If plants have excessively dense growth and are planted too closely, air circulation is impeded and black spot may be more severe. The mildew fungus grows over the upper leaf surface and is easily recognized from the white, powdery patches formed. Severely infected leaves become brittle and wither and die. Infected stem tissue has a withered, spindly appearance and may also be coated with the white, powdery growth of the fungus. Unlike black spot, powdery mildew does not require wet conditions for infection to occur.
Mildew does best when the temperature is cool and relative humidity is high.
However, be on the lookout for mildew throughout the growing season. Fungicides need to be applied at the first sign of powdery mildew. Powdery mildew is most severe in heavily shaded areas. Avoid such locations when planting roses or other plants susceptible to mildew, i.e., lilac, zinnia, dahlias. Follow suggestions for good air circulation as outlined under blackspot.