What kind of viburnum is this, do you know? Most viburnums will lose their leaves in the fall anyway. If this is one of those, be sure to gather up the fallen leaves to avoid spreading any disease to the new foliage next year.

Spraying with fungicides will not cure the fungus once it is already there. Fungicides prevent the disease from spreading to uninfected leaves. If you spray early enough next year (as the leaves begin to unfold) and at the right intervals (according to package instructions)and the nursery center gave you the correct fungicide for your specific disease, you shouldn't have the leaf spot problem next year. Clean up any fallen leaves from around the plant as they may harbor the fungus over winter. Cercospora leaf spot is the one most often associated with Viburnums. I get it on my "Snowball bush"(Viburnum Opulus Sterile)but not so bad as to make me spray (I save that for my roses). And as esh pointed out, most Viburnums are deciduous so they will drop their leaves in the fall. My Viburnum leaves turn a maroonish color before dropping.