In the winter, geraniums may develop yellow spots on upper leaf surfaces with small brown corky, slightly raised spots on corresponding lower leaf surface areas. What causes this condition?
The condition is called oedema. It is caused usually after several cloudy days when roots absorb water normal for sunny days. On cloudy days, stomates are closed somewhat and transpiration (normal water loss through open stomates) is reduced. Leaf cells become too turgid and burst with resulting corky spots developing on lower surfaces. Irrigation should be reduced on cloudy days, if this is possible.
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