I have always had success with baking powder, 1 heaping tablespoon per
gallon as a powdery mildew control, it works immediately and I have very
little subsequent problems.
I just read that
potassium bicarbonate is more effective
mixing a little dormant oil helps keep it on
there is no problem with the sodium accumulating in the soil or the plant (I
think I read that)
that baking soda is more effective than conventional commercial preparationg