Answer: Powdery mildew, caused by the fungus Sphaerotheca pannosa, appears as a white powdery growth on rose leaves, stems, buds, or flowers. It 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 may look badly deformed. The fungus may also infect older leaves. Often, the upper surface of the leaves appears normal, but there is extensive fungus growth on the underside of the leaf.
Fungicides generally recommended for powdery mildew control include: Triadimefon (Bayleton, Strike); Triforine (Funginex), Thiophanate-methyl (Cleary's 3336, Domain). Repeat sprays may be needed so be sure to follow the label directions.
Best wishes with your roses!
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