I have a mature viburnum that flowered beautifully this spring and half of it is suddenly dead i think even though when I snap the twigs, they are still green inside. One half has berries forming, and the other has no leaves and is all grey. Some branches have a couple of small leaves that are curled up and browning. it faces east, is in clay soil that has been amended, I water it regularly and for the last 10 years has been fine. don’t understand what has happended! I don’t see any sign of rot, or fungus on it. So should I prune the grey stuff back or just pull the whole thing out?
I don’t think there’s any need to remove the shrub completely.
You haven’t mentioned that the leaves are skeletonized, so the most common culprit, the viburnum leaf beetle, doesn’t appear to be present.
The leaf curling you describe is usually caused by aphids eating the tender new growth. Even if you can’t see the aphids immediately, you may see quite a bit of ant activity, since ants are attracted to the honeydew secretions of the aphids.
There are other possibilities, thrips, which are exceedingly tiny, but can cause die back, as they feed on the sap – under the bark for adults, in new growth for larvae, and several fungal diseases.
You say you do not see any visible fungal activity, such as mildews. Wilt is a possibility. Wilt is introduced by a soil-borne fungus, such as Verticillium albo-atrum and Verticillium dahliae. The pathogens enter the shrub through its root system and then spread throughout the rest of the plant. The fungus follows the vascular tissue, that which brings nutrients and water from the roots, clogs the system, which in turn, reduces water flow. The shrub’s leaves may suddenly start wilting, yellowing or curling, which in turn, with a bad infestation, can cause premature defoliation, branch dieback, stunted plant growth and death of the entire plant.
For more information on this disease see the University of Minnesota information page here.
However, since the branches are still green on the inside, you are correct in thinking they are not dead. I suspect that aphids are the problem. Open any curled leaves and look for the insects. Also the grey appearance of the branches could be the result of the growth of sooty mold arising from the honeydew.
Plants usually survive this kind of damage, and leaf out again, but you may want to prune the affected areas selectively for appearance.
Natural enemies, such as lady beetles or syrphid fly larvae, will often intervene to feed on the aphids, so you can choose to just let nature take its course.
However, you can also try knocking the aphids off the shrub with blasts of water. Spray leaf undersides vigorously with water three times, once every other day, in the early morning.
For heavy infestations, use insecticidal soap. Spray the foliage thoroughly, including the upper and lower leaf surfaces. Insecticidal soap kills pests by suffocation so the spray must come in contact with the aphids to be effective. Repeat the spray three times at 5- to 7-day intervals. DO NOT apply the soap spray in hot weather; wait until temperatures are below 30C.
Next spring, you can use horticultural oils or insecticidal soaps during the shrub’s dormant stage or soon after bud break to control the aphids and prevent damage next year. I’m including a link to a good article on the use of horticultural oils from the extension services at the University of Nevada.