Pelargonium: Oedema

Problem
Water-soaked spots appear on the leaves. Eventually, these spots turn brown and corky. Affected leaves may turn yellow and drop off. Corky ridges may form on the stems and leaf stalks. In most cases, the soil is moist and the air is cool and humid.

Analysis
Oedema is not caused by a pest, but results from an accumulation of water in the plant. Oedema often develops when the soil is moist or wet, and the atmosphere is humid and cool. Under these conditions, water is absorbed rapidly from the soil and lost slowly from the leaves, resulting in an excess amount of water in the plant. This excess water causes cells to burst. The ruptured cells eventually form spots and ridges. Oedema occurs most frequently in late winter and early spring during cloudy weather, and in greenhouses.

Solution
Plant geraniums in soil that drains well, and avoid overwatering them. (For information on proper watering, see About Watering.)