Answer: Viburnums are potentially subject to a number of leaf spot type diseases both bacterial and fungal. They may also discolor if stressed by weather or by over or underwatering. Based on the information in your question I am not able to tell you specifically what the problem is in this instance. I would suggest you work with your local county extension to obtain a specific diagnosis of the problem, and based on that, determine how to proceed. They may appreciate samples of the affected foliage or photos of the overall plant and close-ups.
If a chemical control is needed they will have the most up to date information on what to use and how/when is best to use it. (I should mention that Viburnums are sensitive to sprays including those containing sulfur, so make sure anything you might use is labeled for use on viburnums.)
Finally, clean up and dispose of all the infected foliage to try to limit sources for reinfection. Good luck with your viburnums!
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