Sounds like a classic case of black spot disease on your roses. This is a fungal disease encouraged by wet foliage, warm weather, and poor air circulation. Black spot is not caused by lack of fertilizer or a nutrient. Remove all the lower, infected leaves and clean up on the ground any fallen leaves. Do not let the plants dry out, but try not to get the foliage wet. A drip system is the best way to water roses. Some roses varieties are more susceptible to black spot so you can try to plant the more disease resistant landscape roses such as Flower Carpet roses to avoid this fungus among us. Rose growers that do not choose disease resistant varieties will sometimes use a fungicide spray like Funginex to keep black spot at bay. Any rose that is stressed from lack of water or poor soil or wet foliage is more likely to become infected with black spot. Allowing lots of space between your rose plants is also helpful as it improves the air circulation. Good luck and keep growing, Marianne Binetti