Answer: Geraniums can develop something called Oedema. Water-soaked spots appear on the leaves and eventually these spots turn brown and corky. The undersides of the leaves are most often affected, and leaves may turn yellow and drop off. These corky ridges may form on the stems and leafstalks, as well. Oedema is not a disease but results from an accumulation of water in a plant. It can develop when the soil is moist or wet and the atmosphere is humid and cool. Under these condtions, 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 and the ruptured cells eventually form spots and rides. Oedema occurs most frequently in late winter and early spring during cloudy weather.
The solution - plant geraniums in soil that drains well and avoid overwatering them.
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