With experience, you will learn which types of powdery mildew are likely to develop in your garden, and this knowledge will take you far in managing this disease. Like other fungicides, milk sprays work best when used preventatively, before the disease can gain a foothold. If you often see powdery mildew on your squash, grapes or zinnias, start milk sprays before the plants show signs of infection. You have nothing to lose beyond a cup of milk.