Answer: Although it sounds as though the growing conditions for both plants are exactly the same, each of your roses is an individual and will react differently to insect and disease problems. Some roses are bred to be resistant to diseases, others do not have that same resistance. What you describe on your yellow rose is a common malady of roses - 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.
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 65F is best for spore germination and the disease develops most rapidly at about 75F.
To control the disease, rake and discard all fallen leaves because they are the main source of spores in the spring. Also 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. Removing infected leaves as they arise early in the season may help to retard the rate of disease spread.
There are several fungicides registered for control of black spot. (Daconil, Funginex, etc.) These products are generally applied at 7-14 day intervals. Be sure to apply according to label directions. Because of the waxy nature of rose leaves, a spreader added to the spray will give better coverage.
Best wishes with your roses!
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