ARS | Publication request: Whole plant inoculations of Viburnum species and cultivars testing for susceptibility to Phytophthora ramorum

Submitted to: Journal of Environmental Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 16, 2010
Publication Date: December 1, 2010
Citation: Widmer, T.L. 2010. Whole plant inoculations of Viburnum species and cultivars testing for susceptibility to Phytophthora ramorum. Journal of Environmental Horticulture. 28:197-202.

Interpretive Summary: The pathogen that causes Sudden Oak Death is also known to cause disease on ornamental plants in nurseries, including Viburnums. Diseased ornamental plants are believed to play a significant role in the spread of this pathogen. Therefore, it is very important to test the susceptibility of different species and cultivars to this pathogen. This study screened 25 different Viburnum species or cultivars for their susceptibility to the pathogen that causes Sudden Oak Death using whole plants. All of the plants tested showed some degree of susceptibility based upon leaf symptoms. There was a wide range of susceptibility towards this pathogen depending upon the species or cultivar. The results of this study will benefit the nursery industry and government regulators in providing information on which Viburnum species or cultivars are more susceptible and likely to advance the spread of this pathogen if infected. Other benefits include giving insight as to which species or cultivars may be better utilized in a breeding program to assist in developing resistant cultivars.

Technical Abstract: Phytophthora ramorum is known to cause Ramorum blight on Viburnum species, which are commonly grown as ornamentals. This study evaluated 25 different species or cultivars for their susceptibility to P. ramorum. Whole plants were inoculated with a zoospore suspension of an NA1 isolate of P. ramorum or buffer alone as a control and placed in dew chambers at 20oC. After 5 days, all of the leaves were removed, scanned, and the percentage of necrotic leaves for each plant and the percentage of necrotic leaf area was calculated for each leaf using computer software. The percentage of necrotic leaves per plant ranged from 95.0% (V. Xcarlcephalum ‘Cayuga’) to 7.9% (V. opulus ‘Notcutt’) and the percentage of necrotic leaf area ranged from 73.0% (V. Xcarlcephalum ‘Cayuga’) to 2.4% (V. trilobum ‘Wentworth’). Enough plant material was available for five species or cultivars to be tested additionally with an EU1 isolate of P. ramorum. The isolate had a significant effect on three of the five cultivars tested for the percentage of necrotic leaves per plant and the percentage of necrotic leaf area although neither one of the two isolates always produced a higher level of necrosis than the other. When comparing leaf type, analyses showed that species and cultivars that have evergreen or semi-evergreen leaves have a higher percentage of necrotic leaves and a higher percentage of necrotic leaf area than deciduous species and cultivars. Conclusions based upon the percentage of leaves per plant that was necrotic and the percentage of necrotic leaf area were made that all Viburnum species and cultivars showed some degree of susceptibility.