Roses - Powdery Mildew
Symptoms
The first signs of Powdery Mildew appear on young leaves, which hold their color but begin to crinkle.
Small patches of mold develop into spore-bearing fungal filaments on foliage, stems and all other parts of the rose, even the buds.
The disease appears as a thin, white powdery substance, and the foliage steadily becomes deformed with the spread of it.
Conditions
Powdery mildew is more severe in warm, dry climates.
This is because the fungus does not need the presence of water on the leaf surface for infection to occur. However, relative humidity of the air does need to be high for spore germination.
Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas.
Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet.
Young, succulent growth usually is more susceptible than older plant tissues.