[From The Art of Gardening with Roses, by Graham Stuart Thomas, p. 14:] Mildew is more often caused by dry soil and damp air than by soil deficiencies.
[From Redoute's Roses, p. 42:] mildew, caused by cold weather in spring... can be cured by rubbing all the affected parts with a sponge soaked in vinegar, according to Boursault.
[From Green Thumb Wisdom: Garden Myths Revealed!, by Doc & Katy Abraham, p. 120:] Most plant diseases are immune to cold weather, although we understand that the cold weather will kill powdery mildew on cherry and apple trees...