Answer: Yellowing leaves can signal some transplant stress, can signal underwatering, and can also signal overwatering. Roses need a well drained location and although the soil should be kept evenly moist while they are becoming established, it should never be sopping wet. If your rose is planted in a poorly drained location or in heavy clay soil then it may be too wet. If this is the case, I would suggest moving it.
Yellowing leaves can also be the beginning of black spot. The Fairy is a very popular rose and is in many ways very easy to grow and undemanding, however it is known for its occasional susceptibility to black spot which is a fungal infection. Damp conditions would of course encourage that to develop. Once black spot appears for the season, it is difficult to control. In general, remove affected leaves as soon as they appear and then replace the mulch immediately beneath the plant to try to keep the disease spores from splashing back up onto it. Keep the plant lightly thinned to promote air circulation and make sure it is planted in a location with good air circulation and in enough sun. If you water, avoid watering the foliage and water in the morning rather than at night. Finally, each winter, remove all of the leaves from the plant and dispose of them. These steps should help limit reinfection.
Once you see signs of it developing, you can pick off the affected leaves, and consider spraying. People have reported success using products containing neem oil or containing sulfur (read and follow the label instructions carefully), and using a home-made spray containing one part milk and two to three parts water. Although some people also report success using a baking soda based spray, it can burn the foliage on some sensitive roses and according to the American Rose Society is better as a preventive than a treatment.
I hope this helps you trouble shoot.
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