Edema
Physiological Abiotic disorder
Last updated
This profile contains basic abiotic disorder data. Regional field notes and expert review are in progress.
What Causes It
Edema (also spelled oedema) is a non-pathogenic disorder in which plant cells take up water faster than they can transpire it off, causing cell walls to rupture and small wart-like or corky blisters to form on leaf surfaces, petioles, or stems. The condition develops when root uptake exceeds leaf water loss, typically when soil is warm and wet but air temperatures are cool, humidity is high, and light is low — a combination that keeps stomata closed while roots continue to push water into the leaves. Greenhouse and container plants are most susceptible because root zone conditions are warmer than ambient air and environmental control can produce the warm-soil cool-air pattern that drives the disorder. Edema is cosmetic on most ornamentals but can be severe on sensitive species like ivy, pelargonium, and tomatoes. [VERIFY]
Quick Reference
Symptoms
Small water-soaked blisters or bumps on the lower surface of leaves, often in lines or patches. Blisters rupture and form corky tan, brown, or rust-colored patches as the damaged tissue dries. Severely affected leaves may yellow and drop. On succulent and thick-leaved species the blisters can be more prominent. On some species (particularly geraniums and ivy) the corky patches appear on upper leaf surfaces, petioles, or stems as well. The disorder is worst on plants grown in greenhouses, cloches, or low-light overwintering conditions when soil is wet. [VERIFY]
Small wart-like or corky blisters without fungal fruiting bodies or bacterial ooze. Association with warm wet soil and cool cloudy air. Pattern on the underside of leaves or on petioles. Most common on greenhouse, indoor, or overwintering plants.
Timeline: Appears under sustained warm-wet soil plus cool-humid air conditions. Resolves when conditions balance again. Damaged tissue does not heal but new growth is typically clean if conditions improve.
Triggers & Conditions
Warm wet soil combined with cool humid air and low light. Over-watering of container or greenhouse plants. Poor air circulation. Low transpiration demand coupled with high root pressure. Most common in spring greenhouse production and winter overwintering of container stock. The core mechanism is a water-uptake/transpiration imbalance: roots absorb water when soil is warm and available, but stomata remain closed (due to cool, dim, humid conditions), preventing transpiration to balance uptake. Cell turgor rises beyond the elasticity of cell walls, rupturing them and producing the characteristic blistering.
Cool cloudy weather with wet soil. Spring greenhouse conditions and winter overwintering indoors are typical scenarios. Indoor houseplants in cold windowsills during winter are most commonly affected.
Edema is most commonly seen on Puget Sound in greenhouse-grown transplants, overwintered container plants, and houseplants moved to cold windowsills during winter. Outdoor landscape cases are less common but do show up on ivy, geraniums, and some succulents during cool wet spring weather. The typical scenario is a houseplant or overwintered perennial in a cool low-light room with wet potting soil; adjusting watering and moving to a brighter location usually resolves the issue. [VERIFY]
Management
Prevention
- Adjust watering to match environmental conditions
- Improve air circulation in greenhouses and indoor growing spaces
- Provide adequate light for overwintering plants
- Use well-drained growing media
Not generally applicable; edema is a production and container-care issue rather than a landscape design issue.
Plant Tolerance
Greenhouse and container-grown plants are most susceptible. Sensitive species include pelargoniums, ivies, tomatoes, cabbage family, and many succulents.
More Sensitive
- Pelargonium species (garden geraniums)
- Hedera helix (English ivy)
- Solanum lycopersicum (tomato)
- Cabbage family greenhouse seedlings
- Many succulent species
Leaf anatomy, transpiration rate under low light, and species susceptibility to the specific water-uptake imbalance that drives edema.
Secondary Effects
Severely affected leaves may yellow and drop, reducing plant vigor temporarily.
Corky blistered tissue provides no additional secondary pathology but is cosmetically damaging on ornamental-quality crops.