But it's still uncertain as to exactly how milk works against the fungus. It appears the milk protein creates an antiseptic effect when exposed to sunlight, which is why you should apply the solution in bright sun. The protein is in the milk fat, though researchers have experimented with both whole and skim milk.

Is milk effective against powdery mildew?

Researchers in Bazil found that weekly sprays of milk controlled powdery mildew in zucchini just as effectively as synthetic fungicides. They also found that milk acted as a foliar fertilizer, boosting the plant's immune systems. A concentration of 10-30% significantly reduced the severity of powdery mildew by 90%.

What kills powdery mildew instantly?

Potassium bicarbonate is a contact fungicide which kills the powdery mildew spores quickly. In addition, it's approved for use in organic growing.