Organic fungicides include diluted milk (any where from 10 to 50% milk, allow to sit until it is room temperature), hydrogen peroxide sprayed straight from the bottle (which is a 3% solution), baking powder solution (1 teaspoon baking soda to 1 quart of water, plus a few drops of liquid dish soap), Neem oil, etc.
You can't mix treatments, but you can rotate/ alternate them. They will need to be repeated every week or two or after it rains.
All of them work best as preventative or if you catch it very early on. If the disease has already progressed, remove the affected leaves first. Spray thoroughly including underside and top of leaves.