I have very healthy viburnum hedging which is very healthy and growing great at the moment. In a small section, about 3 ft in width, the top leaves have all died off. Not sure yet if it is spreading as I have just found it. I have attached a picture of the damage and some leeaves.
This is, almost certainly, a root rot issue. Whether bacterial or fungal, you have a really big issue on your hands.
Check for mushroom fruiting bodies at the base of the shrub. Check for oozing lesions anywhere on the shrub.
fungicides may slow the progression, but this is likely a disease that will require replacing the shrub with something unrelated.
https://www.gardeningknowhow.com/ornamental/shrubs/viburnum/diseases-affecting-viburnums.htm
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
I was pruning a Viburnum plicatum (Japanese Snowball Bush) today and whilst doing so I noticed that there were many small ants (probably sugar ants) on some of the stems. Upon closer inspection I saw that the ants were attracted to some black stuff on the stuff which I couldn't identify. I assume it's some sort of disease or maybe sugar released from the plant itself, but not 100% sure so I came here to ask if anyone knows what exactly is going on. The mystery black stuff was solid, dry, small, oval in shape, and there were many "spots" together along a 2-4 inch section of the stems in question. I forgot to take a picture, but hopefully the information helps!
while the other two look healthy but have no new growth. Should I be concerned or do anything about this? I water and feed them all equally. They get morning sun. The only difference I can think of is that the one with new growth may get more water from my sprinklers, but I don't run them often. I usually water the shrubs manually when there isn't any rain or if it's very hot outside.
I should have mentioned that my soil has a lot of clay. Now that I think about it, the one with the new growth probably does get a bit more sun. Could it possibly be a male/female plant issue? I read that I should plant more than one shrub if I want to get a lot of flowers and berries, but I didn't think that they're specifically male or female, like holly or sumac.
t have any new growth. Should I be concerned or do anything differently? My soil has a lot of clay. We haven't had much rain at all this summer, so I've been watering them all the same amount manually and have sprinkled Preen around them a few times. They get plenty of sun for 6+ hours/day.
I suspect that this is related to the same issue as your Spirea. Clay soil is notorious for being nutrient rich, but largely unavailable. If you can work some organic material in without disturbing all of your plants and shrubs, then this would be ideal. if not, then worms will be your best friends in this situation. I have the same problem in my area, and I solved the issue with these methods. Many of my projects from the mycology lab end up in the yard and garden, as well. Mushrooms and fungi are the BEST at remediating soil. (As long as you avoid putting certain ones out, such as Armillaria.)
These articles will offer some more information:
https://www.gardeningknowhow.com/composting/vermicomposting
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm
https://www.gardeningknowhow.com/special/organic/how-to-easily-improve-your-clay-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/improving-compacted-soil.htm
I need to repot it into a bigger pot. Not sure if its the right time of year to do this. It is nearly a meter wide and about 24 inches tall so far.
It may be a spring blooming viburnum shrub. Wait till after it blooms to repot it into a pot one or two sizes larger.
https://www.gardeningknowhow.com/ornamental/shrubs/viburnum/growing-dwarf-viburnums.htm
After several years I have given up with trying to control viburnum beetle in my viburnum hedge! Can I replant a new Yew hedge? Will the beetle attack it ?
There is no real good way to get rid of them, unfortunately. You can remove all sacks before they hatch around May, and treat the larva, but this will be a yearly task.
Here is an article that will offer more information:
https://www.gardeningknowhow.com/plant-problems/pests/insects/treatment-bagworms.htm
cons of each of these varieties?
Moonlit lace is a hybrid viburnum that grows from 3 to 6 feet in height while David stays below 5 feet. They both are hardy in your zone 7. These articles should help:
https://www.gardeningknowhow.com/ornamental/shrubs/viburnum/growing-david-viburnum-plants.htm
https://hgic.clemson.edu/factsheet/viburnum/
https://www.gardeningknowhow.com/ornamental/shrubs/viburnum/viburnum-shrub.htm
https://jcra.ncsu.edu/horticulture/our-plants/results.php?search=Viburnum