Answer: Unfortunately based on your description I am not certain what is happening to your hedges. It might be powdery mildew, a fungal infection. In many cases this is ugly but not life threatening to the plant and would really not need to be treated. However, to be sure I would suggest you take a sample to your county extension and obtain a specific diagnosis of the white substance. They should also be able to tell if you if it is something that needs to be treated and if so, with what, and how and when to apply it.
Horticultural oil is sold at garden centers and is sometimes used to control specific pests and diseases on certain plants. For example oils are sometimes used in winter as a dormant spray on fruit trees or to smother scale insects on Euonymus plants. It is not a cure-all and should only be used for problems and plants listed in the label instructions. I hope this answers your question.
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