Susceptibility in Viburnum to Phytophthora ramorum

Phytophthora ramorum, the causal agent of sudden oak death on oak and tanoak and Ramorum blight or Ramorum shoot dieback on ornamentals, is a recently emerged pathogen. Viburnum is a genus of commonly grown ornamental shrubs known to be susceptible to P. ramorum. The entire genus has been placed on the P. ramorum host list. The range of susceptibility of genotypes in the genus is currently not understood. We evaluated whether or not cultivars in the genus Viburnum differ in susceptibility to P. ramorum in controlled, detached leaf inoculations using two isolates belonging to the two clonal lineages found in Oregon. The genus Viburnum shows remarkable differences in susceptibility to infection by P. ramorum. Viburnum genotypes differed significantly in susceptibility to P. ramorum in detached leaf inoculations. V. × burkwoodii was consistently the most susceptible host genotype, followed by V. plicatum 'Mariesii' and V. lentago. Most cultivars evaluated for resistance to P. ramorum were not susceptible or developed only small lesions as determined with a detached leaf assay. Our work only provides information on leaf based resistance, and inferences on epidemic development in whole canopies and under field conditions cannot currently be made.