# Oedema

> Small blisters or corky bumps on leaf undersides caused by cells swelling with water and bursting when roots take up more water than the leaves can transpire, typically in cool, humid, wet conditions.

- Canonical: https://genetixbay.com/knowledge/glossary/oedema.html
- Also written: edema, leaf edema, leaf oedema, intumescence, intumescences
- Glossary category: Plant biology · Evidence: Supported
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Oedema (edema, or intumescence) is a physiological disorder, not a disease. When the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md) is wet but the air is cool and humid, roots push in water faster than the leaves can lose it, and some leaf cells swell and rupture, leaving blisters that turn corky and tan. It is documented in many greenhouse crops (UMass Extension); cannabis-specific reports are anecdotal.

## In practice

Look on the leaf underside for raised, water-soaked bumps that later turn brown and scabby, often on lower leaves. Fix the conditions: water less often, raise air temperature, lower [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) and increase airflow so [VPD](https://genetixbay.com/knowledge/glossary/vpd.md) rises toward 0.8-1.2 kPa. Damaged leaves stay marked; new growth will be clean.

## Don't confuse with

- **Mite or thrips damage**: flat [stipples](https://genetixbay.com/knowledge/glossary/stippling.md) or streaks, with pests visible under a lens.
- **Septoria leaf spot**: flat spots with dark margins and black fruiting bodies.
- **Spray burn**: spots where droplets dried in light.

## Related terms

[Transpiration](https://genetixbay.com/knowledge/glossary/transpiration.md), [Overwatering](https://genetixbay.com/knowledge/glossary/overwatering.md), [Relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md), [VPD](https://genetixbay.com/knowledge/glossary/vpd.md)

## Read more

- [Spots, brown patches and dead edges on cannabis leaves: the master decision tree](https://genetixbay.com/knowledge/diagnosis/spots-necrosis-decision-tree.md)
- [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)

## Sources

1. University of Massachusetts Extension. Edema (oedema) of greenhouse crops, fact sheet. https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/fact-sheets/edema