Thanks for writing. There are several common leaf spot diseases that attack nandina. Your best bet is to look for disease-resistant cultivars and make sure your plants are in a sunny spot (the more shade they're in, the more likely they are to become diseased).
During periods of moist weather, spraying them with a fungicide labeled for use on leaf spots can help keep the disease at bay. But fungicides, unfortunately, can't be used to cure disease once it's already attacked.
---Justin, Senior Garden Editor, BHG.com "