Answer: I really can't positively diagnose a problem without seeing it but I suspect a problem called Oedema. It is more common on ivy geraniums that other types, but it can certainly appear on all types of geraniums. Oedema is considered a water imbalance that develops during periods of cool, cloudy weather or when the soil is moist and warm and the air is moist and cool.
The symptoms first appear as chlorotic spots on the upper leaf surface. These develop into water-soaked blisters on the undersides of leaves that later become corky and rusty-brown. The entire leaf may turn yellow, die and drop off.
To control, use a well-drained soil or potting medium; Maintain proper nutrient levels (increased nitrogen and iron have been reported to reduce incidence of Oedema); Avoid over-watering during cool, humid weather; Reduce humidity and space plants to promote good air circulation.
You cannot improve the appearance of the affected leaves but if you improve the growing conditions any new leaves will be healthy and unaffected.
Good luck with your plants.
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