We had several complaints of viburnum problems at the University of Illinois Plant Clinic toward the end of the summer. Usually, the descriptions included branch decline, plants that looked like they were dying, and early defoliation. We rarely hear of problems on viburnum, so it was odd to receive several such complaints on this host. This article describes some of the problems that may appear on viburnum, with comments on how you might use this information to work through a diagnosis. Solving plant problems requires information about the symptom development, the plant site, and the environment … really!
It helps to know what diseases are possible on a given host. Viburnums sometimes have problems with foliar diseases, including bacterial leaf spot, downy mildew, and powdery mildew. Bacterial leaf spot is a disease present in cool, wet weather. It causes angular leaf spots that appear water-soaked. The reported problem appeared in hot, dry weather of late summer. In addition, leaf spots were not apparent. The leaves were dying but not spotted. Downy mildew is another disease that is favored by cool, moist conditions. It causes angular leaf spots much like bacterial leaf spot; but lesions start out yellow, become brown, and may coalesce to form large brown blotches. Downy mildew also forms a fine, downy growth on the underside of these lesions. Again, no leaf spots were reported on the injured plants. Powdery mildew causes a white, dry, powdery growth on the leaves and certainly could be a problem in late summer. Severe cases can cause some leaf deformation and even some leaf drop, but stems should remain alive. Growers reported that stems were dying. Powdery mildew is also a disease that most growers recognize. None of these foliar diseases seemed likely, based on the problem description. Often, however, the description of the problem does not always match the actual symptoms. We received one suspect viburnum sample. In fact, it did not contain any leaf spots. Leaves were sectioned and found to be free of bacteria. Lab incubations of leaves did not yield any fungal pathogens.
Despite the fact that I am a pathologist, I am quick to point out that not all plant problems are caused by disease. It seemed logical to suspect a water deficit or heat problems. It also seemed logical to question root injury, insect borer activity in the stems, and time of planting (to rule out transplant shock). Further questioning revealed that this was an established bed of viburnums and that most of the plants in the front row were affected, while the back row appeared better.
Viburnums are susceptible to Verticillium wilt. The wood submitted was cultured in our lab when it arrived. There was no vascular discoloration. Eventually, cultures proved negative for this vascular pathogen. We could rule out Verticillium wilt.
Lower stems were cankered. These cankers were not extremely obvious, but they were present on the sample. Incubated tissue yielded a form of the Botryosphaeria fungus. Yes, the diagnosis was Botryosphaeria canker, but that is not the entire story. The back row of plants was not yet affected. As is often the case, the fungal pathogen was not able to cause a problem on its own. Botryosphaeria is a stress pathogen. When I looked in the literature, I found that the most common stress factor associated with viburnums and Botryosphaeria was drought. Discussion with the grower revealed that this area had been very dry. Management of the problem was not a matter of spraying a fungicide. In this case, we recommended removal of dead wood, supplemental watering until the ground freezes, and a light application of fertilizer. The grower realizes that he needs to monitor water deficits more closely in the future. Plant problem management depends on an accurate assessment of the problem (site stress and disease in this case) followed by alteration of factors that can be corrected.