Roses also need to be fertilized on a regular basis. Apply fertilizer specific to roses, which is available at full-service garden centers. What gardeners can't control is the weather. Wet, humid conditions increase disease problems in roses, especially weaker ones. If you see a white, talcum powder-like film on the top and bottom of the leaves and stems, your roses are infested with powdery mildew.
Some varieties are more susceptible than others.
Plants also may exhibit signs of stunted and curled leaves. This disease is not a problem with the newer, hybrid tea roses, which have been bred with resistance to this disease. Look for circular black spots with fringed edges on the leaves. The spots may be tiny or more than one-half inch in diameter. If infestation is severe, defoliation may occur.