Epidemiology of Phytophthora ramorum in Oregon tanoak forests

We followed the local intensification and dispersal of Phytophthora ramorum Werres, De Cock, Man In't Veld in Oregon tanoak (Lithocarpus densiflorus (Hook Arn.) Rehd.) forests from its initial detection in 2001 through 2006, co- incident with a continuing eradication effort. The initial infested area included nine scattered sites below 400 m elevation, close to the Pacific Ocean near Brookings, Oregon. In subsequent years, one-half of new infections were within 122 m of a previous infection, and 79% of the newly detected trees occurred within 300 m of a previously identified tree. Dispersal up to 4 km was occasionally recorded. Initial infection occurred in the upper crowns of tanoak trees. The pathogen was re- covered in rainwater collected beneath diseased tanoak trees in every month from November 2006 through October 2007. Twenty-four multilocus microsatellite genotypes were identified among 272 P. ramorum isolates collected from Curry County. Genotypic analysis provided independent estimates of time of origin of the Oregon infestation, its clustered distri- bution, and dispersal distances. In all sampling years, 60%-71% of the isolates belonged to the same multilocus genotype. In 2001, 12 genotypes were detected and new genotypes were identified in each of the subsequent years, but all isolates belonged to the same clonal lineage. Knowledge of local intensification of the disease and long-distance dispersal should inform both Oregon eradication efforts and national quarantine regulations. Resume´ : Nous avons suivi la progression locale et la dispersion de Phytophtora ramorum Werres, De Cock, Man In't Veld dans des forets de lithocarpe de Californie (Lithocarpus densiflorus (Hook. Arn.) Rehd.) en Oregon depuis que la maladie a etedetectee pour la premiere fois, en 2001, jusqu'en 2006, pendant que se poursuivait les efforts d'eradication. Les zones initialement infectees incluaient neuf stations dispersees en dessous de 400 m d'altitude, aproximitede l'ocean Pacifique, pres de Brookings en Oregon. Au cours des annees qui ont suivi, la moitiedes nouvelles infections etaient sit- uees amoins de 122 m d'une infection precedente et 79% des arbres nouvellement infectesetaient situesamoins de 300 m d'un arbre dejainfecte´. La dispersion de la maladie a occasionnellement eteobservee sur une distance pouvant aller jus- qu'aquatre kilometres. L'infection initiale s'est produite dans la partie superieure de la cime des lithocarpes. Le cham- pignon pathogene a eteretrouvedans l'eau de pluie collectee sous les lithocarpes malades achaque mois, de novembre 2006 aoctobre 2007. Vingt-quatre genotypes multilocus de microsatellites ont eteidentifies parmi les 272 isolats de P. ra- morum collectes dans le comtede Curry. L'analyse genotypique a fourni des estimations independantes du moment du de´- but de l'infection en Oregon, de sa repartition en grappes et des distances de dispersion. Durant toutes les annees