Answer: Unfortunately, based on your description I am not certain what is happening. This could be due to over watering. The soil should be slightly damp like a wrung out sponge, not saturated/sopping wet and not dried out. If it is in a container, make sure it is draining freely and empty the saucer of any excess water.
It might also be due to a fungal infection, possibly the beginning of black spot which is so prevalent in the northeast. This is a serious problem. Signs include yellowing foliage and black spots on it. Here is more information about it that you may find helpful.
Unfortunately, many rose varieties have little or no resistance to black spot. Innate susceptibility plus the weather as well as cultural conditions can all contribute to it appearing and to its severity in a given year.
This is a fungal infection. Damp conditions would of course encourage it 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. Then replace the mulch beneath the plants. Also remove any prunings from the area. 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), or 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. It is usually a good idea to test spray a few leaves, wait a few days, and make sure there is no adverse reaction prior to spraying the entire plant -- some roses are extremely sensitive to sprays of any kind.
If your plants are in pots, you may also want to spray the pots and surrounding patio or deck to try to keep the spores from splashing back up onto the plants. (Test and make sure the spray will not damage these surfaces.)
Unfortunately, if you have a plant that is particularly susceptible to black spot you may never get rid of it permanently without a regular spray routine plus some cooperation from the weather and maybe some extra good luck.
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