A tonic for black spot and powdery mildew on roses Roses, while beautiful, are often plagued with black spot or powdery mildew. I mix these ingredients into a tonic, which I spray on my roses: 2 teaspoons of baking soda and 1/2 teaspoon of liquid soap or Murphy's oil soap in 2 quarts of water. The tonic protects the roses for months.
"Aspirin is the remedy for fungal plant problems, Black spot, powdery mildew, and rust are a terrible trio of fungi, which can attack and destroy your plants. Scientists have found that two uncoated aspirin tablets (325 milligrams each) dissolved in 1 quart of water and used as a foliar spray can thwart these diseases."