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Black spot is one of the most common diseases of roses. It is caused by the fungus Diplocarpon rosae. Black spot will cause a weakening of the plant so that fewer and fewer blooms are formed if the disease is left unchecked. Plants are weakened and increasingly subject to winter injury.
Infected leaves show black spots. The spots can be up to 1/2" in diameter and typically have fringed borders. Yellowing of the leaf begins surrounding the spots and the entire leaf may yellow and eventually drop off. Close inspection of the spots will reveal the presence of tiny black spore producing bodies. The fungus may also infect the canes where lesions appear purple at first and later black.
Rake and discard all fallen leaves because they are the main source of spores in the spring. Prune and discard any obviously infected canes. Avoid wetting the foliage especially during dark cloudy days. Grow plants in an open sunny location to promote rapid drying of the foliage.
