Edema is not actually a pest or pathogen. It is a non-pathogenic, physiological disorder that is often mistaken for a leaf spot disease. It results when roots take up water faster than it can be used by the plant or transpired through the leaves. This results in the epidermal cells swelling and eventually collapsing.
Symptoms can include brown or tan wart-like, corky growths that harden and darken with age. Some plants develop water soaked lesions, or fluid filled blisters. The exact symptoms vary with plant species, growing conditions and the tenderness of plant tissue. Most often symptoms occur on leaves and can also occur on stems with severe cases.
Edema occurs most often during autumn and spring, though they may occur at any time of the year under the right conditions. Symptoms may develop during periods of warm or hot day time temperatures occur followed by relatively cold night time conditions and high moisture content in media/soil.
For more information refer to the factsheet available on the Nursery Production FMS website.
Figure caption for edema 2: Close up of edema symptoms on rose grape (Medinilla sp.).