Answer: Viburnums can develop fungal and bacterial leaf blights and they are susceptible to insect problems but I can't really diagnose the problem with your viburnums without actually seeing the plants. I'm confused about the reddish leaves. Some varieties of viburnum develop reddish leaves in the fall and winter months and some produce new growth that is reddish but matures to green so I'm not sure that the reddish leaves you describe are all that unusual on a viburnum. A general decline of a plant can indicate a root problem - either overly wet soils causing root rot or a drought stress where insufficient water is being applied. I can only suggest that you take a sample of the affected plant part to your local Cooperative Extension office where they can test it for pathogens. Once they have diagnosed the exact problem they can give you some treatment or control options.
Best wishes with your viburnums!
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