If your rose starts to develop dark-purple black spots on leaves and the leaves begin to yellow and drop your rose may have developed Black Spot.
Black Spot is common on roses and can be difficult to eliminate once present. It is caused by excessive moisture on the foliage and humidity. To solve the problem, avoid wetting foliage when watering or water only at the soil level. Remove and destroy infected leaves and other debris around the plant. Do not put this material in your compost. Spray your roses with lime sulphur / dormant oil in the winter.
In the spring, begin a spray program using a fungicide or sulphur solution as soon as the disease becomes visible.
Another solution is to purchase roses that are more resistant to black spot in the first place.