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Oedema
Symptoms
Water-soaked spots on the leaves that eventually burst and die, leaving raised, wart-like or corky patches, especially on the undersides. Initially these abnormal growths are similar in colour to the leaf but in time they turn brown.
Plants affected
Many plants, particularly those growing in pots, especially
camellias, eucalyptus, pelargoniums, cacti, succulents, fuchsias
and orchids.
On eucalyptus, oedema scabs are more variable in size and shape when compared with gall wasp galls, however they are not hollow and do not contain insect larvae.
Cause
This is not a disease but a physiological disorder, thought to be caused by excessive moisture in the growing medium and high air humidity. As a consequence, roots take up more water than the leaves can transpire, which results in localised water retention in the leaves.
Control
Water less frequently, and improve air circulation and drainage where possible. Removing affected leaves only exacerbates the situation.