Milk is antifungal. You can mix it 50/50 with water and spray it on infected leaves to treat powdery mildew. I can't recall the science specifics but it's something about how the whey proteins and salts dry in sunlight. Straight whey works more efficiently than milk. I estimate, but I mix it somewhere around 25/75. Since it's antifungal, avoid saturating the soil, but whatever drips from spraying the tops & bottoms of leaves isn't enough to cause catastrophe. I've never tried with whey powders, but I think that's a thing people use for mixes too.