Powdery mildew and black spot are the "one-two punch" on roses. Powdery mildew is a problem in all parts of the U.S.
Powdery mildew usually first appears on new growth, in periods of warm, dry days followed by cool, damp nights. The new leaves may become curled or twisted, and the shoots will look badly deformed. Often, the upper surface of the leaves appears normal, but there is extensive fungus growth on the leaf undersurface.
Do not plant roses in shaded spots, especially in areas which tend to dry out slowly in the morning. Begin spraying with Shield-All II™ Organic Fungicide or Soap-Shield® Fungicidal Soap when weather conditions favor infection, and repeat every 7-10 days for as long as needed to control disease. During times of steady or repeated rainfall, control may not be necessary until the dry months of late summer.
*NOTE: Soap-Shield may cause copper toxicity on some rose varieties. Copper toxicity appears as purple spots and is more likely to occur outdoors than in greenhouses.