Oedema
Risk near you, next 48h
See whether your local forecast favours this disease right now.
When it strikes
Fix the cause, not the leaf
No spray will help here. The trigger is soil, water supply, nutrition or temperature, so change the growing condition and judge the result on new growth, not on damaged leaves.
Not contagious
Nothing spreads from plant to plant here. Neighbouring plants showing the same signs are reacting to the same soil, water or weather, so fix the cause rather than isolating plants.
Safe to compost
No pathogen carries over on this material, so it can go on an ordinary heap with the rest of the garden waste.
When and where to look
Scout the planting once temperatures drop below 18°C, with humidity above 85%. Risk builds after roughly 12 unbroken hours of those conditions, so the morning after such a night is the moment to look.
The signs that lead here
About
Oedema happens when roots take up water faster than the leaves can transpire it away. Cells on the leaf underside swell and burst, leaving corky bumps. It is a watering and humidity problem, common on greenhouse plants in dull cool weather and on succulents and pelargoniums in winter.
Symptoms
Small water-soaked blisters on the undersides of leaves and on stems, which later turn tan or rust-coloured and corky. Affected leaves may yellow and drop. The bumps do not rub off and no fungus or insect is present.
Organic Treatment
Reduce watering, water in the morning rather than evening, and increase ventilation and light. The affected leaves will never recover, but new growth will be clean once conditions change. Do not remove healthy leaves — the plant needs the leaf area.
Chemical Treatment
No treatment exists or is needed; oedema is not a pathogen. Spraying only adds moisture, which makes it worse.
Prevention
Let compost dry slightly between waterings in cool weather, space plants for air movement, ventilate greenhouses on mild days even in winter, and avoid cold water on warm rootballs.
Related problems
Oedema risk by location
Each location has a month-by-month profile computed from the weather archive: how many hours a year actually meet this disease’s thresholds.