Fungicide for edible plants

Hi all.

My Sage has white powdery mould on some of its leaves. It seems happy enough and is growing well, and the younger leaves are unaffected. It is getting ready to flower so it must be reasonably happy. However the mouldy leaves are a bit unsightly, and picking them off is only a temporary solution because it just affects more leaves as they mature.

I've had a look at some fungicides in the garden shops but they all state they are only for ornamental plants. Anyone know of an off the shelf product I can use to tackle this?

The plant in question gets lots of sun (or as much as can be expected in Britain at least), has plenty of room for air circulation and is in very free draining soil.

 

Maybe I am thinking of something else here, but for some reason I am thinking soda water.

 

Thanks Claire.

I googled soda water mildew and found this:
http://gardening.about.com/od/gardenproblems/qt/PowderyMildew.htm

It seems to be saying it is more of a preventative than a cure, but I'll give it a try and see what happens.