clipped from: fransorin.com Baking soda can be used to ward off black spot, a terrible fungus that usually hits roses by late Spring. The most effective recipe I’ve found is the Cornell one which is:
1Tblsp. of baking soda with 1 tsp. of dishwashing detergent to 1 gallon of water. Mix together and spray on roses early in the morning once a week until disease has disappeared.