There have been a lot of reports doing the rounds recently that dipping cuttings in cinnamon can aid rooting, and that spraying plants with water which has been steeped in Cinnamon helps to combat disease.
I am reminded that some of the early fungicides worked in a very subtle way, slightly changing the pH (acidity or alkalinity) of the plants leaf. The resultant change in pH could influence whether fungal spores germinated on the leaf and so could arguably protect the plant from disease.
Powdery Mildew development could therefor be reduced with a treatment of milk or baking soda.
With this in mind the possibility that cinnamon could reduce fungal attack does not seem very far fetched at all.
To try Cinnamon as a preventative fungal spray mix some ground cinnamon into warm water, allow the solution to steep overnight, strain and use a spray. I will most certainly be trying this to see whether it is effective.
The same reported fungicidal property of cinnamon is probably behind its use in increasing the take of cuttings. The cutting is simply dipped in cinnamon before being inserted. Whilst not convinced that this will stimulate rooting, it could help to protect the cutting from disease.