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Tree and Shrub Water Injury

July 14, 1999

Another year of abundant rainfall has set the stage for root problems, often referred to as “wet feet” (feet meaning roots). Symptoms are often the same as those resulting from a lack of water or other root injuries. They include withering of leaves, little terminal growth, yellowing of foliage, and dieback of shoots and roots. Some woody plant species are particularly sensitive to such conditions, including yews (which are injured by as little as 12 hours of saturated soil), rose, white birch (which has seen much injury the last two years), Norway and sugar maples, flowering dogwood, and forsythia, to name only a few. Water tolerance of many plants is discussed in Diseases of Trees and Shrubs by Sinclair, Lyon, and Johnson. Most good tree-identification books list such sensitivities as part of the species description.

Interestingly enough, woody plants often show injury from water damage when a hot, dry spell occurs after heavy rains. The roots have been damaged. The water deficit set up by heat, sun, and wind pulls on roots to provide water to the foliage. Even if plants are watered, the injured roots cannot take up water fast enough to meet the demands of such environmental conditions.

Roots need oxygen to grow and to absorb nutrients. In a water-saturated soil, the oxygen content is low and, without oxygen, roots cannot respire or take up water properly. Even though water is abundant, the plant cannot absorb it. 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 that holds water.

If you are not sure that water is the problem, dig up some of the soil around the suspect plant. Typically, if there is too much water, the soil is saturated and standing water may be evident. Roots are black or brown internally instead of the white color of healthy, new roots. Usually, fungicides do not help. They pro-tect 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 and cause root rot. If the water problem has been eliminated and root rot is still present, a root- rot fungus might be involved as well. This is particularly true if not all plants in a bed are affected. In such cases, consult a lab or specialist who is trained to identify the root-rot fungi. Root rots cause roots to be discolored brown, black, or pink. In the early stages, root tips are discolored or there are lesions on the roots. In more advanced stages, entire roots rot and, at that point, plant decline is very noticeable. Wash roots carefully to remove soil without removing rotted root cortex tissues. Root rots are more evident without soil coverings. 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 Reports on Plant Disease (RPD) 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.

Author: Nancy Pataky


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