Spray with Copper sulphate (Cheshunt compound) which is allowed by organic growers I think. Or spray with a systemic fungicide if you do not mind chemicals. There are various ones around, the ones recommended for mildew on roses are as good as any.
Now I have read somewhere that you can spray with milk. But I cannot say I have ever tried it and h ave no idea of the type of milk needed (full cream, semi skimmed, skimmed or even Soya!), nor the dilution required.