Powdery mildew on petunias

I have got powdery mildew on some petunias in my hanging baskets.  I have looked online and suggestions for stopping it range from milk & water to a mix of baking soda, dormant oil and liquid soap.  This last one seems the most popular but I haven't got a clue what dormant oil is.  Has anybody tried any of these remedies or have any other suggestions that work please.  My baskets are looking really good and I would like to keep them that way for a bit longer. Also, would any of the above mixes do anything to those petunias that don't have powdery mildew?