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. Galls are most frequently seen as round, rough-surfaced growths near the plant's crown or on roots. Crown gall is a difficult disease to control once a plant is infected. It is best to rogue diseased plants as soon as they are detected.