If its powdery mildew, you could have used the milk spray - 1 part milk to 9 parts water to treat it in the early stages. Dryness at the roots encourages mildew, but its always about in warm dry weather. If its the other type of mildew, more air flow round the plants helps, and a fungicidal spray.

Thanks for that...I obviously didn't water my potted plants enough, they're the mildewy ones, those in the garden are much better. I will remember your advice for next year...regarding the milk mix. Too late for this !
Thanks again...Q