Oedema: how to recognise and treat it

Growing conditions

Symptoms

Small water-soaked spots on the leaves, mostly on the undersides, that swell into raised, blister-like or pimple-like bumps.
The bumps break open and turn tan or brown, corky and wart-like; they can look white and powdery or rusty and scaly.
Appears when the soil is wet and the air is cool and humid, so the roots take up more water than the leaves can give off.
In severe cases corky growths also form on leaf stalks and stems, and the leaves turn yellow, droop and fall.

Water less and keep the plant slightly on the dry side, but do not let the compost dry out completely.
Do not pick off the affected leaves — this makes oedema worse; improve the conditions and let healthy new growth develop.
Space plants farther apart and ventilate well so air moves over the leaves.
If a pot plant is too wet, repot it into fresh, free-draining compost; give the plant more light.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Avoid overwatering, especially in cool weather; never let the soil or compost stay waterlogged.
Use free-draining compost; in the garden improve heavy soil with organic matter and stand permanent pots on pot feet.
In winter keep the relative humidity around indoor and greenhouse plants below 70%.
Ventilate greenhouses and conservatories well, especially in hot, sunny weather.

Figures checked against these sources.