Black spot disease on roses can ruin the beauty of the plant and cause defoliation. The fungus appears as round black spots on the leaves of the roses, progressing to large lesions on the leaves and stems and often the blooms themselves. While many of the treatments for black spot rely on toxins, there are a number of ways to treat black spot organically.
A number of studies, including one from the Texas A&M Research Station, have shown cornmeal to be effective as a fungicide, and rose gardeners have found it to be particularly effective on black spot disease. You can use the common cornmeal that is available at the grocery store, but it is far less expensive to purchase horticultural cornmeal from a feed and seed store. Sprinkle a handful of cornmeal around the base of each rosebush, gently working the meal into the loose soil with your fingers. You can also make a solution by soaking 1 cup of cornmeal in 1 gallon of water for eight to nine hours. Spray the liquid on any of your roses that are affected by black spot.
Milk and Water Solution
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Some gardeners swear by using a milk and water solution. This method calls for mixing 80 to 90 per cent water to 10 to 20 per cent milk. Apply using a spray bottle. You must take care to spray your roses with this solution every day without fail. Be sure to cover the entire rose, including the top and undersides of each leaf.
The University of Maine Pest Management Office recommends neem oil, the essential oil of the neem tree, which is native to India. Mix 2 tablespoons of neem oil (usually 70 per cent neem oil and 30 per cent inert ingredients) to 1 gallon of water. Apply every seven to 14 days, completely covering every inch of the foliage including the undersides of the leaves, and reapply after every rainfall.
Cornell University Solution
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A team of researchers at Cornell University, led by Dr. R. Kenneth Horst, studied the effects of baking soda spray on plants as a fungicide. After extensive testing and some modification by others, the basic solution that emerged as effective for black spot is 4 teaspoons of baking soda per 1 gallon of water. You should also add a teaspoon of canola or liquid soap to this mix to help the baking soda adhere to the plant. Spray every week, or after every rainfall. If the leaves seem to be getting burnt, do not use it as often.
Although sulphur is not always considered organic, many gardeners consider it to be a natural addition to the garden because it's commonly found in soil and is necessary for plant growth. Sulphur prevents fungal infection and will kill off any fungus in its earliest stages. Micronised sulphur holds the sulphur particles in a suspension that allows it to penetrate the plant better and hold to the plant better during the rain.
A number of organic sprays are available commercially. Kaligreen and Remedy are based on the Cornell University baking soda solution. Yellow Jacket and Bonide are micronised sulphur compounds, and Wilt-Pruf is a natural pine oil emulsion.
Preventive Garden Hygiene
One of the best treatments for black spot is to prevent it. Because the disease tends to quickly infect wet leaves, leave plenty of space between your roses and other plants. Always water roses from the bottom of the plant. As soon as you notice a black spot, immediately remove that leaf and a few leaves surrounding it. Always bag old leaves and pruned material and remove from the garden and burn. This will prevent any fungi spores from spreading to healthy foliage.