As a general rule, any time you notice dark spots appearing on your plants which weren't there before, it's time to reach for the Disease Control. You can use anything commonly available that's ok to use on ornamentals, and Ortho, for example, even makes something for diseases and insects, all in one. Just be sure you do your spraying outside, (in the morning is best) and that the stuff has dried off before you bring the plants inside again. Wait ten days, then repeat the application. Also, be sure you treat the entire plant, not just the tops of the leaves.