There are a number of diseases that can affect geraniums, and, yes, the extra rain may be causing the problem. Keeping the foliage as dry as possible, especially in late afternoon and nighttime hours, will help to prevent these diseases. Removing dead foliage helps too; it can prevent the disease from spreading so quickly. If keeping the foliage drier doesn't help, you may need to resort to spraying a fungicide labeled for use on geraniums. Check for such a product at your local garden center, and use it just as the label says.