Posted: 03/04/2016 at 06:44
There's not really any such thing as an eco friendly fungicide so I don't use them.
Powdery Mildew is usually the result of the roots of a plant suffering from drought and weakening the plant which lets the mildew take hold. Is the Viburnum in a pot or in the ground?
If it's in the ground I would clear all the fallen leaves from around the base of the plant, feed it with Fish Blood and Bone and mulch well with organic material which will help the soil to retain moisture.
If it's in a pot, how large is the pot and what sort of compost is it in?