More than one pathogen?
Although Viburnum expert, Professor Emeritus Michael Dirr states that there are few insects and diseases that seriously affect Doublefile viburnum (Viburnum plicatum f. tomentosum), environmental conditions may cause problems. It has been hot and dry at times during the summer. If a plant becomes stressed due to lack of water, it is susceptible to insects and disease that it would not normally affect it. Most plants require an inch of water a week and often, during the summer, one to two weeks went by with little or no rain and so plants needed supplemental water. As a result many diseases and insects may have attacked your viburnum. I’m attaching a bulletin from Clemson University which lists such diseases and insects and how to manage them.
This will help you treat the plant. Be sure to provide ample water to prepare it for the winter.
Thank you for your question and contacting Ask an Expert.
Thank you for this response!