Ag Notes: High rainfall levels can cause plant problems

July 07, 2004|JERRY LITTLE

High rainfall levels in Kentucky during late May resulted in high soil moisture levels which may cause symptoms to appear on some landscape plants. Prolonged soil saturation and localized temporary flooding which occurred in some regions of Kentucky would reduce soil oxygen levels, causing roots to function abnormally, thus stressing plants.

It has been our experience that when excess soil moisture is present in landscapes due to frequent, heavy rainfall, landscape plants can show symptoms of damage caused by excess water. Landscape professionals and homeowners can be looking for symptoms, which may include:

* Chlorosis - particularly on plants such as flowering crabapple that don't normally get Chlorosis from iron deficiency;

* Edema - many plants such as day lily, euonymus, holly, and spruce can show edema;

* Red or purple coloration - euonymus and kousa dogwood sometimes show this, but be aware that flowering dogwoods with powdery mildew may also show purple coloration;


* Marginal leaf browning or sunscald - can occur on some plants;

* Wilt - sometimes the wilt is associated with Phytophthora root rot, but excess water alone can cause wilt;

* Twig dieback - again canker-causing pathogens, active on flood-stressed plants, can be involved; and

* Plant death - Phytophthora root rot, especially on hemlock, taxus, and rhododendron is likely to be associated with excess soil moisture.

* Epinasty - (downward bending of leaf petioles), stem swelling, and leaf drop are also symptoms of excess water damage to plants.

Some trees and shrubs show symptoms or die if only flooded for a few days during the growing season. Local short-term flooding and soil saturation have certainly occurred in many instances these past several weeks in Kentucky.

Roots in flooded or waterlogged soils often die of oxygen deficiency. In flooded soil, plant roots and microorganisms use up the available oxygen while adding to a buildup of carbon dioxide. As redox (chemical reduction and oxidation) potential decreases due to low oxygen levels, some mineral elements may be reduced to toxic forms. In addition, a variety of toxic organic substances can form in the soil. The metabolism of the tree is changed and adversely affects the tree by using energy less efficiently, producing toxic by products, inadequately taking up water and mineral elements, closing stomata, and depressing photosynthesis and translocation.

After the soil drains, plants with killed roots may subsequently suffer drought stress and death. For many of these plants, the only functioning roots are near the soil surface, and when dry weather follows wet, those surface roots quickly dry out. Plants enduring a flooding episode may also become abnormally susceptible to Phytophthora root rot or collar rot. Excess water promotes susceptibility of roots to this disease and aids the fungus in moving to new infection sites during its swimming phase. So, if the roots don't die of lack of oxygen, water molds such as Phytophthora are waiting for their turn to kill the plant by causing root and collar rot.


Little Jimmy was in the garden filling in a hole when his neighbor peered over the fence. Interested in what the cheeky-faced youngster was up to, he politely asked, "What are you up to there, Jimmy?"

"My goldfish died," replied Jimmy tearfully, without looking up, "and I've just buried him."

The neighbor was concerned, "That's an awfully big hole for a goldfish, isn't it?"

Little Jimmy patted down the last heap of earth then replied, "That's because he's inside your cat."

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