I have a Korean Spice Viburnum in my back yard in mid-town Toronto. It’s in mostly shade in good loamy soil and I dig in compost around it every year. Some of the leaves have spots and have turned yellow. I’ve had it a few years now, but usually the leave haven’t been turning all yellow. What is this and and what can I do about it?
Thank you for contacting Toronto Master Gardeners.
Korean Spice Viburnum is a shrub rated for zone 6a, so with the right growing conditions, it should grow in Toronto (zone 7a in your location).
The shrub does best in full sun to partial sun. You said that the plant is mainly in shade, and imply that the leaves in past years been turning at least partly yellow. The white leaf in the photo suggests insufficient light to allow photosynthesis. I suspect that there may be insufficient light where your shrub is located to allow it to really thrive.
This shrub prefers to grow in average to moist conditions, and shouldn’t be allowed to dry out. You did not mention your watering routine. Inadequate soil moisture is plant stressor.
Stressed plants are more susceptible to disease.
We cannot easily identify the exact cause of the spots on the leaf is difficult to discern from the photo. The best course of action is to remove diseased leaves, and make sure the plant has sufficient water and light to meet its needs. You may need to relocate the shrub to a sunnier area.