Vulnerable Viburnums

By Susan Littlefield

In the early 1990s, a previously unknown fungus-like organism called Phytopthora ramorum was discovered infecting rhododendrons and viburnums in parts of Europe. Then this same organism was discovered in the U.S. causing the new disease "sudden oak death" in the San Francisco Bay area. In the years since, it has spread to various locations in western North America, infecting not only oaks and tanoaks, but over forty species of ornamentals, including viburnum, rhododendron, camellia, pieris, mountain laurel and honeysuckle.

On viburnums, ramorum blight causes leaf blight and shoot dieback; severe infections can kill plants. Recently, Timothy Widmer of the Foreign Disease and Weed Science Research Unit of the USDA Agricultural Research Service evaluated twenty-four species or cultivars of viburnum to assess their susceptibility to this emerging disease threat.

The results of the study showed a wide range of vulnerability. The most susceptible plants tested were Viburnum x carlcephalum 'Cayuga' and V. tinus. The most tolerant were V. x rhytidophylloides 'Willowwood' and V. opulus 'Notcutt.' In this study, above-ground parts of the plants were inoculated with the disease-causing organism, in contrast to previous studies using detached leaves. This is important because, while the individual leaves of some viburnums show extensive necrosis when they are infected, some of these same viburnums show the fewest number of infected leaves overall. In general the evergreen species and cultivars were more susceptible to infection than deciduous ones.

While no one yet knows whether this disease can be contained or how far it may spread geographically, if you are in or near an area where ramorum blight has been found, it may be a good idea to choose the most resistant species or cultivars when planting viburnums in the landscape.

For more information on the susceptibility of viburnums to Phytopthora ramorum, go to : ARS.

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