From: https://web.extension.illinois.edu/roses/disease.cfm
Blackspot:
This fungal disease can cause almost complete defoliation of bushes by early fall, resulting in a weakened bush on which cane die-back and cankers become severe. Blackspot is identified as circular black spots that appear on the upper surface of the leaves, starting at the bottom of the plant and moving upward. Infected leaves turn yellow and fall off prematurely. The fringed margin and black color distinguish this leaf spot from others. Infections on canes are identified as reddish-purple spots. Splashing water spreads the blackspot. Infection occurs after leaves are wet for several hours, making it more serious during rainy periods. Some roses are less susceptible than others, so cultivar selection is important. The fungus overwinters in fallen leaves and stem cankers. Raking and removing these leaves and pruning out affected canes by spring before the buds swell may help provide some control. Avoid wetting the leaves when watering, and locate plants with good air circulation. Fungicide spray programs must be started as soon as new leaves appear in the spring.
Powdery Mildew:
Powdery mildew is a fungus disease that affects young leaves, causing them to curl and twist and develop a purple coloration. As the disease progresses, leaves become covered with white powdery fuzz. Whereas blackspot is usually most severe on the lower part of the plant, mildew affects the top part. Mature leaves are less likely to be affected. Mildew is spread by wind and develops rapidly during periods of warm, dry days followed by cool, humid nights. Infections of mildew are actually discouraged by the presence of water on the leaves. However, keeping plants wet all night to avoid mildew provides an environment that allows other diseases to develop. Infection can be reduced through sanitation and fungicide spray programs. Prune out all dead and diseased canes to reduce initial fungus infection. Because new growth is especially susceptible, thorough coverage of new growth with fungicides is important. Plant roses where they receive good air circulation and the foliage can dry off rapidly in the early morning to prevent many diseases.