminal growth, small leaf size, yellowing of foliage, and dieback of shoots and roots. Some woody plant species are particularly sensitive, including yews (injured by12 hours of saturated soil), rose, white birch, Norway and sugar maples, flowering dogwood, and forsythia. Water tolerance of many plants is discussed in Sinclair, Lyon, and Johnson’s book, Diseases of Trees and Shrubs. Most comprehensive tree ID books also list water sensitivities in the species description.
Water damage injures roots, limiting ability to absorb water and nutrients. For this reason, woody plants (trees and shrubs) often show injury when a hot, dry spell occurs after heavy rains. Aboveground plant parts need water, but the roots cannot absorb it fast enough. Watering helps; but in hot weather, the injured roots cannot absorb water fast enough to meet demands of such environmental stress.
Roots need oxygen to grow and to absorb nutrients. When soil is saturated, its oxygen content is low. Without oxygen, roots cannot respire properly and absorb water. Despite abundant water, the plant cannot effectively use it. For long-term management, improve drainage of the soil and drainage away from the plants.
To determine whether water is the problem, dig up some 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 inside, instead of a healthy white. In most cases, fungicides do not help once roots lose the outer layer to water damage: Fungicides are developed to protect healthy plants from root-rot pathogens, not to revive dead roots. They may be helpful to plants along margins of water-damaged areas. The water problem must be alleviated for new roots to form. Root-rot fungi commonly infect stressed plants, especially those stessed by excess water. See Report on Plant Disease, no. 602, “Armillaria Root Rot of Trees and Shrubs,” and no. 664, “Phytophthora Root Rot and Dieback of Rhododendrons and Azaleas,” available in University of Illinois Extension offices or on the Vista Web site.