I was checking around and found a recipe for a fungicide developed at Cornell University. Here's what you do:
In a sprayer mix 1 tablespoon baking soda, 1 tablespoon oil and 1 or 2 drops dishwashing liquid with 1 gallon of water and shake well, then spray.
They recommend using horticultural oil, either dormant oil as is used to spray peaches and other fruit
trees in late winter, or summer oil, which is a lighter oil, but they say that if you can't get these, vegetable oil can be used. They say this recipe has been proven to be as effective as any chemical spray. Spray in early morning or late evening to avoid sun scorch.