that type of mildew is related to dryness at the roots, so watering when its necessary will help, as will using the milk treatment - mix up 9 parts water to one part milk in a spray bottle, and spray the foliage and stems with it, including under the leaves. The milk doesn't have be skimmed or full fat, so long as its cows milk, but skimmed is less smelly on the plants - no cream to go rancid.

Alternatively, spray with a suitable fungicide treatment.