Your thinking of baking SODA (sodium Bicarbonate).  It doesn't work very well.  Potassium Bicarbonate is what is sold as "Green Cure" to home users and "Mil-stop" to commercial growers.  It's the exact same stuff, they just charge consumers 400% more for the small container.  It only treats what is on the plant and has ZERO residual.  I would really doubt it is powdery mildew this early in the season.  Need a picture.