Before using fungicides, carefully read the label and follow directions. Be sure to wear protective clothing. Do not apply if other people are in the same area. Check the National Pesticide Information Center for more information: http://npic.orst.edu/
Biological fungicides are generally safe to use and have little or no effect on beneficial insects. Bacillus amyloliquefacius and bacillus subtillus are the most common ones and are sold under various trade names. They need to be applied every 3 to 5 days as the bacteria will eventually degrade after being exposed to air and sunlight. Used as a preventative against many common fungi such as powdery mildew but not effective at stopping the spread once plant is infected. Some can also be used as a soil drench to protect roots from soil borne pathogens.
Wettable sulfur is generally effective at protecting plants from powdery mildew and other air borne pathogens. It is incompatible with horticultural oils such as neem oil. It is not effective at stopping the spread of the disease once the plant is infected. Sulfur is harmful to some beneficial insects such as parasitic mites.
Neem oil is an effective fungicide and can be used to slow or stop the spread of some fungi such as powdery mildew. Incompatible with sulfur. Neem oil can harm a few beneficial insects. Try not to use in areas where bees are active.