Edema
Also known as Oedema, Intumescence
In short
Edema is a non-infectious disorder in which roots take up water faster than the leaves can lose it, so leaf cells swell and burst. It leaves small raised blisters that turn into tan, corky scabs, mostly on leaf undersides. It is cosmetic and does not spread, but it signals that watering and conditions need adjusting.
How it looks
Tiny blisters about 1–2 mm (roughly 1/16 inch) across appear, especially on the undersides of leaves and on petioles. At first they may look pale green or water-soaked; they then rupture and turn tan, brown or rust-coloured and rough, corky or scabby. Blisters can form in clusters that look like larger patches. Badly affected leaves may yellow and fall, and on thin leaves the dead tissue can drop out, leaving holes.
Signs
Raised, corky or scabby bumps on leaf undersides; yellowing and dropping of heavily affected leaves; outbreaks after cloudy, cool, humid spells with wet soil.
What favours it
Warm, wet potting mix combined with cool, humid air, low light or overcast weather, when transpiration is low. A plant kept very dry and then watered heavily can also develop it. Overwatering, high humidity and low light all favour edema.
Prevention
- Feel the mix before watering and avoid extremes: neither drying out hard nor keeping it soggy.
- Water less in cool, cloudy or humid periods, keeping susceptible plants slightly on the dry side.
- Use a well-drained mix and pots with drainage holes.
- Give plants brighter light and better air movement; space them apart.
Treatment
Start with the first step and move down only if the problem persists.
- Damaged leaves cannot be restored; they can stay on the plant unless badly affected.
- Correct watering, light and airflow so new leaves grow without blisters.
- Repot into a better-drained mix if the mix stays wet for long periods.
Pros and cons
Cons
- Scarring is permanent and unsightly.
- Severe cases cause leaf yellowing and drop.
Look-alikes
- Rust
- Rust forms powdery orange-brown spore pustules; edema scabs are corky tissue of the leaf itself and do not spread.
- Bacterial leaf spot
- Bacterial spots are flat, dark and water-soaked with yellow halos and can spread between plants.
Affected plants
Especially ivy-leaved geraniums (Pelargonium), jade plants and other thick-leaved plants, pansies, kalanchoe, eucalyptus and Agave.
Related
Sources
- Edema — UC Statewide IPM Program
- Oedema Is a Corky Quirk — Purdue Indiana Yard and Garden
- Houseplant Diseases & Disorders — Clemson HGIC
Written in our own words from these sources. Check a product label before using any treatment.