Using vinegar as an herbicide reminded me of a fantastic natural fungicide for seedlings: chamomile tea.
One summer I worked on a farm that grew dried flowers. The gal was attempting to be as sustainable as possible and did a little test of chemical fungicide versus the chamomile tea and the tea prevented seedling stem rot just as well, if not better than the chemical! I've used it too, with great results.
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.