Came across this recently. The person who posted it says she’s used it to great effect:
Once your seedlings are up with 2 to 4 leaves, give them the royal treatment ASAP!
Mix together: 1 aspirin + 1/2 teaspoon liquid dish soap per gallon of water. Add 1/4 cup nonfat powdered milk and 1 tablespoon of baking soda.
The soap makes it stick. Aspirin triggers a defense response and stimulates growth. Nonfat powdered milk is a natural germicide and boosts the immune system. Baking soda makes the leaves alkaline and inhibits fungal spores – that’s MILDEW, the nemesis of beans (and a big problem for us).
Use weekly on your young bean plants, all your cucurbits – cukes, zuchs – any mildew prone plant. Use a watering can that has a rose (nozzle) that turns upward to get the undersides of leaves as well as their tops.
Concord Village EarthLab 
Great! How soon can we prepare it??? Would be great to hold a workshop in the community room for this and other natural home/garden remedies…I’ve got citronella, lavender and lemon essential oils 🙂
I’d make it as needed; the powdered milk is likely to turn otherwise. In the past I’ve attempted to make large batches of natural sprays and they all have spoiled in some way or other before being used up, even if refrigerated.
Perhaps we can figure out a way to mix the base and the add-ins and store them separately, then mix on the spot as we go.