Answer: What you describe sounds like a disease called black spot. Black spot is caused by the fungus Diplocarpon rosae. Black spot will cause a general weakening of the plant so that progressively fewer and fewer blooms are formed if the disease is left unchecked. Plants so weakened are increasingly subject to winter injury.
As the name implies, infected leaves show black spots especially on the upper leaf surface. 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.
As is true with most fungi, this fungus requires free water for infection to occur. The spores must be wet for at least 7 hours before they can germinate. A temperature of 65oF is best for spore germination and the disease develops most rapidly at about 75oF. Temperatures of 85oF and above inhibit the spread of the disease.
To manage the problem, 1) Rake and discard all fallen leaves because they are the main source of spores in the spring. 2) Also prune and discard any obviously infected canes. 3) Avoid wetting the foliage especially during dark cloudy days. 3) Grow plants in an open sunny location to promote rapid drying of the foliage. 4) Do not plant in dense plantings and avoid windbreaks to allow good air circulation. 5) Roses vary in their resistance to this disease. Use resistant varieties for low maintenance plantings. 6) Removing infected leaves as they arise early in the season may help to retard the rate of disease spread. 7) Several fungicides are registered for control of black spot. Among them are Funginex, Captan and Daconil. Apply according to label directions and you'll be able to control black spot disease on your roses.
Best wishes with your garden!
Q&A Library Searching Tips