I have an old viburnum bush that has been growing well, even flowered this spring. Now the leaves are curling, turning browns and yellow and falling. I googled it, but couldn't find exactly the same. I saw no traces of bugs and the stems look normal. We do have a runoff problem around there and with all the rain could that be affecting it? These leaves come from the same bush. Thank you.
Your viburnum probably is suffering from too much water if it is in a location where the water does not run off quickly. It may have a root problem (root rot disease possibly.) At any rate, this is not an insect or disease problem that you can solve.
This does not mean that the entire plant will die, but you may lose some of it.
If it is standing in wet soil for long periods, try to change the grade or divert the water somewhat. In the spring, prune off any dead branches. If the shrub has mulch, be sure it is pulled back from the trunk. Mulch should never be piled on bases of any plants. Keep mulch shallow, no more than 2-3" deep.