Viburnum - American Cranberry - leaf spots
Hi. We have 6' viburnum, planted 2 years ago. Last year they were attacked by the leaf beetles and spider mites so we are watching them closely. I looked at every branch and cut out anything suspected egg infested twigs. Yesterday, I noticed some spots on some of the leaves. Could this be the beetles? Something else? Not sure what to do at this point. We have 9 of these as a privacy fence and they are very important.
Although viburnum rarely have diseases, some viburnum may get powdery mildew, downy mildew or fungal leaf spot. In fungal leaf spot, spots on leaves begin small but gradually grow into an irregular shape which is reddish or grayish brown. It tends to occur in warm, wet summer months. Proper siting is necessary to prevent disease where there is adequate air circulation and good water practices. For information on leaf spot diseases see www.extension.umn.edu/leaf spot diseases ). The spots on your leaves don't appear to be a fungal disease. It is likely that an insect caused them.
As you are aware of the damage done by viburnum leaf beetle (and for in-depth information about the viburnum leaf beetle, I refer you to Paul Weston, at Cornell University who has studied the pest at www.hort.cornell.edu/vlb/ ) and spider mite, you can probably rule these out at the moment. If the damage becomes more severe, a certified arborist may be able to help you.