Answer: Based on your description, I am not certain what has affected the viburnums. It might possibly be sooty mold, growing on honeydew secreted by insects such as aphids. The aphids might have infested an overhead tree for instance rather than the viburnum shrubs themselves, so the source may not be obvious. Sooty mold can be washed off, and once the insect problem is controlled the mold should stop. I would strongly suggest consulting with your local county extension to obtain a firm diagnosis prior to planting the shrubs. They may appreciate a photo of the overall plant as well as closeups of the affected foliage, and possibly a freshly taken sample of the foliage kept cool and enclosed in a clear plastic bag. I hope it is nothing too serious.
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