Nowadays there are lots of fungicides, both organic and otherwise. Someone who used to post a lot on here always recommended keeping a can of aerosol Lysol on hand for that, and I haven't had any since I followed her advise. Some people recommend baking soda, but I don't remember the formula--maybe a tbsp to a gallon of water, plus a spreader/sticker. There's also things like Neem oil, which is excellent stuff, and any number of standard commercial fungicides.