How to recognise it
Visible through raised, cork-like spots on the underside of affected leaves which start out the same colour as the leaf but later turn brown. Could be mistaken for the pustules of the rust fungus.
What causes it
It is caused by the plant’s roots providing a higher volume of water than the leaves can effective transpire (ie get rid of). This is most likely to happen in late winter and early spring in periods of cool, cloudy weather. Where the soil is moist and warm and the surrounding air is cool and moist the condition is most prevalent.
Why it’s a problem
Damages leaves (although it shouldn’t do sufficient damage to kill them).
Where you are likely to find it
All plants, but particularly camellias, eucalyptus, pelargoniums, rhododendrons, begonias, pansies, violets and other fleshy leaved plants (eg succulents). Can occasionally affect orchid petals.
Commonly found in unheated greenhouses.
How to prevent it
In greenhouses ensure appropriate heating and ventilation is installed to control the environment. Ensure plants are well spaced for good air circulation. Watering should also be reduced where there is a risk of excess moisture being available to the plant.
How to get rid of it
You can’t get rid of it, but you can stop it worsening by reducing the amount of watering and, where possible, improving drainage. Don’t remove damaged leaves as they should survive.
Is it good for anything?!
No.
Other useful information
Oedema is also a condition affecting humans, where water is retained and causes swelling, often in the hands and feet.
