Susceptibility of Viburnum Species and Cultivars to Phytophthora ramorum1

Phytophthora ramorum causes ramorum blight on Viburnum species, which are commonly grown as ornamentals. This study evaluated 24 different species or cultivars for susceptibility to P. ramorum. Whole plants were inoculated with an NA1 isolate of P. ramorum and placed in dew chambers at 20C (68F). After 5 days, the percentage of necrotic leaves for each plant and the percentage of necrotic area for each leaf were calculated. The percentage of necrotic leaves per plant ranged from 96.1% (V. tinus) to 7.9% (V. opulus ‘Notcutt’) and the percentage of necrotic leaf area ranged from 73.0% (V. × carlcephalum ‘Cayuga’) to 2.4% (V. trilobum ‘Wentworth’). In addition, six species or cultivars were evaluated for their susceptibility to a EU1 isolate of P. ramorum. The isolate had a signifi cant effect on three of the six species or cultivars tested, although one isolate did not always yield the greatest necrosis. Evergreen species and cultivars had a higher percentage of necrotic leaves and higher percentage of necrotic leaf area than semi-evergreen or deciduous species and cultivars. All Viburnum species and cultivars showed some degree of susceptibility to P ramorum. Index words: host range, ornamental, ramorum blight, sudden oak death. Species used in this study: Allegany lantanaphyllum viburnum (V. × rhytidophylloides ‘Allegany’); arrowwood viburnum (V. dentatum); blue muffi n arrowwood viburnum (V. dentatum ‘Blue Muffi n’); Burkwood viburnum (V. × burkwoodii); Cayuga hybrid viburnum (Viburnum × carlcephalum ‘Cayuga’); Chesapeake service viburnum (V. utile ‘Chesapeake’); Conoy Burkwood viburnum (V. × burkwoodii ‘Conoy’); Cree leatherleaf viburnum (V. rhytidophyllum ‘Cree’); Cunningham’s white rhododendron (Rhododendron ‘Cunningham’s White’); dwarf Koreanspice viburnum (V. carlesii ‘Compactum’); Erie linden viburnum (V. dilatatum ‘Erie’); European snowball viburnum (V. opulus ‘Roseum’); fragrant snowball viburnum (V. × carlcephalum); Judd viburnum (V. × juddi); laurustinus viburnum (V. tinus); linden viburnum (V. dilatatum); Maries doublefi le viburnum (V. plicatum var. tomentosum ‘Mariesii’); Mohawk Burkwood viburnum (V. × burkwoodii ‘Mohawk’); Notcutt European cranberrybush viburnum (V. opulus ‘Notcutt’); Prague viburnum (V. × pragense); Sarcoxii hybrid viburnum (V. × burkwoodii ‘Sarcoxii’); Shasta doublefi le viburnum (V. plicatum var. tomentosum ‘Shasta’); summer snowfl ake doublefi le viburnum (V. plicatum var. tomentosum ‘Summer Snowfl ake’); Wentworth American cranberry viburnum (V. trilobum ‘Wentworth’); willowwood lantanaphyllum viburnum (V. × rhytidophylloides ‘Willowwood’). 1Received for publication March 30, 2010; in revised form July 15, 2010. The author wishes to thank Steven Dodge for all of his help in the collection of the data and John Phillips, Dana Berner, and Nina Shishkoff for their helpful discussions on statistical analyses of the data. I would also like to thank Nina Shishkoff for her review of the draft manuscript and helpful suggestions. In addition, I would especially like to thank Jim Kohl of Angelica’s Nursery and Margaret Pooler of the National Arboretum, for providing plant material. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Dept. of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable. 2Research Plant Pathologist. [email protected]. Signifi cance to the Nursery Industry Viburnums are important landscaping shrubs to the nursery industry. While they are considered relatively pest-free, they are in a high risk host group for P. ramorum, the causal agent of sudden oak death. However, variations in susceptibility are expected since there are more than 160 Viburnum species. Whole plants of 24 commercially available Viburnum species or cultivars were tested by inoculation followed by evaluation of the number of symptomatic leaves and area of symptomatic tissue on each leaf. All plants tested exhibited some symptoms; however, some had more severe symptoms than others. Based upon the two evaluation criteria, the most susceptible species and cultivars were V. tinus and V. × carlcephalum ‘Cayuga’, while the most tolerant were V. opulus ‘Notcutt’ and V. × rhytidophylloides ‘Willowwood’. This research is important as it increases our knowledge on which plants should be avoided in pathogen infested areas and it identifi es more tolerant or resistant species and cultivars that can be used in a breeding program to develop new tolerant varieties, helping reduce the risk of long distance spread of this pathogen.