Actinovate
Liquid Sulfur
Potassium Bicarbonate
Neem Oil

Which of these products would you recommend for a general Powdery Mildew preventative? and Why

Answer #1 · Brent Wilson's Answer · For "prevention," I wouldn't recommend using any chemical. I only resort to sprays when the powdery mildew is present. In this case, I've found Neem oil to be very effective. Haven't used the other one's you mentioned above so can't comment on them. I've also heard that a mild solution of hydrogen peroxide works well to eliminate powdery mildew but not sure about the the mixing ratio?

The best ways to "prevent" powdery mildew are by selecting plants that are resistant to it, spacing plants properly so there will be good air circulation between the plants, avoid planting plants that prefer sun and well-drained soils in locations that are too shady and have consistently damp or wet soil, and never water foliage late in the day or at night. It's best to water in the early morning hours, especially if you are using an overhead sprinkler system. These are most often the cause of powdery mildew on plants that are susceptible to it.

Hope this info helped. Brent)