It might work on the peach tree fungus - I don't know. I'm pretty sure it wouldn't hurt to try it!
I used to have a baking soda recipe for a spray that controlled black spot on roses. (Lots of fungus, mold and mildew here in the NW!). It might have included a bit of
soap to help it stick to the leaves, but I can't find the recipe at the moment. Though Sue mentioned in the acidic soil
thread that baking soda can add salt to the soil, so I'm not sure if that's not the best option.