According to author and horticultural professor Jeff Gillman, who has conducted extensive research on blackspot remedies, a spray composed of one part milk and two parts water is the best answer to the disease. He says this simple solution, if applied weekly, will control blackspot as well as any synthetic fungicide, including Chlorotalonil.
Milk and Water Use: To control black spots on roses. How to make it: Add 1 cup of milk to 2 cups of water. Spray mixture directly onto rose leaves with a handheld spray bottle to protect them from black spots. Why it works: No one knows exactly why milk helps control certain plant diseases, but it does contain lactoferrin, a protein that has been shown to help control diseases in animals.
Rose Gardening Tips I add used coffee grounds to the soil. It’s supposed too make the color/scent intense. And do not water from above.” Add epsom salt in the ground. It’s a tip from Martha Stewart!” Teresa said: “Water, combined with additives: iron, root stimulator, and miracle grow. Works like magic.”
Black Spot Fungus on Rose Bushes: Pull off leaves with black spots. Mix 1/2 a cup of baking soda and a gallon of water and rinse over the rose bushes on a sunny day. Or you can mix 1/4 cup of skim milk in a gallon of water in your watering can and rinse over the rose bushes too. It will last longer and not leave a chalky white residue like baking soda does, plus the rose bush will sprout many new growth stalks since it responds to milk just like a good fertilizer. Repeat every 4 weeks.
ROSE SPRAY-helps prevent diseases such as rust, blackspot, and powdery mildew. Mix together: •1-tablespoon baking soda •1-teaspoon mild dishwashing detergent •1-tablespoon vegetable oil •1 dissolved aspirin Add: •1-gallon water Mix together and pour into a spray bottles. Spray roses every 1-2 weeks.