From your description I suspect that your geraniums have botrytis, also known as gray mold. This fungal disease affects dying flowers and older leaves. Its development is favored by high humidity and poor air circulation. Remove dead leaves and flowers (as you have been doing) to slow its spread. When conditions become warmer and drier, the problem will likely subside. In the meantime, do what you can to promote rapid drying of plant foliage. Water only early in the day; space plants farther apart, if possible; and improve airflow around the plants.