|Larsen, Meredith - Meg|
|Grunwald, Niklaus - Nik|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2010
Publication Date: 1/15/2011
Citation: Goss, E.M., Larsen, M.M., Vercauteren, A., Werres, S., Heungens, K., Grunwald, N.J. 2011. Phytophthora ramorum detections in Canada: evidence for migration within North America and from Europe. Phytopathology. 101:166-171. Interpretive Summary: The fungus-like pathogen Phytophthora ramorum causes non-lethal disease on popular ornamental plants and sudden oak death disease of oak and tanoak trees in coastal California and southern Oregon forests. Phytophthora ramorum occurs as three distinct genetic groups that likely represent three introductions of the pathogen to North America and Europe. All three groups are present in United States nurseries, but the group responsible for the forest epidemic (named NA1) has been the most commonly detected group. British Columbia, Canada has also had P. ramorum-infected plants in nurseries, but the genetic diversity of P. ramorum in Canada had not been examined. We found all three P. ramorum groups in a sample of Canadian isolates. Unexpectedly, the most common genetic group in Canada was the group that has had the most limited distribution in the United States (NA2). Migration analysis of the group that is found in the United States, Canada, and European countries (EU1) suggests that this group was introduced to North America from Europe.
Technical Abstract: Phytophthora ramorum, the cause of sudden oak death on oaks and ramorum blight on woody ornamentals, has been reported in ornamental nurseries on the West Coast of North America from British Columbia to California. Long distance migration of P. ramorum has occurred via the nursery trade, and shipments of host plants are known to have crossed the United States-Canadian border. We investigated the genotypic diversity of P. ramorum in Canadian nurseries and compared the Canadian population to United States and European nursery isolates for evidence of migration among populations. All three of the P. ramorum clonal lineages were found in Canada, but unexpectedly the most common was the NA2 lineage. The NA1 clonal lineage, which has been the most common lineage in United States nurseries, was found relatively infrequently in Canada and these isolates may have been the result of migration from the United States to Canada. The EU1 lineage was observed almost every year and shared multilocus genotypes with isolates from Europe and the United States. Estimation of migration rates between Europe and North America indicated that migration was higher from Europe to North America than vice versa, and that unidirectional migration from Europe to North America was more likely than bidirectional migration.