Answer: This sounds like edema (also oedema), a physiological problem. It is caused by overwatering and is common on begonias and geraniums. The extra water in the tissues causes corklike patches. It doesn't usually kill the plant, but causes stunting and poor blooming. Since it is physiological, it will not spread to other plants. However, if you don't correct the watering, it will spread on the existing plants. Try growing them a bit drier, and make sure your watering is consistent -- not heavy water one time and sparse water the next.
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