Mildews are moulds that are specific to a particular plant. The spores germinate when the leaves are dry but the air is still and humid. Good ventilation is important for preventing mildew, but also keeping the roots moist. Mildew spores do not germinate, in fact they are killed, by water and mildew starts during dry weather.

Mildew is difficult to treat once the symptoms are visible, try to remove and destroy affected foliage to reduce the number of spores. You might try a horticultural oil which can actually control mildew after you see it. Most other treatments only prevent infections and do not control them once they have started. Try baking soda, potassium bicarbonate, or sulphur. The bicarbonates can be mixed with oil, sulphur should not. Chemical fungicides are not particularly effective.