How Do I Prevent Geranium Leaves from Turning Brown?

It sounds as though your geraniums are infected with botrytis blight, or gray mold. This fungal disease attacks many plants, especially during cool, wet weather. The problem can be severe in an enclosed greenhouse, during cloudy conditions, or when air circulation is poor and humidity is high. Outdoors, the problem is usually temporary. 

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When the weather warms up and dries out, the problem goes away on its own. You can help prevent the spread of the disease by clipping off and destroying affected flowers and leaves. Give plants wider spacing so they’ll have better airflow around them and dry out more quickly. Use drip irrigation, if necessary, to avoid wetting foliage and flowers when watering. If the problem persists, spray with a protective fungicide.