Edema is the development of raised, scabby areas, especially on the underside of leaves. This abiotic disorder, or noninfectious disease, is also called corky scab or oedema.
Affected tissue is blisterlike and often brown or tan. Typically, symptoms appear as single, blisters or warts, about 1/16 inch in diameter. They may occur in small groups or cover large areas of a leaf. Two or more spots may merge to form a larger affected area.
Edema often develops when soil is especially wet or waterlogged and transpiration is reduced, such as during cloudy weather or exceptionally high humidity, such as in greenhouses. These conditions apparently cause the roots to absorb water at a rate faster than is lost through transpiration. Excess moisture accumulates in leaves, damaging tissue and causing leaf blisters.
Edema damage is aesthetic and does not threaten plant health.
Injured tissue cannot be restored. No methods are known to definitely prevent edema in landscapes.
Excess soil moisture apparently promotes edema, so avoid irrigating too frequently and improve soil drainage where warranted. Avoid irrigation during cool, overcast humid weather, such as persistent foggy conditions. Avoiding prolonged excess soil moisture will at least help to prevent certain other common maladies.
Edema blisters on kalanchoe leaf
Scabby blisters (edema) on eucalyptus