Edema
Water imbalance (physiological disorder, not pathogenic)
Overview
Edema (also spelled oedema) is a physiological disorder, not an infection. It occurs when roots take up water faster than leaves can transpire, so cells in leaves or stems burst and form watery blisters. Cool, humid, cloudy weather, overwatering, and poor airflow are classic triggers. Succulents, geraniums, brassicas, tomatoes, and many greenhouse crops show edema. Gardeners often mistake the corky spots for pests or disease and spray unnecessarily, which does not correct the water balance problem.
Symptoms
Small water-soaked bumps appear on leaf undersides or along stems, then turn tan, corky, or white as they heal. Leaves may cup, distort, or look scarred. On cabbage and related crops, blistering can be widespread after muggy nights. Succulents develop corky patches on leaves and stems. Growth otherwise continues, and there is no pathogen ooze or fungal mycelium. Symptoms often appear suddenly after a stretch of cool, wet, low-light days following heavier irrigation.
Environmental Conditions
Humid
Prevention
Water based on plant need and weather, not a fixed daily schedule. Use well-drained media and pots with open drainage holes. Provide morning sun and good spacing so leaves dry and transpire. In greenhouses, vent humidity and avoid overnight soggy media. Harden transplants gradually so they are not moved from humid propagation to cold, wet outdoor nights without adjustment.
Management
Reduce irrigation frequency when weather is cool and cloudy. Increase airflow with spacing, pruning, or fans in greenhouses. Raise light levels if plants are shaded. Avoid heavy fertilization that produces soft, watery growth during low-transpiraton periods. Damaged leaves do not heal cosmetically; remove the worst ones if appearance matters, and wait for new growth under better conditions. No fungicide or insecticide will fix edema.