Apparently (I have no personal experience of this) you use 1 tablespoon of baking soda and half a teaspoon of washing up liquid to a gallon of water.
You should water the plants a day or so ahead of spraying them with this and use caution (test a bit) as it can sometimes burn the leaves. Avoid doing it in bright sunlight. Apply to top and bottom of leaf.
you can also add 1 tablespoon of horticultural oil, which smothers the fungus.
I found this on About.com (Gardening).