I am not sure about powdered milk but I believe skim milk has some beneficial bacteria that helps a little with mildew but the pure flower is a big help. For milk to work as a mildew killer you need to use 10-12 ounces per gallon. Far to much milk for me to go this route. I just use 1 ounce of milk per gallon of water as a wetting agent. I read a note from Dave and Deb Boyd from Montana and it stated that if they sprayed the foliage with water two times a week they controlled mildew. I concur with this idea. It is important to point out that I spray my roses once a week with nutrient solutions. So that may account for clean foliage. One week earth juice microblast half strength, next week half strength liquid seaweed kelp, next week pure flower full strength 1.5 teaspoon/gal water, next week liquid iron half strength, next week half strength B-1. Next week Pure flower again. Then I repeat the list again which I believe most of these products they recommend that you spray once every 3-4 weeks so it works out perfect. All of these spray sessions you need to add 1 ounce of milk per gallon of water. So if you spray with 15 gallons you use 15 ounces of milk. I have 400 roses so a power sprayer with high capacity turns an all day job into two hours. After adopting this method I have not had any occurences of rust or black spot. Be careful not to over fertilize by adding other fertilizers to the soil as well. Keep track of your delution rates and adjust accordingly if you want to add to the soil as well. The only product I add to the soil every three months is 8 ounces of alfalfa pellets.