Viburnums can be susceptible to a canker disease called Botryosphaeria which is a natural soil borne fungal disease that can attack the plant, usually when it is experiencing drought stress symptoms. You can prune the dead stems out and look at the cross section. If it has some dark inner wood that usually means that is they issue. The canker grows and ends up choking the sap flow of water and nutrients out to the leaves and causing die back. If you do notice that, you can prune back an additional 6 inches until you see healthy inner wood. 

Be sure to monitor for irrigation needs. Did you water regularly through the drought last summer and now in this years drought symptoms? You can review how to water deeply on our Watering Trees and Shrubs page. 

There is not a cure for the canker disease and a fungicide will not be beneficial so just pruning out the dead branches and watering and reducing stress symptoms is what is best. 

Let us know if you have additional questions. 

Emily