How to Treat Black Spot on Roses Organically*1-Spray milk mixture on roses.Combine milk with water & spray on roses. 4- Blend cornmeal with some water & form a paste. Rub this on roses. Let the paste stay on the flowers until it comes off naturally due to the weather & other elements. Cornmeal is a coating substance that can lift the black spots from the flowers. 5- Mix 1 gallon of water with 3 tsp of baking soda & 1 tsp canola oil. Coat the leaves until the mixture naturally dissolves.