Black spot, or diplocarpon rosae, is one of the most dreaded diseases of roses. Black spots form on leaves and stems causing them to yellow and drop. A serious infection can cause complete defoliation of the plant. Adopting a regular schedule of applying fungicidal sprays can greatly reduce the scope and damage the fungus can do to your roses. You do not have to buy expensive potions from the garden center. Just mix up one of these proven recipes from ingredients you may have on hand at home.
Black Spot Spray Using Baking Soda
1 heaping tsp. baking soda 1 tbsp. horticultural dormant oil or vegetable oil 1 tsp. insecticidal soap or dish soap 1 gallon water 1 gallon-sized jug with cap for mixing and storage 1 quart spray bottle
Cornell University led the research years ago for using common baking soda as an anti-fungal agent. They found that the addition of oil is essential to the effectiveness of the recipe. Many other versions of the original recipe add a little dish soap, or insecticidal soap, to help the solution cling to the leaves, and to help control insect pests. If you want to keep the recipe organic, use dish soap instead of insecticidal soap.
Black Spot Spray Using Milk
1 cup of milk, any type 2 cups of water 1 quart spray bottle
Milk is a folk home-remedy for black spot on roses, but modern testing has proved it is an effective deterrent for fungus. The downside to using it is that it can get smelly, when applied too thickly, from decomposition of the milk fats.
Black Spot Spray with Mouthwash
1 tsp. vegetable oil 1 gallon water 1 tbsp. apple cider vinegar 1 tsp. unflavored antiseptic mouthwash, such as Listerine 1 tsp. liquid dish soap 1 ½ tsp .baking soda 1 quart spray bottle
Mouthwash is formulated to retard the growth of bacteria and fungus in the mouth. When combined with the other known fungicides like baking soda and vinegar, it becomes a powerful weapon in your arsenal against black spot.
Instructions for Making the Recipes and Applying Fungicidal Sprays for Black Spot
Combine all ingredients in the gallon-sized jug and secure the cap. Shake vigorously until ingredients are thoroughly mixed. Pour enough of the mixture from the gallon jug into the spray bottle to fill it up. Spray the roses weekly, in the morning, wetting them completely with the spray.
Tips for Battling Black Spot on Roses
Water roses well a day or two in advance of spray treatment. Start spraying in spring, when the leaves first emerge, and continue until frost. Reapply after rain. Avoid spraying in the heat of the day or at night. Remove and destroy any leaves or stems that show signs of black spot. Buy disease resistant varieties of roses. Do not allow plants to become crowded. Decreased air circulation will encourage the formation of black spot. Keep them pruned and trimmed for optimal air circulation.
- Cornell University Cooperative Extension of Schenectady County: Black Spot on Roses
- National Sustainable Agricultural Information Service: Use of Baking Soda as a Fungicide
- University of Minnesota: Professor Jeff Gillman: Some Interesting Projects and Fun Experiments
- Rose Magazine: Mildew
- Earth R. O. S. E. Care, Mildew and Black Spot Control Options
- Washington State University: The Myth of Milk and Roses
Diane Cass began writing professionally in 1982, creating theatrical productions for churches and schools. She now enjoys sharing her knowledge by writing for various websites. Cass earned a Bachelor of Arts in music and teaching credentials at Vanguard University in Southern California.