When soils remain saturated with too much water, the plant roots cannot function properly and drown. As the roots die, the bark on the root begins to rot. Continued drowning causes the inner tissue to turn brown (on most plants, the tissue right under the bark is creamy white) and die while the bark softens and separates from the inner tissue. Meanwhile, above ground, the leaves begin to wilt, and leaves start going off color. As more and more roots drown, the leaves turn brown and die and eventually the entire plant dies. While simple drowning can cause the death of a plant, the drowning cycle can be problematic in other ways. Many root rot organisms attack when soils are wet and roots are stressed. Once most of these organisms get into the root, the fungi or fungi-like organisms will infect and kill root cells as long as the soils stay too wet. Other pathogens (Verticillium or Fusarium wilt pathogens) attack stressed roots. Drowning roots are stressed roots.
Some edema symptoms are bumps, or blisters on the lower leaf surface. Occasionally on some plants edema symptoms are seen on the upper leaf surface and/or on the petioles and stems. Eventually the bumps/blisters turn tan and corky when the cells rupture. The tan corky bumps are easily seen on lower epidermis of the needles of yews (Taxus spp.) when they develop edema. Depending on the plant other symptoms may be seen. Outside, many cool humid/damp/rainy days may result in edema occurring on several herbaceous plants as well as the yew and hibiscus. The following herbaceous flowers are prone to edema:begonia, ivy geraniums, cactus, cleome, ivy, ipomoea and annual thunbergia. Several vegetable plants such as broccoli, cabbage, cauliflower and tomato (leaves or fruit) are prone to edema.
Plants drown when the soil stays saturated to long for the root system of that particular plant. Where standing water occurs for hours after a rain or irrigation or where puddles occur on a regular basis are indications that most plants would not survive very long in these soils. There are multiple factors that can cause a soil to be poorly drained. Salt used to clear streets, driveways and sidewalks can create a structural problem. As sodium chloride gets into the soil, the sodium replaces the calcium in the soil. As the calcium is replaced the soil structure starts collapsing and drainage declines. Soil compaction can also be due to mechanical reasons, for example, a vehicle constantly driving on the soil especially when the soil is wet will compact the soil and damage the soil drainage. Eventually the soils become "puddled" and few plants can grow in affected soil. Consistent foot or animal traffic over time can also create a poorly drained soil.
Standing water or puddles are not the only indications of poorly drained soils. If soil is still "squishy" six or more hours after a rain or watering, it is another indicator of soils that drain too poorly for most trees (including fruit trees), shrubs, and herbaceous plants (including vegetables) to survive in that location. Plants need soils that not only contain nutrients and moisture but also oxygen.
Sometimes when soils are wet and uptake of moisture is faster than transpiration (water loss through the leaves), plants may show edema symptoms. Edema is a non-infectious disease, typically caused by the excess moisture available to the plant.
Improve drainage. This may be as simple as core aerating the soil or filling in low spots. More severe drainage problems may require re-grading or tiling to remove excess subsoil moisture. Elevating annual and perennial plants by raising the soil in a planting bed or planting trees and shrubs more shallow in a wet prone area may also help.
Drowning and diseases - management is more difficult. First drainage must be improved. If the disease is a root rot, the pathogen needs to be identified so the correct fungicides can be applied (not all fungicides control root rot pathogens). If the disease is one of the wilts, drainage still needs to be improved but there are no effective fungicides that control soil borne wilt diseases at this time. Waiting three or more years for the pathogen to decline in the soil is recommended before planting susceptible plants back into the area.
Most plants recover from edema as soon as growing conditions improve naturally. If the conditions that are causing edema are caused by an irrigation system, modification to the irrigation cycle would be necessary.