Variation in phenotype for resistance to Phytophthora ramorum in a range of species and cultivars of the genus Viburnum

Phytophthora ramorum is a recently introduced plant pathogen causing a range of diseases including sudden oak death, Ramorum shoot dieback and Ramorum blight (Rizzo and others 2002, 2004; Werres and others 2001). P. ramorum also attacks several nursery crops including viburnum and rhododendron (Werres and others 2001). Since its discovery P. ramorum has been shown to have a remarkably wide host range including many nursery crops (Parke and others 2002; Linderman and others 2002). It is well established that the ornamental plant Viburnum plicatum var. tomentosum cv. Mariesii and other Viburnum species or cultivars are highly susceptible to P. ramorum. What is not known is whether all cultivars in the genus Viburnum are equally susceptible, whether they are field- or container-grown. The objective of our research was to evaluate nine species of field-grown Viburnum for resistance to P. ramorum.