Powdery mildew rose

Powdery mildew, the third common rose disease, occurs
during early summer and early autumn when warm, humid
days (64 to 75°F) coupled with cool nights (50 to 55°F)
enhance germination of powdery mildew spores. Prolonged
rainy periods are not favorable for spore germination because
it is inhibited by standing water. The disease first appears as a
whitish-gray, powdery coating at the tips of canes. Young
leaves often become culled, distorted and dwarfed. With
severe infections, tip dieback can occur.
Controls include sanitation and fungicidal sprays. The
fungus is wind-borne and sprays will provide better coverage
than dusts. The spores will overwinter on fallen leaves and
inside stems and bud scales.
Other diseases that can be a problem are Botrytis blight,
crown gall (a bacterial disease), rust, spot anthracnose, and
several viral diseases including witches’ broom, mosaic, and
spring stunt.