Edema

also called Oedema, Dropsy

Small corky blisters or warty bumps on the undersides of leaves, where the plant has taken up more water than it can breathe out.

Disorder Disfiguring Doesn't spread

What you'll see

Water-soaked bumps on the leaf underside that turn corky, brown and warty
Rows of raised blisters along leaf veins and on leaf stalks
Corresponding pale or yellow dimples on the upper leaf surface
Badly affected leaves yellow and drop; the bumps do not rub off and hold no pest inside
Older, lower leaves worst affected on soft, sappy plants

Why it happens

The roots take up water faster than the leaves can transpire it away, usually because the air is cold and humid while the compost is warm and wet. The excess water swells cells on the leaf underside until they burst, and the plant seals the wounds with corky tissue. It is a watering and air problem, not a pest or disease.

Typical under glass in dull, cool, humid weather, especially in late winter and spring when days are short. Overwatering, poor ventilation and crowded plants all make it worse.

Treating it

Reduce watering and let the compost dry a little between waterings
Improve ventilation and air movement around the plants
Space plants out and raise the temperature on cold, dull days
Leave affected leaves in place unless badly disfigured, as removing them does not undo the damage

Keeping it away

Water in the morning so leaves are drier and warmer through the cool of the day
Keep greenhouses and windowsills well ventilated and avoid stagnant, humid air
Do not overwater, particularly in dull spells and over winter
Give crowded plants more room so air can move freely between them

Who gets it

Pelargoniums are the classic sufferers, along with brassicas, succulents, camellias, ivy and fuchsias grown under glass or on cool windowsills.