I can't see anything that looks like powdery mildew anywhere in that picture - and its unlikely to be that at this time of year, unless these seedlings are in a warm, moist environment without proper airflow, such as a closed and heated greenhouse. Try dusting with sulphur dust. Can't imagine what water and milk will do...

Jerseydirect, you are in direct contravention of the rules of this site in directing people to your commercial website. I have flagged your answer under this question, and under a previous question.