Mildew on spinach

[dinah gale]

Just as my perpetual spinach is getting to a good size for picking I have noticed mildew (a powdery white substance, anyway) on the leaves.

Anyone have any experience of this please? If so, can you make suggestions for dealing with it? For the moment I'll pick off the affected leaves and hope it doesn't spread. I have a gloomy fear that I might lose all my lovely spinach.