According to the University of Maine Insect and Plant Disease Diagnostic Lab, black spot is one of the most common and detrimental disease for roses to contract. The disease, which affects rose bushes all around the word, causes the rose bush to deteriorate over time, leading the plant to produce fewer roses each season. Black spot is easy to detect, because, as the name implies, black spots actually appear on the leaves of the bush. In order to prevent your rose bush from succumbing to black spot disease, you must stop the spots as soon as possible.
Thin out the rose bush every year at the beginning of the growing season, which is generally early spring. According to PennLive, thinning the plant allows air to circulate through the leaves, keeping them dry and preventing the growth and spread of black spots. Pick several long stems on the bush and prune back to only 8 to 10 inches or down to 1/3 of their original length.
Remove any leaves on the rose bush that already have black spots on them. Once the leaves have contracted black spot, they cannot generally be saved, and leaving them on the plant may cause the disease to spread. Wearing gloves, pinch leaves at the base near the stem to remove them from the bush.
Water the ground around the base of the rose bush. Do not water the rose flowers and leaves. Watering in the morning also gives the leaves time to dry in the daytime sun, where watering at night, especially when nights are cool, causes water to sit around the plant, which lends to the spread of the disease.
Spray the rose bush with a baking soda mixture if the black spots are widespread on the rose bush. The baking soda mixtures recommended by Go Organic Gardening and PennLive both contain baking soda, vegetable oil and water, though PennLive recommends adding dish soap as well. Try a mixture of 1 tbsp. baking soda, 1 tbsp. vegetable oil, 1 tbsp. dish soap and 1 gallon of water. Put the mixture in a spray bottle and cover the entire plant. The mixture doesn’t generally heal the spots on infected leaves, but can prevent black spots from spreading